create_cdf

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Overview

overview

CDFs are useful when sampling from distributions. For example, you could create a CDF of light source power. This would allow sampling of lights with a probability based on power. This is an example of a discrete CDF, where sampling selects among a fixed set of probabilities. (See the example below.)

Use the sample_cdf function to sample values from the returned CDF array.

Usage

usage

float [] create_cdf(float pdf[])

Returns a CDF for the input PDF as an array of floats.

pdf

Array of PDF values to create the CDF for.

Examples

examples

// Iterate over all lights, sampling their power
int[] li = getlights();
float values[];
resize(values, len(li));
int nsamples = 256;
int sid = israytrace ? SID : newsampler();
vector s, pos, clr;
float scale;
for (int i = 0; i < len(li); i++)
{
    for (int j = 0; j < nsamples; j++)
    {
        nextsample(sid, s.x, s.y, "mode", "nextpixel");
        sample_light(li[i], P, s, Time, pos, clr, scale);
        values[i] += luminance(clr);
    }
    values[i] /= nsamples;
}

// Create a CDF of the power distribution
float cdf[] = create_cdf(values);

// Randomly select a light based on power distribution
nextsample(sid, s.x, s.y, "mode", "nextpixel");
int index = 0;
sample_cdf(cdf, s.x, index);

// Do something with the selected light
// li[index] ...