Simple ppm heatmap

heatmap.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define COLORMAP_STEPS 2
typedef double colormap[3][COLORMAP_STEPS + 1][3];

/* interpolate 'val' from range [inlow, inhigh) into range [outlow, outhigh) */
static double interpolate(double val, double inlow, double inhigh, double outlow, double outhigh)
{
    return (val - inlow) / (inhigh - inlow) * (outhigh - outlow) + outlow;
}

/* apply colormap 'map' to value 'val', and write it to 'outstream' */
static void apply_colormap_and_write(double val, colormap map, FILE *outstream, int newline)
{
    for(int c = 0; c < 3; c++)
        for(int i = 0; i < COLORMAP_STEPS; i++)
            if(val >= map[c][i][0] && val < map[c][i+1][0]) {
                fprintf(outstream, "%3d ",
                        (int)(256.0 * interpolate(val, map[c][i][0], map[c][i+1][0], map[c][i][2], map[c][i+1][1])));
                break;
            }
    if(newline)
        fputc('\n', outstream);
}

int main()
{
    const int NROWS = 10, NCOLS = 10, SQUARE_SIZE_PX = 50;
    double m[NROWS][NCOLS];
    colormap cmap = { { { 0.0, 0.0, 0.0 },
                        { 0.5, 1.0, 0.7 },
                        { 1.0, 1.0, 1.0 } },
                      { { 0.0, 0.0, 0.0 },
                        { 0.5, 1.0, 0.0 },
                        { 1.0, 1.0, 1.0 } },
                      { { 0.0, 0.0, 0.0 },
                        { 0.5, 1.0, 0.0 },
                        { 1.0, 0.5, 1.0 } } };

    srand48(time(NULL));
    for(int r = 0; r < NROWS; r++)
        for(int c = 0; c < NCOLS; c++)
            m[r][c] = drand48();

    printf("P3\n%d %d\n255\n", NCOLS * SQUARE_SIZE_PX, NROWS * SQUARE_SIZE_PX);
    for(int r = 0; r < NROWS; r++)
        for(int rr = 0; rr < SQUARE_SIZE_PX; rr++)
            for(int c = 0; c < NCOLS; c++)
                for(int cc = 0; cc < SQUARE_SIZE_PX; cc++)
                    apply_colormap_and_write(m[r][c], cmap, stdout, c == NCOLS - 1 && cc == SQUARE_SIZE_PX - 1);
}

result.md

After converting .ppm output to .png with ImageMagick: