Image::thumbnail and color profile normalisation

[binaryfire]

Hi. I saw this in the libvips docs:

Final suggestion
Putting all this together, I suggest this as a sensible set of options:

$ vipsthumbnail fred.jpg \
    --size 128 \
    --export-profile srgb \
    -o tn_%s.jpg[optimize_coding,keep=none]

I'm using Image::thumbnail() to optimize user uploads to my CMS. Should I be converting color profiles to sRGB as well? I need to make sure images display correctly in the browser but I'm not sure if this is already taken care of under the hood. Or if it's necessary.

These are the options I'm using at the moment:

        'jpeg' => [
            'quality' => 80,
            'keep' => 'none',
            'optimize_coding' => true,
            'export_profile' => 'srgb', // <------------ is this correct / necessary?
        ],

Also, are the options the same for JPG2000 files? Just wondering if I need to handle them differently or if I can use the same settings.

Cheers

[jcupitt]

Hi @binaryfire,

For thumbnails you generally want to either strip profiles (and get the default sRGB rendering), or transform to a known space, like adobe98 or one of the HDR spaces.

This is because colour profiles can be extremely large, for example:

$ vipsheader -a Apple_Group_with_Labels_V3.jpg | grep icc
icc-profile-data: 557168 bytes of binary data

You absolutely do not want 500kb thumbnails.

The --export-profile srgb option transforms to sRGB, replacing any profile on the image with a sRGB profile, then --keep none removes that as well, since default rendering will be sRGB.

You can experiment at the CLI:

$ vipsheader Apple_Group_with_Labels_V3.jpg
Apple_Group_with_Labels_V3.jpg: 2625x1837 uchar, 4 bands, cmyk, jpegload
$ vipsthumbnail Apple_Group_with_Labels_V3.jpg -o x.jpg
$ vipsheader x.jpg 
x.jpg: 128x89 uchar, 3 bands, srgb, jpegload
$ vipsheader -a x.jpg | grep icc
$ 

So the CMYK JPEG is transformed to profile-less sRGB.

[jcupitt]

The vipsthumbnail command does keep=none for you if it's an sRGB profile. The JPEG2000 library we use has no colour management support, so these options will have no effect there.

[binaryfire]

Hi @jcupitt. I'm still a little confused. I'm not very knowledgeable about colour profiles :)

I'm using Image::thumbnail() to optimise everything - shrink the originals a bit to save on storage, generate responsive variants and generate the thumbnail. I'm already using keep=none to make the images as small as possible and I know that will strip colour profiles.

What I'm unsure about is whether keep=none does automatic colour conversion of non-sRGB images to sRGB? If not, then my understanding is non-sRGB images would look different in the browser afterwards? That's what I need to avoid, since users expect their uploads to look the same on their site as on their device. If that's case, I should probably always use export-profile srgb to make uploads are always converted to the "web standard" first, right?

Regarding the format for PHP, do I pass it in via the options array as export_profile => 'srgb' (using underscores)? It's set using a
--flag in the cli and not via the -o array so I wasn't sure how to use it in PHP.

Regarding JPG2000 images, can I still pass them the set of optimization options as normal JPGs without it throwing any errors? When you say the colour management options won't work there, I'm assuming they'll just be ignored for JPG2000s as opposed to causing an exception? I've created optimizer classes for each mime type and I'm wondering if I can use my JpegOptimizer for JPG2000s or whether I need to make a separate class that doesn't attempt any colour conversion.

Thanks again!

[jcupitt]

Setting the export profile does the colour conversion, so if you set p3, for example, you'll get an image in p3 space and with a p3 profile attached to it.

Yes, it's export_profile in PHP.

Yes, you can use the colour options for jp2k load and save, they just won't do anything, since profile support in jp2k is so poor. You can't set things like optimise_coding for jp2k though.

[binaryfire]

Cheers!

[binaryfire]

Hi again. I'm hitting an optional argument 'export_profile' does not exist exception when I pass export_profile as an option to Image::thumbnail()

[
    'Q' => 75,
    'export_profile' => 'srgb',
    'keep' => 'none',
    'optimize_coding' => true,
] 

It works fine without it. Is it not available on thumnail() ?

[jcupitt]

Q, keep and optimize_coding are jpegsave options, not thumbnail.

[binaryfire]

I'm an idiot - I've only ever passed options to writeToFile() and had no idea thumbnail() accepted its own options array. Spent a couple of hours reading the docs yesterday but somehow that didn't register. Tunnel vision... Working beautifully now👍