Add ability to specify target size in "number of pixels"

[clairem-sl]

For reasons beyond my control, I am sometimes required to submit images "less than N megapixels in total pixels size"

Because the images I need to submit oftentimes do NOT have uniform aspect ratio, currently I have to manually calculate the proper bounding box for each image I want to submit:

In pseudo-code:

for each source_file in directory:
  width = get_width(source_file)
  height = get_height(source_file)
  
  desired_pixels = desired_mp * (10 ** 6)
  
  target_width = int(sqrt(desired_pixels * width / height))
  target_height = int(sqrt(desired_pixels * height / width))

  preview_file = (replace-suffix source_file ".preview.png")

  vipsthumbnail source_file --size "{target_width}x{target_height}>" -o preview_file

I think it will be great to add a new option, let's say --size-mp M, or --size-pixels P, wherein vipsthumbnail will perform all the necessary scaling calculation above internally per processed file.

Example usages

vipsthumbnail source1.png --size-mp 20 -o source1_preview.png
# Equivalent to
vipsthumbnail source1.png --size-pixels 20M -o source1_preview.png

# Or apply to a bunch of files, wherein the calculation will be done per-file by vipsthumbnail
vipsthumbnail *.png --size-mp 10 -o %s.preview.png

# Should also be able to use '>' or '<' to resize conditionally
vipsthumbnail *.png --size-mp "10>" -o %s.preview.png  # Only resize images larger than 10 MP

(Maybe limit the suffixes for --size-pixels to K, M, and maaaybe G. I don't think anyone will want terapixel-sized images.)

Optionally, for --size_pixels should also be able to ignore _ (underscore) inserted for visiblity, e.g.:

vipsthumbnail *.png --size-pixels 7_500_000 -o %s.preview.png
# should be equivalent to
vipsthumbnail *.png --size-pixels 7500000 -o %s.preview.png

Of course the desired number of pixels is 'approximately'; depending on the ratio between width and height, an exact number might be impossible to achieve. But using the formula in the pseudocode above, a 'reasonably accurate' number should be achievable. If the "number of pixels limit" is a hard limit, and the calculation method has the possibility of doing a rounding up, it will be the responsibility of the user to target a smaller pixel-size. E.g. --size-pixels 9_800K or --size-mp 9 if the hard limit is exactly 10 million pixels and not a single pixel more.

And finally, this option definitely conflicts with --size; so if both are specified, an error should be raised.

[jcupitt]

Hi @clairem-sl,

We could perhaps implement more of imagemagick's size specifications:

https://imagemagick.org/command-line-processing/#geometry&gsc.tab=0

They use the syntax:

$ vipsthumbnail something.jpg --size 1000@

ie. appending @ means "size to this many pixels or less".

vipsthumbnail currently implements x, >, < and !, so it'd just be adding one further option.

[jcupitt]

I made a quick PR, any comments very welcome.

[jcupitt]

#5084

[clairem-sl]

Sweet! Did not realize that the --size syntax is derived from IM's.

Yes, adopting the @ suffix would be perfectly suitable for my needs!