Can't figure out how to properly discard the alpha channel

[Nakilon]

I again appear to be dumber that I suppose when I deal with images and my laziness or lameness plays with me. So I still can't figure out the alpha channel thing.

I want to just properly convert 4 channels to 3 channels.

The issue is... see these two images:

• https://drive.google.com/file/d/1SeYIbaawN5zaR4K3gsoHZyjcb-oy5gri/view?usp=sharing
• https://drive.google.com/file/d/1AMa-1Q8hlfwevHSM1nXHSLZ6l4Dc_rth/view?usp=sharing

If I do

image = image.has_alpha? ? image.flatten : image

one image becomes just black.

If I do:

image = (image.has_alpha? ? image.flatten.composite2(image.bandsplit[3], :screen) : image)

another one becomes just white.

I guess I totally misunderstand the thing.

UPD: Note: one image is downloaded from tg stickers, another one is a html5 base64 exported from p5js canvas after using the text and geometry primitives. I.e. none is made by me band by band so I suppose the images aren't "corrupted" in any way.

[jcupitt]

Hey @Nakilon,

Your first image has RGB all zero, and only has non zero values in the alpha channel. I find vipsdisp useful for looking at images like this: it will show you exactly what's in every pixel in the image:

By default, vips_flatten() will flatten against black (ie. 0), so the output is a black image on a black background :( If you flatten against 255 instead, you'll be able to see the alpha:

image = image.flatten(background: 255) if image.has_alpha?

That should work for your second image too.

Maybe the black default for flatten wasn't a great idea :(

[Nakilon]

Woah, 4 channel eyedropper! Is it available on macOS though?

Thank you for the prompt response and making me see the problem.

I believe I saw the mention of the "default background" somewhere in https://en.wikipedia.org/wiki/Alpha_compositing when I was solving the issue with that image few weeks ago, can't find it now.
That is interesting because on canvas the default color is white I guess (while as you showed it stores the color as black at least in the export by document.getElementsByTagName('canvas')[0].toDataURL()). Also it's white in macOS preview. It's white in Telegram. But black in Skype (when you click/zoom it).

[jcupitt]

You'd need to compile from source I guess :( Though I've not tried.

https://github.com/jcupitt/vipsdisp