Targa Image Loading for Common Lisp
A Truevision (TGA) image loader for Common Lisp.
This package makes no attempt at trying to put the TGA image loaded into a format that is good for use with any OS. It simply parses the TGA and extracts the pixel data into an easily accessible format.
It is then assumed that it can be used by the developer to create OpenGL textures, converted into another image format, etc.
Just use the
tga-read functions to load TGA files off disk or to read one from a stream.
CL-USER > (setf tga (tga-load #p"tests/FLAG_T32.tga")) #<TARGA::TGA 32 bit, 124x124, TRUE-COLOR-IMAGE>
If you use
tga-read, be sure that the stream is an input stream of element-type
(unsigned-byte 8), otherwise the read will fail.
That's pretty much it. ;-)
Once you have the TGA structure, you can use the reader functions to find out everything about it. Most of the time what will be of most importance are the width, height, and the pixel data.
CL-USER > (tga-header-width (tga-header tga)) 124 CL-USER > (tga-header-height (tga-header tga)) 124 CL-USER > (tga-get-pixel tga 34 16) (248 248 0 255)
The TGA pixels is a 1D array of all the pixels in scanline-order (read: row-major). The size of the array is the width * height of the image.
No matter what format the pixels were originally stored in (color mapped, run-length encoded, 8-bit grayscale, 16-bit, 24-bit, 32-bit, etc.), each element of a scanline array is a list of 4 color components: red, green, blue, and alpha. Each of which is in the range of 0-255.
A TGA file can have its screen origin on the y-axis be the bottom or top, the x-axis origin on the left or right. When reading in the TGA, no effort is made to keep a consistent origin for the pixels array. But, as long as you use the
tga-get-pixel function, it will ensure that you get the "correct" pixel (originating from the top-left).
(tga-get-pixel tga x y) ;=> (r g b a)
Getting extension values
tga-read, there is an optional (defaulting to
T) keyword parameter: read-ext-and-tags. If this is non-nil, then, if this is a new TGA file with an extension area and developer tags, they will be read in.
If the TGA has an extension, there are several other optional areas that can then be read from the stream as well:
(tga-read-color-correction-table stream tga) ;=> table (tga-read-scanline-table stream tga) ;=> scanlines (tga-read-postage-stamp-image stream tga) ;=> tga
The table is an array of 256 lists, where each list is comprised of 4 floating point values for red, green, blue, and alpha. It is up to you to use these values in conjunction with
tga-get-pixel however you'd like.
The scanlines is simply an array of offsets into the stream (1 per row for the image). Generally speaking it isn't very useful since all the scanlines have been pre-read. But it's here if you are interested.
The postage image is probably the most interesting. It will parse a smaller sized version of the TGA and return a new TGA that has the same format as its parent, but a different header and pixels array.
I might add support in the future for writing TGA files, but that's a low priority unless I happen to get a lot of requests for it.
To test this library, I use the images found at http://www.fileformat.info/format/tga/sample/, which have targas saved with the various color formats, RLE compression, and different screen originations.