Every process flips images vertically on Mac OS with the Metal API

[pgervais]

To reproduce

• Open Score, create a new project

• Create a Window device use the Device Explorer -> a video window opens

• Drag and drop and image on the canvas, connect the output to the Window device using the inspector

• Play -> The image is flipped vertically

• Add a "Flip V" process at the output of the image process, connect its output to the Window device

• Play -> The image is still flipped vertically

• Add a "Flip H" process at the output of the Flip V process (or the image process), connect its output to the Window device

• Play -> The image is now rotated 180°

I also created a no-op process, with this fragment:

void main() {
    vec2        normSrcCoord;
    normSrcCoord.x = isf_FragNormCoord[0];
    normSrcCoord.y = isf_FragNormCoord[1];
    gl_FragColor = IMG_NORM_PIXEL(inputImage, normSrcCoord);
}

Every time this process is applied the input is flipped vertically. Applying it an even number of times is a no-op.

Bug found on Mac OS M2, pre-compiled DMG, version 3.1.11 downloaded from https://github.com/ossia/score/releases/tag/v3.1.11 It only exists when using the Metal backend. The OpenGL backend doesn't have this bug.

The above behavior can be explained by every image-process flipping the image vertically no matter what the processing is:

• Flip V becomes a no-op
• Flip H becomes "Flip H+V", which is a 180° rotation
• A no-op process is a vertical flip.

[jcelerier]

thanks, yes this is actually a thing I haven't managed to fix properly yet in the general case - OpenGL, Vulkan, Metal and D3D all have:

• different coordinate systems for input textures
• different coordinate systems for output attachments
• different coordinate systems for NDC

see e.g. https://github.com/gpuweb/gpuweb/issues/416

there's some work to do in ISF filters so that depending on the backend, IMG_NORM_PIXEL & friends get the pixel in the "right" direction... and even then I'm not sure that would fix the multipass case if there are some direct texture reads