envutil - a utility program to convert between lat/lon and cubemap environment maps and extract partial images from them
This is a stand-alone repository for envutil, which started out as a demo program for my library zimt. The program has grown beyond the limits of what I think is sensible for a demo program, and I also think it's a useful tool.
This program takes a 2:1 lat/lon environment or a 1:6 cubemap image as input and produces output in the specified orientation, projection, field of view and extent. For CL arguments, try 'envutil --help'. Panorama photographers may not be familiar with the term 'lat/lon environment' - to them, this is a 'full spherical panorama' or 'full equirect'. The difference is merely terminology, both are the same. Cubemaps are rarely used in panorama photography, but I aim at reviving them by building a code base to process them efficiently, honing the standard, and integrating processing of single-image cubemap format (as it is used by envutil) into lux, my image and panorama viewer - currently lux cubemaps need six separate images and a special litle script and the code is less optimized than the code I offer here. The single-image cubemap format I process in envutil leans on the openEXR standard as far as the order and orientation of the cube face images is concerned: the images are expected stacked vertically (in this sequence: left, right, top, bottom, front, back; top and bottom align with the 'back' image), but the precise geometry of the images may differ from openEXR's standard - I measure cube face image field of view 'edge-to-edge', meaning that I consider pixels small square areas and X degrees field of view extend over just as many of these small areas as the image is wide and high. A different notion is 'center-to-center' measuring, where the X degrees refer to the area enclosed between the marginal pixels seen as points in space (the convex hull). Cubemaps with this type of field of view mesurement can be processed by passing --ctc 1 (for 'center-to-center'). envutil can also process cubemaps with cube face images with larger field of view than ninety degrees (pass --cmbfov). Cubemaps can also be passed as six single cube face images. The images have to follow a naming scheme which the caller passes via a format string. You pass the format string as input, and it's used to generate six filenames, where the format string is expanded with "left", "right"... for the six cube faces. All six image file names generated in this fashion must resolve to square images of equal size and field of view.
The output projection can be one of "spherical", "cylindrical", "rectilinear", "stereographic", "fisheye" or "cubemap". The geometrical extent of the output is set up most conveniently by passing --hfov, the horizontal field of view of the output. The x0, x1, y0, and y1 parameters allow passing specific extent values (in model space units), which should rarely be necessary. To specify the orientation of the 'virtual camera', pass Euler angles yaw, pitch and roll - they default to zero: a view 'straight ahead' to the point corresponding to the center of the environment image with no camera roll. The size of the output is given by --width and --height. You must pass an output filename with --output; --input specifies the environment image. Note that you can use 'envutil' to reproject environment images, e.g convert a cubemap to a lat/lon image and the reverse. To get a full 360 degree lat/lon image from a cubemap, you must pass --hfov 360 and --projection spherical, and for the reverse operation, pass --hfov 90 and --projection cubemap. Export of a cubemap as six separate images can be achieved by passing a format string - the same mechanism as for input is used.
envutil can also produce image series. The individual images are all created from the same environment, but the horizontal field of view and yaw, pitch and roll of the virtual camera for each image are taken from a 'sequence file', passed as --seqfile. The four values are given four in a line, each line provides settings for one image. If you pass a format string for the output, the images will be named accordingly; use format specs like %03d (to make the images lexically sortable). If output is not a format string, it's interpreted as a video file name, and the consecutive images are used to create a video with ffmpeg. This is a handy way to create video sequences of zooms and pans, but you'll want a script to produce the seqfile - it's a bit like a slicer file for a 3D printer - you don't want to write that by hand, either. Default is H265 with 60 fps and 8 Mbit/sec.
You can choose several different interpolation methods with the --itp command line argument. The default is --itp 1, which uses bilinear interpolation. This is fast and often good enough, especially if there are no great scale changes involved - so, if the output's resolution is similar to the input's. --itp -1 employs OpenImageIO (OIIO for short) for interpolation. Without further parameters, OIIO's default mode is used, which uses sophisticated, but slow methods to produce the output. All of OIIO's interpolation, mip-mapping and wrapping modes can be selected by using the relevant additional parameters. Finally, --itp -2 uses 'twining' - inlined oversampling with subsequent weighted pixel binning. The default with this method is to use a simple box filter on a signal which is interpolated with bilinear interpolation; the number of taps and the footprint of the pick-up are set automatically. Additional parameters can change the footprint and the amount of oversampling and add gaussian weights to the filter parameters. Twining is quite fast (if the number of filter taps isn't very large); when down-scaling, the parameter 'twine' should be at least the same as the scaling factor to avoid aliasing. When upscaling, larger twining values will slighly soften the output and suppress the star-shaped artifacts typical for bilinear interpolation. Twining is new and this is a first approach. The method is intrinsically very flexible (it's based on a generalization of convolution), and the full flexibility isn't accessible in 'envutil' with the parameterization as it stands now, but it's already quite useful with the few parameters I offer. Automatic twining is also used for video output, where fixed twining parameters would fail to adapt to changing hfov. Use of twining or OIIO's lookup (which also adapts to the field of view) is strongly recommended for video output to avoid aliasing.
The program uses zimt as it's 'strip-mining' and SIMD back-end, and sets up the pixel pipelines using zimt's functional composition tools. This allows for terse programming, and the use of the functional paradigm allows for many features to be freely combined - a property which is sometimes called 'orthogonality'. What you can't combine in 'envutil' is twining and interpolation with OIIO - this is pointless, because OIIO offers all the anti-aliasing and quality interpolation one might want, and using twining on top would not improve the output - it's meant as an alternative to OIIO's lookup code, but it's a new development and hasn't been tested much. I'd welcome enterprising users to compare it with other anti-aliasing and interpolation methods and share the results!
Currently, the build is set up to produce binary for AVX2-capable CPUs - nowadays most 'better' CPUs support this SIMD ISA. When building for other (and non-i86) CPUs, suitable parameters should be passed to the compiler (you'll have to modify the CMakeLists.txt). I strongly suggest you install highway on your system - the build will detect and use it to good effect. This is a build-time dependency only. Next-best (when using i86 CPUs up to AVX2) is Vc, the fall-back is to use std::simd, and even that can be turned off if you want to rely on autovectorization; zimt structures the processing so that it's autovectorization-friendly and performance is still quite good that way.
The program is built with CMake.
The only mandatory dependency is OpenImageIO - OIIO in short. envutil also links to some ffmpeg libraries, but these should come with an installation of OpenImageIO - OIIO can use ffmpeg to open videos, but at the time of this writing it could not write to videos, so I had to do it 'by hand', using ffmpeg code directly.
If you have highway installed, the build will use it, if not it tries for Vc, and if that isn't present either it uses std::simd. If you dont' want any of the three, set the option USE_GOADING=ON. The functionality and results should be the same with either of the back-ends, but processing time will vary, especially if you are using 'twining'. My own library code (from the zimt library) is provided here as a stripped-down copy - it's header-only C++ template metacode and doesn't link to anything, so it's a compile-time dependency only.
It's recommended to build with clang++
To build, try this:
mkdir build
cd build
cmake [options] ..
make
recommended options for the build:
-DCMAKE_CXX_COMPILER=clang++
-DCMAKE_C_COMPILER=clang
-DCPACK_GENERATOR=DEB
the last option is only useful if you want to build debian packages with 'make package'. Compilation with g++/gcc may or may not work - I don't check regularly.
With version 0.1.1, on top of building on debian12, I have also managed to build envutil on on an intel mac running macOS 12.7.5 (using macPorts for the dependencies) and on windows 11 using mingw64. I haven't yet managed to produce video files with the mac build which played on my mac, but otherwise the builds seem viable.
'make' should produce a binary named 'envutil' or 'envutil.exe''.
envutil --help gives a summary of command line options:
--help Print help message
-v Verbose output
mandatory options:
--input INPUT input file name (mandatory)
--output OUTPUT output file name (mandatory)
important options which have defaults:
--projection PRJ projection used for the output image(s) (default: rectilinear)
--hfov ANGLE horiziontal field of view of the output (default: 90)
--width EXTENT width of the output (default: 1024)
--height EXTENT height of the output (default: same as width)
additional input parameters for cubemap input:
--cbmfov ANGLE horiziontal field of view of cubemap input (default: 90)
--support_min EXTENT minimal additional support around the cube face proper
--tile_size EXTENT tile size for the internal representation image
--ctc CTC pass '1' to interpret cbmfov as center-to-center (default 0)
additional parameters for single-image output:
--yaw ANGLE yaw of the virtual camera
--pitch ANGLE pitch of the virtual camera
--roll ANGLE roll of the virtual camera
--x0 EXTENT low end of the horizontal range
--x1 EXTENT high end of the horizontal range
--y0 EXTENT low end of the vertical range
--y1 EXTENT high end of the vertical range
additional parameters for multi-image and video output:
--seqfile SEQFILE image sequence file name (optional)
--codec CODEC video codec for video sequence output (default: libx265)
--mbps MBPS output video with MBPS Mbit/sec (default: 8)
--fps FPS output video FPS frames/sec (default: 60)
interpolation options:
--itp ITP interpolator: 1 for bilinear, -1 for OIIO, -2 bilinear+twining
parameters for twining (with --itp -2):
--twine TWINE use twine*twine oversampling - default: automatic settings
--twine_width WIDTH widen the pick-up area of the twining filter
--twine_sigma SIGMA use a truncated gaussian for the twining filter (default: don't)
--twine_threshold THR discard twining filter taps below this threshold
parameters for lookup with OpenImageIO (with --itp -1):
--tsoptions KVLIST OIIO TextureSystem Options: coma-separated key=value pairs
--swrap WRAP OIIO Texture System swrap mode
--twrap WRAP OIIO Texture System twrap mode
--mip MIP OIIO Texture System mip mode
--interp INTERP OIIO Texture System interp mode
--stwidth EXTENT swidth and twidth OIIO Texture Options
--stblur EXTENT sblur and tblur OIIO Texture Options
--conservative YESNO OIIO conservative_filter Texture Option - pass 0 or 1
The input can be either a lat/lon environment image (a.k.a. 'full spherical' or 'full equirect' or '360X180 degree panorama') - or a 'cubemap' - a set of six square images in rectilinear projection showing the view to the six cardinal directions (left, right, up, down, front, back). The cubemap can be provided as a single image with the images concatenated vertically, or as six separate images with 'left', 'right' etc. in their - otherwise identical - filenames, which are introduced via a format string.
envutil only processes sRGB and linear RGB data, the output will be in the same colour space as the input. If you use the same format for input and output, this will automatically be the case, if not, you may get faulty output if the default coulour spaces of the formats don't match - your output will look too bright or too dark. The number of colour channels will also be the same in the output as in the input - it's recommended you use the same file format for both, e.g. produce JPEG output from JPEG input, but if the formats are compatible, you're free to move from one to the other. To get mathematically correct results, note that your input should be in linear RGB (e.g. from an openEXR file), because internally all calculations are done as if the image data were linear RGB. This is not mathematically correct for sRGB data. Future versions of envutil may add a stage of processing to deal with non-linear-RGB input. Linear RGBA is fine; the import with OIIO should provide associated alpha which can be processed correctly by envutil as four-channel data, producing correct output for images with alpha channel. Single-channel data are also fine, and even two-channel data should work, though it's hard to find them 'out there' - maybe anaglyphs? Channels beyond four are ignored.
The options are given with a headline made from the argument parser's help text. The capitalized word following the parameter is a placeholder for the actual value you pass.
Any image file which has 2:1 aspect ratio will be accepted as lat/lon environment map, and any image file with 1:6 aspect ratio will be accepted as a cubemap. If you pass a string containing a percent sign, envutil will consider this as a format string which can be used to specify six separate cube face images. The string will be used like a 'normal' C format string to generate six filenames, passing the strings "left", "right", "top", "bottom", "front" and "back", in turn, to replace the format sequence in the string - you'd typically just use '%s' here. Only one percent sign is allowed, and the images are all treated alike. And they must, of course, all exist and have 1:1 aspect ratio. Once they've been loaded into the internal representation, processing continues as if the input had been a single cubemap image. When invoked with an input containing a percent sign, envutil first tries to find the left cube face image. If that fails, the attempt to load six images is aborted and instead envutil will assume you have actually passed a filename containing a verbatim percent sign and proceed accordingly - and likely fail.
The output will be stored under this name. If you are generating cubemaps (pass --projection cubemap) you may pass a format string, just as for input. You'll get six separate cube face images instead of the single-image cubemap. When generating image sequences and you want single-image output, you also have to pass a format string, but with a format element specifying an integer. This is replaced with the output image's sequential number. Use a format element like %03d to get alphabetically sortable images - in preference to e.g. %d which does not produce leading zeroes. If you are generating image sequences and your output parameter is not a format string, envutil will produce video output to the given filename.
Pass one of the supported output projections: "spherical", "cylindrical", "rectilinear", "stereographic", "fisheye" or "cubemap". The default is "rectilinear".
The default here is ninety degrees, which all projections can handle. Spherical, cylindrical and fisheye output will automatically periodize the image if the hfov exceeds 360 degrees. Rectilinear images can't handle more than 180 degrees hfov, and at this hfov, they won't produce usable output. Stereographic images can theoretically accomodate 360 degrees fov, but just as rectilinear images aren't usable near their limit of 180 degrees, they aren't usable when their limit is approached: most of the content becomes concentrated in the center and around that a lot of space is wasted on a bit of content which is stretched extremely in radial direction. For cubemaps, you should specify ninety degrees hfov, but you can produce cubemaps with different hfov - they just won't conform to any standards and won't be usable with other software unless that software offers suitable options. envutil supports wider-angle cubemaps, see --cbmfov
in pixel units. For lat/lon environment images, this should be precisely twice the height, so this value should be an even number. I recommeend that you pick a multiple of a small-ish power of two (e.g. 64) to make it easier for software wanting to mip-map the data. When producing cubemaps from full sphericals, I recommend using a width which is ca. 1/pi times the width of the input. For the reverse operation, just use four times the cubemap's width. These factors preserve the resolution.
in pixel units. For cubemaps, this is automatically set to six times the width. For spherical output, if height is not passed, it is set to half the width, increasing 'width' to the next even value. For other projections, if you don't pass 'height', the default is to use the same as the width, so to render a square image.
If the environment given as input is a cubemap, you can specify the horizontal field of view of the cube face images with this parameter. The default is precisely ninety degrees, but values greater than ninety are allowed as well. You can create such wider-angle cubemaps by passing --hfov greater than ninety when creating a cubemap with envutil.
This is a technical value, probably best to leave it at it's default of 8. when a cubemap is converted to it's internal representation, the ninety degree 'cube face proper' is surrounded with a frame of additional 'support' pixels which help interpolation. You can specify here how wide this frame should be at the least.
Also best left at the default of 64. This value takes care of widening the support frame further - if necessary - to make the size of the square images in the internal representation a multiple of this size. This does not magnify the image but adds pixels reprojected from other cube faces, so it doesn't affect image quality. Using a small-ish power of two here is especially useful when using OIIO for lookup, to help it mip-map the texture generated from the internal representation - OIIO can't natively process cubemap environments, so envutil generates a texture file and feeds that to OIIO's texture system for the look-up.
The standard way of measuring the field of view of cube face images in envutil is to consider pixels as small square areas of constant colour with an extent of one pixel unit. If an image is W pixels wide, a field of view of D degrees is taken to coincide with the angle between rays to the left margin of the leftmost pixel and the right margin of the rightmost pixel (same for top and botttom). If you pass --ctc 1, D will instead coincide with the angle between rays to the centers of the marginal pixels. So usually (--ctc 0), we have D = atan ( f * W / 2 ), but with --ctc 1, we have D = atan ( f * ( W - 1 ) / 2 ) This is hard to see, but some cubemaps seem to use this convention, and using them without --ctc will lead to subtle errors. Internally, envutil uses the first notion, and simply recalculates the field of view to be used internally to the slightly larger value which results form the edge-to-edge notion; You could do the same 'manually' and pass a slightly higer value for cbmfov - using --ctc is merely a convenience saving you the calculation.
Note that --ctc does not affect the processing of lat/lon environment maps - I may add a separate option for that. lat/lon environment maps are always processed with edge-to-edge semantics, assuming the image is periodic in the horizontal - the first column follows again after the last. The other assumption about lat/lon images is that they also follow edge-to-edge semantics for the vertical: the image lines at the very top and bottom of the image represent a (very small) circle around the pole with half a pixel width radius, opposed to some lat/lon formats which use 'center-to-center' semantics and repeat a single value (namely that for the pole) for all pixels in the top, and another for the bottom row. From this it shoud be clear why envutil expects lat/lon images to have precisely 2:1 aspect ratio. Envutil honours the peculiarities of the spherical (a.k.a. equirectangular) projection and interpolations near the poles will 'look at' pixels which are nearby on the spherical surface, even if they are on opposite sides of the pole, so you can e.g. safely extract nadir caps.
These three angles are applied to the 'virtual camera' taking the envutiled view. They default to zero. It's okay to pass none or just one or two. yaw is taken as moving the camera to the right, pitch is taken as upward movement, and roll as a clockwise rotation. Note that the orientation of the virtual camera is modified; when looking at the resulting images, objects seen on them seem to move the opposite way. Negative values have the opposite effect. Panorama photographers: to extract nadir patches, pass --pitch -90 These angles are known as the 'Euler Angles' and are easy to understand, as opposed to the quaternions which envutil uses internally to represent rotations.
These are special values which can be used to specify the extent, in model space units, of the output. This requires some understanding of the inner workings of this program - if you use -v, the verbose output will tell you for each rendering which extent values are generated from a field of view parameter, given a specific projection. This can help you figure out specific values you may want to pass, e.g. to produce anisotropic output or cropped images.
Alternative output of an image sequence with hfov, yaw, pitch and roll given as four consecutive numbers per line, one line per image. This is a new experimental feature, output files are numbered consecutively when 'output' can be interpreted as a format string (use something like --output img%03d.jpg). If 'output' is not a format string, it's interpreted as the name of a video file, which is put together from the image sequence using ffmpeg and the codec given with --codec (default is libx265).
Single images can be combined into a video with ffmpeg, like so:
ffmpeg -f image2 -pattern_type glob -framerate 60 -i 'seq*.jpg' -s 960x540 -c:v libx264 foo.mp4
This may be preferable, because all ffmpeg options can be exploited that way, and the code in envutil is derived from a very old example program which may not be at the height of time.
This is a string passed to ffmpeg to specify the video codec. Internally, frames for video are encoded in YUV, which may not work with other codecs than the hevc family (H264, H265). If direct video output fails, pass a format string as output and then combine the separate images with ffmpeg (see --seqfile, above) for H264 output, pass libx264. This is a bit enigmatic - I haven't yet figured out which strings to pass here for all the codecs which ffmpeg supports, and which of the codecs work with the pixel type I supply.
Megabits/second value for the codec. This determines how much the data will be compressed by the video codec. Note that this only affects video output - if you generate separate images, they aren't affected by this parameter.
This does not change the image sequence, but it changes the speed of the video when it's displayed.
This is an integer value determining the interpolation method. There are currently three modes of interpolation:
1 - use simple bilinear interpolation directly on the source image
this is the fastest option, and unless there is a significant scale
change involved, the output should be 'good enough' for most still
image renditions. This is the default.
-1 - use OpenImageIO's 'environemnt' or 'texture' function for lookup.
without additional arguments, this will use a sophisticated
interpolator with good antialiasing and bicubic interpolation.
-2 - use 'twining' - this is a method which first super-samples and then
combines several pixels to one output pixel ('binning'). This is my
own invention. It's quite fast and produces good quality output.
This method should see community review to compare it with other
methods. If you only pass --itp -2 and not --twine, envutil will
set up twining parameters calculated to fit well with the
transformation at hand, and that's also done for image sequence
output, where the parameters have to adapt to the changing geometry.
Why use negative values for ITP for the second and third mode? This is similar to the values used in lux for 'decimators' - in lux, positive values are reserved for degrees of a b-spline reconstruction filter used as low-pass filter. bilinear interpolation is the same as a degree-1 b-spline, hence the value 1 for bilinear interpolation. I may add other spline degrees to envutil, so I reserve the positive numbers for future use.
These options control the 'twining' filter. These options only have an effect if you activate twining with --itp -2. envutil will use bilinear interpolation on the source image for single-point lookups, but it will perform more lookups and then combine several neighbouring pixels from the oversampled result into each target pixel.
The effect of 'twining' is the same as oversampling and subsequent application of a box filter. The filter is sized so that the oversampling is uniform over the data, but the direct result of the oversampling is never saved - all samples falling into a common output pixel are pooled and only the average is stored. This keeps the pipeline 'afloat' in SIMD registers, which is fast (as is the arithmetic) - especially when highway or Vc are used, which increase SIMD performance particularly well.
If you activate twining by selecting --itp -2, but don't pass --twine (or pass --twine 0 explicitly), envutil will set up twining parameters automatically, so that they fit the relation of input and output. If the output magnifies (in it's center), the twine width will be widened to avoid star-shaped artifacts in the output. Otherwise, the twine factor will be raised to avoid aliasing. You can see the values which envutil calculates if you pass -v. If the effect isn't to your liking, you can take the automatically calculated values as a starting point, but even if you pass --twine, the values will adapt in an image sequence. --twine only has an effect for single-image output
A second parameter affecting 'twining'. If the source image has smaller resolution than the target image, the output reflects the interpolator's shortcomings, so with e.g. bilinear interpolation and large scale change (magnification) the output may show star-shaped and staircase artifacts. A 'standard' twine with twine_width 1.0 will pool look-ups which all correspond to target locations inside the target pixel's boundaries. For magnifying views, this becomes ever more pointless with increasing magnification - the look-up locations will all fall into a small area of the source image - so, they'll be very much one like the other, and pooling several of them is futile. So for magnifying views, you want to widen the area in which look-ups are done to an area which is in the same order of magnitude as a source image pixel. To get this effect, try and pass a twine_width up to the magnitude of the scale change, or rely on automatic twining, which calculates a good value for you.
Input with low resolution is often insufficiently band-limited which will result in artifacts in the output or become very blurred when you try to counteract the artifacts with excessive blurring. There's little to be gained from scaling up anyway - the lost detail can't be regained. If you don't get satisfactory results with twining and adequate twine_width, you may be better off with one of the better OIIO interpolators, e.g. bicubic.
The twine_width parameter also only affects single-image output - for image sequences, it's set automatically to fit the relation of input and output. If --twine is not passed (or passed 0) this parameter will be calculated automatically, and any value passed here is then overridden by the automatics.
If you don't pass --twine_sigma, envutil will use a simple box filter to combine the result of supersampling into single output pixels values. If you pass twine_sigma, the kernel will be derived from a gaussian with a sigma equivalent to twine_sigma times the half kernel width. This gives more weight to supersamples near the center of the pick-up. If you pass -v, the filter is echoed to std::cout for inspection.
You can combine this parameter with automatic twining - the twine factor and the twine width will be calculated automatically, then the gaussian is applied to the initially equal-weighted box filter. Keep in mind that applying gaussian weights will 'narrow' the filter, so you may need to pass a larger twine_width to counteract that effect. This may be a bit counterintuitive, because gaussians are commonly associated with blurring - but a gaussian kernel produces 'sharper' output than a box filter. Also consider the next parameter which eliminates weights below a given threshold to save CPU time.
If you pass twine_sigma, marginal twining kernel values may become quite small and using them as filter taps makes no sense. Pass a threshold here to suppress kernel values below the threshold. This is mainly to reduce processing time. Use -v to display the kernel and see which kernel values 'survive' the thresholding. This parameter makes no sense without --twine_sigma (see above): if all weights are equal, they'd either all be above or below the threshold.
You can combine this parameter with automatic twining - the twine factor and the twine width will be calculated automatically, then the twine_sigma is applied, and finally the thresholding eliminates small weights.
After thresholding, the weights are 'normalized' to produce a filter with 'unit gain' - you can see that all the weights add up to 1.0 precisely. Without the normlization, just eliminating the sub-threshold taps would darken the output.
These options can be used to configure the look-up with OpenImageIO's 'environemnt' and 'texture' functions. You can read up the options in the OpenImageIO documentation
This argument can be used to pass a set of comma-separated key=value pairs to the 'catch-all' OIIO texture system attribute 'options'. It's a handy way to handle the transfer, because OIIO has a parser for such lists.
Separate wrapping modes, determining how texture access outside the texture area proper is handled.
mip-mapping mode. I use a default of 'automip=1' for tsoptions
interpolator. This is the interpolator OIIO uses for it's lookup, not the interpolator specified with the --itp option. To use OIIO's lookup, you pass --itp -1. Then you can use --interp to specify OIIO's interpolator, like --interp InterpSmartBicubic
I lump together the swidth and twidth options - OIIO allows to pass them separately, but I don't see a need to do so in envutil. The most useful value here is to pass zero, which makes OIIO ignore the pickup-point's derivatives. This can speed up the calculations.
I also lump together pre-blur along the s and t axis. This is to add more blur to the processing.
pass this to switch on OIIO's 'conservative filter' on or off (pass 0 or 1); the default is to have it on.
One problem with cubemaps is that they are normally stored as concatenations of six square images with precisely ninety degrees fov (field of view). This makes access to pixels near the edges tricky - if the interpolator used for the purpose needs support, marginal pixels don't have it all around, and it has to be gleaned from adjoining cube faces, reprojecting content to provide the support. envutil uses an internal representation (IR for short) of the cubemap which holds six square images with a fov slightly larger than ninety degrees, where the part exceeding the 'ninety degrees proper' cubeface image is augmented with pixels reprojected from adjoining cube faces. With this additional 'frame', interpolators needing support can operate without need for special-casing access to marginal pixels. The formation and use of the IR image is transparent, it's used automatically. There are two parameters which influence the size of the 'support margin', namely --support_min and --tile_size - usually it's best to leave them at their defaults. To access pixels in lat/lon environment maps which are marginal, envutil exploits the inherent periodicity of the lat/lon image - simple periodicity in the horizontal and 'over-the-pole' periodicity in the vertical (that's reflecting in the vertical plus an offset of half the image's width in the horizontal). One can in fact generate an image from a full spherical which is periodic vertically by cutting of the right half, rotating it by 180 degrees and pasting it to the bottom of the first half. lux does just that for it's 'spherical filter', so that the image pyramids used internally can be created with geometrically correct down-scaling. envutil does not use mip-mapping or other pyramid-like code itself (even though OIIO's texture system code does so), so this is merely a technical hint.
As an alternative to the antialiasing and interpolation provided by OIIO, envutil offers processing with bilinear interpolation and it's own antialiasing filter, using a method which I call 'twining'. Both for OIIO-based lookup and for twining, the beginning of the pixel pipeline receives not just one 3D directional 'ray' coordinate pointing 'into' the environment, but three: the second and third ray are calculated to coincide with a target coordinate one discrete step toward the right or downwards, respectively, in the target image. The pixel pipeline which receives these sets of three ray coordinates can glean the difference between the first ray and the two other two rays and adapt the lookup based on this additional information. For lookup, simple differencing produces an approximation of the derivatives, or, to interpret the difference differently, a pair of 3D vectors which can be used to construct a plane and place several lookup points on that plane, to combine their results to form the output as a weighted sum.
From visual inspection of the results, envutil's use of OIIO's lookup seems to produce quite soft images. OIIO's lookup is - with the settings envutil uses - conservative and makes an effort to avoid aliasing, erring on the side of caution. Of course I can't rule out that my use of OIIO's lookup is not coded correctly, but I'm quite confident that I've got it right. As of this writing, OIIO's lookup offers functions with SIMD parameter set, processing batches of coordinates. Internally, though, the coordinates are processed one after the other (albeit with use of vertical SIMDization for the single-coordinate lookups). This is one of the reasons why this process isn't very fast. OIIO's method of inspecting the derivatives has the advantage of being perfectly general, and the lookup can decide for each pixel whether it needs to be interpolated or antialiased - and how much.
In contrast, using bilinear interpolation (--itp 1) is very fast, but it's not adequate for all situations - especially not if there are large differences in resolution between input and output. If the scale of the output is much smaller than the input's, the use of 'twining' should provide adequate antialiasing. When scaling up, bilinear interpolation only produces visible artifacts if the scale change is very large (the typical 'star-shaped artifacts'), which makes little sense, because there's no gain to be had from scaling up by large factors - the content won't improve. Up-scaling with OIIO's lookup uses bicubic interpolation, which may be preferable. Ultimately it's up to the user to find a suitable process by inspecting the output. envutil's 'twining' can cover a certain range of input/output relations with a given parameter set, but for extreme distortions, it may be suboptimal. This should be analyzed in depth, for now, it's just a promising new method which seems to work 'well enough'.
I think that twining offers a good compromise beween speed and quality. I'm sure I'm not the first one to think of this method, but I haven't done research to see if I can find similar code 'out there'. What I am sure of is that my implementation is fast due to the use of multithreaded horizontal SIMD code through the entire processing chain, so I think it's an attractive offer. I'd welcome external evaluation of the results and a discussion of the methods; please don't hesitate to open issues on the issue tracker if you'd like to discuss or report back! In my tests, images generated with twining seemed to come out somewhat 'crisper' than renditions with OIIO's default mode, so they may be preferable for photographic work, whereas OIIO's renditions make extra sure there is no aliasing, which is good for video output.
There seems to be ambiguity of what constitutes a 'correct' cube face image with ninety degrees field of view. In envutil, I code so that each pixel is taken to represent a small square section of the image with constant colour. So the 'ninety degrees proper' extends form the leftmost pixel's left margin to the rightmost pixel's right margin. Some cubemap formats provide images where the centers of the marginal pixels coincide with the virtual cube's edges, repeating each edge in the image it joins up with in the cube. If you process such cubemaps with envutil, pass --ctc 1 (which stands for center-to-center). Otherwise, there will be subtle errors along the cube face edges which can easily go unnoticed. Make sure you figure out which 'flavour' your cubemaps are.
I use a coding style which avoids forward declarations, so at times it's better to read the code from bottom to top to follow the flow of control. My code formatting is quite 'old-school', with white-space-separated tokens and curly braces in separate lines - and generally a lot of white space. I think this style makes the code easier to grasp and also helps with reading it.
The code itself is 'optimistic' - I don't make efforts to prevent errors from happening - if they happen, most of the time an exception will terminate the program. I tend not to analyze parameters and trust the user to pass sensible ones.
The code is complex - it's template metacode, and control flow may be hard to grasp, due to the use of functional programming. Functional components follow the pattern I have established in vspline and zimt: the functors have an 'eval' method which takes a const reference for input and writes output to another reference. The eval method itself is void. Usually I code it as a template to avoid a verbose signature - the types are implicit from the functor's template arguments, and oftentimes the template can be used for scalar and SIMDized arguments alike.