one image becomes just black.
\nIf I do:
\nimage = (image.has_alpha? ? image.flatten.composite2(image.bandsplit[3], :screen) : image)another one becomes just white.
\nI guess I totally misunderstand the thing.
\nUPD: 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.
","upvoteCount":1,"answerCount":1,"acceptedAnswer":{"@type":"Answer","text":"Hey @Nakilon,
\nYour 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:
\n![\"image\"](https://flashblogg.com:443/je/cache.php/https://github.com/libvips/ruby-vips/discussions/\"https:/user-images.githubusercontent.com/580843/209337495-3c1df3ea-58ae-460d-908b-3a3be630a1f7.png\")
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.
\nMaybe the black default for flatten wasn't a great idea :(
","upvoteCount":1,"url":"https://github.com/libvips/ruby-vips/discussions/355#discussioncomment-4484576"}}}
-
|
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:
If I do image = image.has_alpha? ? image.flatten : imageone 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. |
Beta Was this translation helpful? Give feedback.
Replies: 1 comment 2 replies
-
|
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, 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 :( |
Beta Was this translation helpful? Give feedback.
-
|
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. |
Beta Was this translation helpful? Give feedback.
-
|
You'd need to compile from source I guess :( Though I've not tried. |
Beta Was this translation helpful? Give feedback.
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:That should work for your second image too.
Maybe the black default for flatten wasn't a great idea :(