Stack of scaled layers breaks at certain window size

[dominikh]

When rendering with a base color other than black, the following scene

scene.push_layer(BlendMode::new(Mix::Normal, Compose::SrcOver), 1.0, Affine::scale(2.0), &Rect::new(0.0, 0.0, 1024.0, 1024.0)); 
scene.push_layer(BlendMode::new(Mix::Normal, Compose::SrcOver), 1.0, Affine::scale(2.0), &Rect::new(0.0, 0.0, 1024.0, 1024.0)); 
scene.push_layer(BlendMode::new(Mix::Normal, Compose::SrcOver), 1.0, Affine::scale(2.0), &Rect::new(0.0, 0.0, 1024.0, 1024.0)); 
scene.push_layer(BlendMode::new(Mix::Normal, Compose::SrcOver), 1.0, Affine::scale(2.0), &Rect::new(0.0, 0.0, 1024.0, 1024.0)); 
scene.push_layer(BlendMode::new(Mix::Normal, Compose::SrcOver), 1.0, Affine::scale(2.0), &Rect::new(0.0, 0.0, 1024.0, 1024.0)); 

scene.pop_layer();
scene.pop_layer();
scene.pop_layer();
scene.pop_layer();
scene.pop_layer();

will display the base color for small window sizes, but black for large window sizes. In my case, "small" is any window with a pixel area less than 2**20.

To reproduce the problem, I've needed at least 5 layers, and a scale transform that's >1.0; but even 1.00001 was sufficient.

[DJMcNab]

I strongly suspect that this is a duplicate of #366. The reason it displays black is because the rendering never occurs, because you run out of memory in the blend stack.

However, I don't know why a different scale transform would have any impact here, so I'm leaving this open.

[dominikh]

Indeed, changing

vello/vello_encoding/src/config.rs

Lines 403 to 404 in a00cd9f

	// 16 * 16 (1 << 8) is one blend spill, so this allows for 4096 spills.
	let blend_spill = BufferSize::new(1 << 20);

to 1<<22 "fixes" it for this particular scene with the scale of 2, and 1<<21 suffices for a scale of 1.00001. I suppose the scale matters simply because it changes how many tiles are covered? Though that doesn't explain why a scale of 1 didn't run into this problem at any window size.

[dominikh]

I'll chalk this up to getting unlucky with what tiles were occupied at different scales and close it as a duplicate of #366.