4 ms·

What part of a pixel/fragment shader does not map very well to pixmaps?

bsder 3y ago
Only interpolating based on the edge? Needing to carefully map float coordinates so you don't wind up with fractional pixels mapping incorrectly? Inability to look at pixels to each side to compute something? You can work around the limitations in vertex or fragment shaders, but, in many cases, you're having to unwind a lot of what the vertex and fragment processing does. On of my favorite examples is drawing a screen/pixel-space rectangle and then drawing a border of the rectangle in a different color and specifying the number of pixels that border should be wide. Oof. You have to specify 4 triangles, make sure they don't overlap, specify them in a particular vertex order or annotate them with extra data so you only draw one of the three edges, make sure that you only draw each pixel just once or your blending goes all to crap, etc. Whereas, you can divide the rectangle into chunks, send each chunk through the compute pipeline, and it's stupidly straightforward. Or, if you want simpler, just draw a line n pixels wide. If you don't have an explicit extension to do this, it's really a pain. 2D graphics simply wants very different graphic and computational primitives compared to 3D.
- persnickety 3y ago
  Interpolation works on neighboring pixels though? Not sure what you mean there. Looking at neighboring pixels of the input pixmap is trivial as well. The fractional pixel mappings do get you - but I think I recall an integer mode somewhere in GLSL 3.2. Your example steps outside the shader boundary, though. If you try to use triangles to draw pixmap, of course you'll have trouble. That's why pixel shaders are the right tool for the job. The only disadvantage they have is that the pixmaps are immutable within a single pass, so you can't (easily) draw the border together with the inner rectangle in one pass. But if you're used to functional programming, you won't even notice this. I agree that shaders are not the right thing when you need mutability, as you seem to, but working with 2D graphics doesn't necessarily require mutability.

Only interpolating based on the edge? Needing to carefully map float coordinates so you don't wind up with fractional pixels mapping incorrectly? Inability to look at pixels to each side to compute something? You can work around the limitations in vertex or fragment shaders, but, in many cases, you're having to unwind a lot of what the vertex and fragment processing does. On of my favorite examples is drawing a screen/pixel-space rectangle and then drawing a border of the rectangle in a different color and specifying the number of pixels that border should be wide. Oof. You have to specify 4 triangles, make sure they don't overlap, specify them in a particular vertex order or annotate them with extra data so you only draw one of the three edges, make sure that you only draw each pixel just once or your blending goes all to crap, etc. Whereas, you can divide the rectangle into chunks, send each chunk through the compute pipeline, and it's stupidly straightforward. Or, if you want simpler, just draw a line n pixels wide. If you don't have an explicit extension to do this, it's really a pain. 2D graphics simply wants very different graphic and computational primitives compared to 3D.

persnickety 3y ago
Interpolation works on neighboring pixels though? Not sure what you mean there. Looking at neighboring pixels of the input pixmap is trivial as well. The fractional pixel mappings do get you - but I think I recall an integer mode somewhere in GLSL 3.2. Your example steps outside the shader boundary, though. If you try to use triangles to draw pixmap, of course you'll have trouble. That's why pixel shaders are the right tool for the job. The only disadvantage they have is that the pixmaps are immutable within a single pass, so you can't (easily) draw the border together with the inner rectangle in one pass. But if you're used to functional programming, you won't even notice this. I agree that shaders are not the right thing when you need mutability, as you seem to, but working with 2D graphics doesn't necessarily require mutability.