Writing bitmap 24 bit color odd behavior

Question

I'm writing my own bitmap generator in c and I've run into some behavior that I can't figure out.

I'm storing pixel data in a 2D array of a pixel struct I wrote. I write my structs directly to the file after the headers.

typedef struct {
    uint8_t blue;
    uint8_t green;
    uint8_t red;
} pixel_24;

Solid color bitmaps were working correctly but I ran into a problem when trying to produce gradients. The images were coming out corrupt. After some experiments I found that any image containing a pixel with a r, g, or b value of 10 would display corrupt. I altered my code to avoid all 10's in my color channels like this:

void load_pixels(pixel_24 pixels[VSIZE][HSIZE])
{
    unsigned int y, x;

    for (y = 0; y < VSIZE; y++)
    {
        for (x = 0; x < HSIZE; x++)
        {
            uint8_t b = (x+y)/4;
            uint8_t g = 255 - (x+y)/4;
            uint8_t r = 0;
            pixels[y][x] = (pixel_24) {b, g, r};
            if (b==10)
            {
                pixels[y][x].blue = 9;
            }
            if (g==10)
            {
                pixels[y][x].green = 9;
            }
            if (r==10)
            {
                pixels[y][x].red = 9;
            }
        }
    }
}

This produces a correct gradient:

When I remove the if statements I get:

What do I need to know about bitmaps to avoid problems like this?

Answer 1

RogerRowland got it. I was opening my file in text mode when I needed binary.