// RGBA, BGRA, ...
#define CHANNEL_COUNT 4
int x, y;

// Get the scanline (starts at finalbits and goes finalpitch bytes every line)
float *scanline = (float*)(((char*)bmp->finalbits) + y * bmp->finalpitch);
float channels[CHANNEL_COUNT];
for(i = 0; i < CHANNEL_COUNT; i++) {
    // Index each channel at x
    channels[i] = scanline[CHANNEL_COUNT * x + i]; // Offset in the scanline
}