Basic GLSL fragment shader with palette lookups and palette shift

Question

I am remaking an old game, which animates water by rotating its palettes over time.

I have written a basic fragment shader to replicate this behavior. It checks which color we are trying to render, and if that color is known to be water, we apply a palette shift.

This works well, but there is a lot of branching involved. Should I be concerned about this? How could it be improved?

#version 330 core
uniform sampler2D tex;
uniform sampler2D palette;
uniform float palette_txy;
uniform int transparent_index = 0;
uniform int water_shift_1;
uniform int water_shift_2;
in vec2 tex_coords;
out vec4 frag_color;
int add_wrapped(int value, int addend, int range_min, int range_size)
{
 return (value + addend - range_min) % range_size + range_min;
}
void main() {
 float palette_index = texture(tex, tex_coords).r;
 if (palette_index == transparent_index || palette_index == 1)
 {
 discard;
 }
 // Get the size of 1 pixel
 int tex_width = textureSize(palette, 0).x;
 float px_size = 1.f / float(tex_width);
 
 // Apply a palette shift for water pixels
 int palette_index_int = int(palette_index * 256);
 if (palette_index_int >= 224 && palette_index_int < 228)
 {
 // Water
 palette_index_int = add_wrapped(palette_index_int, water_shift_1, 224, 4);
 palette_index = palette_index_int * px_size;
 }
 else if (palette_index_int >= 228 && palette_index_int < 232)
 {
 // Wakes
 // Use the inverse of water_shift_1
 int shift = 3 - water_shift_1;
 palette_index_int = add_wrapped(palette_index_int, shift, 228, 4);
 palette_index = palette_index_int * px_size;
 }
 else if (palette_index_int >= 232 && palette_index_int < 239)
 {
 // Coastlines
 palette_index_int = add_wrapped(palette_index_int, water_shift_2, 232, 7);
 palette_index = palette_index_int * px_size;
 }
 vec2 palette_lookup = vec2(palette_index, palette_txy);
 frag_color = texture(palette, palette_lookup);
}
```

• Tristen Woodruff: Not all branching in shader programs will immediately lead to poor performance.

Answer 1

This code works, it isn't so bad.

DRY

There's an obvious cleanup of consolidating those px_size multiplies:

 if (224 <= palette_index_int && palette_index_int < 239)
 {
 palette_index_int = get_water_effect(palette_index_int);
 palette_index = palette_index_int * px_size;
 }
 ...
int get_water_effect(int palette_index_int) {
 [deal with water, wake, coast]
}

lookup table

Alternatively, you might init a 256-element lookup table which holds enums, or three distinct add_wrapped base indexes, or pointers to {water, wake, coast} helper functions.

Perhaps entries less than 224 would point to a no-op helper.

profiling

There's not a lot going on here. It's unclear if most of the time would be spent on branch mispredictions, lookup latencies, or other things that are just unavoidable. Bench the alternatives and post the measurements.