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Asked: 2025-05-08T14:17:27+05:30

How can I optimize the palette cycling function in my Unity shader for better performance?

I’m diving into some shader work for a Unity project, and I hit a wall with my palette cycling function in the background shader. I’m hoping someone can help me figure out how to optimize it before the project hits any performance issues.

So here’s the deal: I’m using two grayscale sprites for my background animation—one is a 256×256 texture that I set to “Repeat”, and the other is a 17×1 palette texture with “Clamp” wrapping. The shader I’ve written has a function for palette cycling (which I suspect is pretty inefficient), and it looks like this:

“`hlsl
float4 paletteCycle(float4 inCol, sampler2D paletteCycle, float paletteCount)
{
float4 outCol = inCol;

int paletteIndex = -1;
for (int i = 0; i < paletteCount; i++) { if (inCol.a == tex2D(paletteCycle, float2(i / paletteCount, 0)).a) { paletteIndex = i; } } if (paletteIndex >= 0)
{
int paletteOffset = (paletteIndex + _Time.y * 12) % paletteCount;
outCol = tex2D(paletteCycle, float2(paletteOffset / paletteCount, 0));
}
return outCol;
}
“`

The main inefficiency I see stems from the loop that scans each color in the palette to find a match based on the alpha value. Not only does this seem slow, but it also feels like a hacky way to approach it. I have a sneaking suspicion that this could lead to performance drops, especially when it comes to rendering lots of objects with this shader.

I’ve tried a couple of things, like simplifying the texture lookups or reducing the number of unique colors in my palette, but I haven’t really seen any significant improvements. I could definitely use some advice on how to make this function more performant. Should I switch to a different way of handling color matching? Or maybe use texture atlases instead?

Any tips or suggestions would be greatly appreciated! What are some strategies you’ve used to optimize shader functions like this in Unity, especially when it comes to palette cycling? Thanks for your help!

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    2 Answers

    1. Your current approach relies heavily on iterative searching through the palette texture per pixel, which indeed can become a substantial bottleneck in shader performance. A more efficient strategy is to eliminate the loop entirely by leveraging textures indexed directly via grayscale values rather than matching alpha channels repeatedly. Consider restructuring your background sprites so that pixel intensity (grayscale) directly corresponds to your palette index; then, you can sample the palette texture using a simple UV offset, avoiding costly per-pixel loops and conditional checks. This change effectively transforms palette cycling into a single texture lookup after some minor arithmetic, significantly improving shader efficiency.

      Additionally, a common performance optimization used in similar scenarios is employing a small lookup texture, often a 1D palette (implemented as a 2D texture of height 1, with wrap-around enabled), providing fast and direct indexing without conditionals. Animating palette cycling then becomes just a matter of shifting texture coordinates along the indexing dimension based on elapsed time or uniform inputs, avoiding direct indexing by pixel-comparisons. Another effective method is precomputing a color remapping on the CPU or utilizing texture arrays or atlases to avoid dynamic shader-side matching entirely, thus greatly enhancing rendering performance, especially when multiple objects or backgrounds rely on this approach.

    2. Hey! So, I totally get where you’re coming from with the palette cycling issue in your shader. It sounds like that alpha comparison loop could definitely be slowing things down, especially if you have a lot of objects using this shader.

      One approach I can think of is to avoid that loop entirely. Instead of checking alpha values one by one, you might consider using a texture that maps the grayscale values directly to their corresponding palette indices. This way, you can just sample the texture to get the palette index directly instead of having to loop through all the colors.

      Here’s a rough idea of how you might implement this:

      
float4 paletteCycle(float4 inCol, sampler2D paletteCycle, sampler2D indexMap, float paletteCount)
{
    float4 outCol = inCol;

    // Use a texture to get the palette index based on the grayscale value
    float index = tex2D(indexMap, float2(inCol.r, 0)).r; // Assuming input color is in grayscale
    int paletteOffset = int(index * paletteCount + _Time.y * 12) % paletteCount;
    
    outCol = tex2D(paletteCycle, float2(paletteOffset / paletteCount, 0));
    return outCol;
}

      This way, you avoid the loop entirely, and each frame, you’re just doing texture lookups which are generally faster. You’ll need to create that indexMap texture, but it should be worth it for the performance boost!

      Also, keep an eye on your palette size. If you can limit the number of colors in your palette or perhaps compress them down even further while still maintaining visual fidelity, that might help too.

      Hope that helps! Good luck with your shader work – you got this!