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.

There are a few other questions on BMP

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.

Answer 2

Something like:

...

int originalLineSize = width * 3;
int workLineSize = originalLineSize + 4 - originalLineSize % 4;

for (int a = 0; a < bmpArraySize; ++a) {
    if ((a % workLineSize) >= originalLineSize)
        bmpArray[a] = 0x00;
    }
    else if (bmpArray[a] < 127) {
        bmpArray[a] = 0;
    ...
}

Answer 3

The "padding bytes" are the bytes following the pixels of a scanline. You are not so interested in the padding as in the scanline size and pixel size:

iScanlineSize  = ((width * bitsperpixel) + 31) / 32 * 4;
iBytesperPixel = bitsperpixel / 8;

Now you can loop over scanlines and adress pixels and pixel parts (colors) as follows:

for (int a = 0; a < height; a++) {
    for (int b = 0; b < width; b++) {
        for (int c = 0; c < iBytesperPixel; c++) {
            pixelPart= bmpArray[a*iScanlineSize + b*iBytesperPixel + c];
        }
    ]
}