gamma.c

#include <stdio.h>
#include <string.h>
#include <math.h>


#define clamp(d, min, max) ((d) < (min) ? (min) : ((d) > (max) ? (max) : (d)))

float
sRGB_encode(float v)
{
    if (v <= 0.0031308f)
        return 12.92f * v;
    return 1.055f * powf(v, 1.0f / 2.4f) - 0.055f;
}

int
encode(float v)
{
    return (unsigned char)roundf(sRGB_encode(clamp(v, 0.0f, 1.0f)) * 255.0f);
}

static FILE *
open_ppm(char const *name, int width, int height)
{
    FILE *fp = fopen(name, "wb");
    fprintf(fp, "P3\n");
    fprintf(fp, "%d %d\n", width, height);
    fprintf(fp, "255\n");

    return fp;
}

static void
write_pixel_u8(FILE *fp, int r, int g, int b)
{
    fprintf(fp, "%d %d %d\n", r, g, b);
}

int
main()
{

    FILE *fp = open_ppm("image.ppm", 255, 200);
    {
        int x, y, r, g, b;
        for (y = 0; y < 100; ++y) {
            r = 0, g = 0, b = 0;
            for (x = 0; x < 255; ++x) {
                write_pixel_u8(fp, r++, g++, b++);
            }
        }

    }

    {
        int x, y, r, g, b;
        for (y = 100; y < 200; ++y) {
            r = 0, g = 0, b = 0;
            for (x = 0; x < 255; ++x) {
                write_pixel_u8(fp,
                    encode(r++ / 255.0f),
                    encode(g++ / 255.0f),
                    encode(b++ / 255.0f));
            }
        }

        fclose(fp);
    }

    return 0;
}