Dealing with padding in a BMP file in C

Question

I am trying to edit a BMP file in C. My code works for BMP files with no padding but I am having trouble dealing with padding.

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Answer 1

The problem is in this part:

int c = 0;
for (int a = 0; a < height; a++) {
    for (int b = 0; b < width*3; b++) {
        if (bmpArray[a*(width*3)+b] < 127) {
            bmpArray[a*(width*3)+b] = 0;
        } else {
            bmpArray[a*(width*3)+b] = 255;
        }
    }
    for (int pad = 0; pad < padding; pad++) {
        bmpArray[c++] = 0x00;  /* ONLY HERE is 'c' updated! */
    }
}

At the end of each line, you fill out the padding starting at c, which starts out at 0 and so overwrites the first few bytes of the first line. Then, each next line gets copied but you continue overwriting from the start (where c initially pointed to).

The padding should be added on each line. In the loops, you adjust a and b but you forget to adjust for the padding.

I suggest the more straightforward code (untested!):

for (int a = 0; a < height; a++) {
    for (int b = 0; b < width*3; b++) {
        if (bmpArray[a*(width*3 + padding)+b] < 127) {
            bmpArray[a*(width*3 + padding)+b] = 0;
        } else {
            bmpArray[a*(width*3 + padding)+b] = 255;
        }
    }
    for (int pad = 0; pad < padding; pad++) {
        bmpArray[a*(width*3 + padding) + 3*width + pad] = 0x00;
    }
}

is there a way I can identify the padding pixels ..

Yes – in my loop above with adjustments for padding, it automatically skips the padding itself. You can safely remove the explicit 'set padding to 0' loop at the end.