- Yaoi Gagarin
- Feb 20, 2014
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I'm following vulkan-tutorial.com on an old mac with integrated graphics. I ran into a weird issue, and I wonder if any of y'all have a clue about what could be going on.
The first part of the tutorial involves drawing a single triangle to the screen without using a vertex buffer at all, so the vertex shader just hardcodes the vertex positions and colors into the shader, using gl_VertexIndex to select the appropriate position/color. I'm following the code exactly to the tutorial, but my triangle's colors are wrong: only one corner is red, and the other two corners are black.
code:// Vertex shader
#version 450
layout(location = 0) out vec3 vertColor;
vec2 positions[3] = vec2[](
vec2(0.0, -0.5),
vec2(0.5, 0.5),
vec2(-0.5, 0.5)
);
vec3 colors[3] = vec3[](
vec3(1.0, 0.0, 0.0),
vec3(0.0, 1.0, 0.0),
vec3(0.0, 0.0, 1.0)
);
void main() {
gl_Position = vec4(positions[gl_VertexIndex], 0.0, 1.0);
vertColor = colors[gl_VertexIndex];
}
code:// fragment shader
#version 450
layout(location = 0) in vec3 vertColor;
layout(location = 0) out vec4 outColor;
void main() {
outColor = vec4(vertColor, 1.0);
}
code:// <above main, all the code is the same>
void main() {
gl_Position = vec4(positions[gl_VertexIndex], 0.0, 1.0);
colors[0] = vec3(1.0, 0.0, 0.0);
colors[1] = vec3(0.0, 1.0, 0.0);
colors[2] = vec3(0.0, 0.0, 1.0);
vertColor = colors[gl_VertexIndex];
}
e: I realize that this specific shader is totally unrealistic, but I'm stuck on this because in the future if I want to use hardcoded constant arrays for some reason, I'm not gonna trust 'em if they can exhibit weird behavior I don't understand!!!
Suppose I have a compute shader with a really sparse SSBO array that is ostensibly a tree structure that fills itself up (fixed depth, fixed size). Is it fine to just no-op any indexes that haven't been populated yet?
Edit: Found better leverage of my acceleration structure to still do everything parallel but get 90% of the benefit. Still posing the question whether a single if-then-no-op is potentially slow just because it needs to split the execution weirdly or something.
If you mean something like:
code:
if (is_populated[threadID]) {
// Do stuff
}
It'll really depend on how much work is done on that branch. If it's really quick then it's probably not a big deal but if it's long then you could be hurting your utilization over time.
Note that if you have an else branch on that it gets even worse because all the threads in each warp will have to run both if even one is different
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