Intrinsics1 min read
log2
Computes the base-2 logarithm.
Reading Time
1 min
Word Count
140
Sections
12
Try It Live
Test log2 in a live shader
Open the playground, start from a visual preset, and wire log2 into the fragment stage to see how it behaves with real values.
Open PlaygroundLive Demo
The log2 function returns the base-2 logarithm of the input value.
Signature
bwsl
log2 :: (T x) -> T {...}
Where T can be float, float2, float3, or float4.
Parameters
| Parameter | Type | Description |
|---|---|---|
x | T | The input value (must be > 0) |
Return Value
Returns the base-2 logarithm of x.
Example
bwsl
pipeline MipmapLevel {
fragment {
// Calculate appropriate mipmap level
float2 texSize = float2(textureWidth, textureHeight);
float2 duvdx = ddx(input.uv) * texSize;
float2 duvdy = ddy(input.uv) * texSize;
float maxDelta = max(dot(duvdx, duvdx), dot(duvdy, duvdy));
float mipLevel = 0.5 * log2(maxDelta);
// Sample with calculated LOD
float4 color = sample_lod(albedoTex, input.uv, mipLevel);
output.color = color;
}
}Common Use Cases
Mipmap Level Calculation
bwsl
// Determine which mip level to sample
float coverage = max(ddx(uv).x, ddy(uv).y) * textureSize;
float mip = log2(coverage);Power-of-Two Detection
bwsl
// Check if value is power of 2
float logVal = log2(float(value));
bool isPowerOf2 = fract(logVal) < 0.001;Bit Position
bwsl
// Find highest set bit position (approximate)
int bitPos = int(floor(log2(float(value))));LOD Bias
bwsl
// Calculate LOD with sharpening bias
float baseLOD = log2(screenSpaceDerivative);
float sharpLOD = baseLOD - sharpnessBias;Octave Calculation
bwsl
// Determine octave from frequency
float octave = log2(frequency / baseFrequency);Performance
log2 is typically faster than log on GPU hardware since it maps directly to the native instruction. Prefer log2 when the base doesn't matter or can be adjusted.
Domain
log2(x) is undefined for x <= 0. Returns negative infinity for x = 0.
Compiled Output
When compiled to GLSL:
glsl
log2(x)
When compiled to HLSL:
hlsl
log2(x)
When compiled to Metal:
metal
log2(x)