invert SkImage

[Noel Grandin]

Hi

Newbie question - what is the best way to invert an SkImage? 

    i.e. dest = 1 - src

Thanks, Noel Grandin

[craste...@gmail.com]

Do you want to draw it to the screen inverted?

Or do you want to write it to a file with the pixel values inverted?

[John Stiles]

You can do this pretty easily with a Runtime Effect:

https://fiddle.skia.org/c/26b417697e9996b0bce3b32ab62cbd5b

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[Michael Katz]

Here's another Runtime Effect but with a custom blender instead of a custom shader:

https://groups.google.com/g/skia-discuss/c/AYpB0Z-fX94?pli=1

[craste...@gmail.com]

You can use the blendmode "kDifference" if the area being drawn to is white first.

https://fiddle.skia.org/c/64dd24ecff8c907b14612791f0f6081b

[Noel Grandin]

Thanks so much for all the help, that is very useful.

[Noel Grandin]

On Saturday, July 1, 2023 at 5:10:17 PM UTC+2 craste...@gmail.com wrote:

Do you want to draw it to the screen inverted?

Or do you want to write it to a file with the pixel values inverted?

I'll be drawing it to the screen. 

I have an existing (single-channel) SkImage, and I want to invert it (because its transparency and I want it to be alpha or vice-versa).

Currently I read it back from the GPU, construct a new one in memory, and push it back to the GPU, which is obviously pretty hideous for performance.

PS. If you think this sounds pretty awful, you should see the rest of my graphics stack :-)

 

[craste...@gmail.com]

I also imagine that there is a way to use a color filter with a transformation matrix that reverses the colors by translating the RGB values by 1 and then set the scalars to -1.

Though I have not tried it... And it wouldn't require that you draw the image on top of white.

Or you could use the shader, like in the first example.  Not sure which would be more straight-forward.