What are successful strategies to optimize HLSL shader code in terms of computational complexity (meaning: minimizing runtime of the shader)?
I guess one way would be to minimize the number of arithmetic operations that result from compiling the shader.
How could this be done a) manually and b) using automated tools (if existing) ?
Collection of manual techniques (Updated)
- Avoid branching (But how to do that best?)
- Whenever possible: precompute outside shader and pass as argument.
An example code would be:
float2 DisplacementScroll;
// Parameter that limit the water effect
float glowHeight;
float limitTop;
float limitTopWater;
float limitLeft;
float limitRight;
float limitBottom;
sampler TextureSampler : register(s0); // Original color
sampler DisplacementSampler : register(s1); // Displacement
float fadeoutWidth = 0.05;
// External rumble displacement
int enableRumble;
float displacementX;
float displacementY;
float screenZoom;
float4 main(float4 color : COLOR0, float2 texCoord : TEXCOORD0) : COLOR0
{
// Calculate minimal distance to next border
float dx = min(texCoord.x - limitLeft, limitRight - texCoord.x);
float dy = min(texCoord.y - limitTop, limitBottom - texCoord.y);
///////////////////////////////////////////////////////////////////////////////////////
// RUMBLE //////////////////////
///////////////////////////////////////////////////////////////////////////////////////
if (enableRumble!=0)
{
// Limit rumble strength by distance to HLSL-active region (think map)
// The factor of 100 is chosen by hand and controls slope with which dimfactor goes to 1
float dimfactor = clamp(100.0f * min(dx, dy), 0, 1); // Maximum is 1.0 (do not amplify)
// Shift texture coordinates by rumble
texCoord.x += displacementX * dimfactor * screenZoom;
texCoord.y += displacementY * dimfactor * screenZoom;
}
//////////////////////////////////////////////////////////////////////////////////////////
// Water refraction (optical distortion) and water like-color tint //////////////////////
//////////////////////////////////////////////////////////////////////////////////////////
if (dx >= 0)
{
float dyWater = min(texCoord.y - limitTopWater, limitBottom - texCoord.y);
if (dyWater >= 0)
{
// Look up the amount of displacement from texture
float2 displacement = tex2D(DisplacementSampler, DisplacementScroll + texCoord / 3);
float finalFactor = min(dx,dyWater) / fadeoutWidth;
if (finalFactor > 1) finalFactor = 1;
// Apply displacement by water refraction
texCoord.x += (displacement.x * 0.2 - 0.15) * finalFactor * 0.15 * screenZoom; // Why these strange numbers ?
texCoord.y += (displacement.y * 0.2 - 0.15) * finalFactor * 0.15 * screenZoom;
// Look up the texture color of the original underwater pixel.
color = tex2D(TextureSampler, texCoord);
// Additional color transformation (blue shift)
color.r = color.r - 0.1f;
color.g = color.g - 0.1f;
color.b = color.b + 0.3f;
}
else if (dyWater > -glowHeight)
{
// No water distortion...
color = tex2D(TextureSampler, texCoord);
// Scales from 0 (upper glow limit) ... 1 (near water surface)
float glowFactor = 1 - (dyWater / -glowHeight);
// ... but bluish glow
// Additional color transformation
color.r = color.r - (glowFactor * 0.1); // 24 = 1/(30f/720f); // Prelim: depends on screen resolution, must fit to value in HLSL Update
color.g = color.g - (glowFactor * 0.1);
color.b = color.b + (glowFactor * 0.3);
}
else
{
// Return original color (no water distortion above and below)
color = tex2D(TextureSampler, texCoord);
}
}
else
{
// Return original color (no water distortion left or right)
color = tex2D(TextureSampler, texCoord);
}
return color;
}
technique Refraction
{
pass Pass0
{
PixelShader = compile ps_2_0 main();
}
}