从位图数据中指定坐标[x，y]获取像素颜色

Question

I want to get pixel color on raster coordinates like for example:

[0,0] - pixel in first row and first column (top left)

[0,1] - pixel in first row and second column and so on.

I'm loading my bitmap like so:

BitsPerPixel = FileInfo[28];
width = FileInfo[18] + (FileInfo[19] << 8);
height = FileInfo[22] + (FileInfo[23] << 8);
int PixelsOffset = FileInfo[10] + (FileInfo[11] << 8);
int size = ((width * BitsPerPixel + 31) / 32) * 4 * height;
Pixels.resize(size);
hFile.seekg(PixelsOffset, ios::beg);
hFile.read(reinterpret_cast<char*>(Pixels.data()), size);
hFile.close();

and my GetPixel function:

void BITMAPLOADER::GetPixel(int x, int y, unsigned char* pixel_color)
{
    y = height - y;
    const int RowLength = 4 * ((width * BitsPerPixel + 31) / 32);
    pixel_color[0] = Pixels[RowLength * y * BitsPerPixel / 8 + x * BitsPerPixel / 8];
    pixel_color[1] = Pixels[RowLength * y * BitsPerPixel / 8 + x * BitsPerPixel / 8 + 1];
    pixel_color[2] = Pixels[RowLength * y * BitsPerPixel / 8 + x * BitsPerPixel / 8 + 2];
    pixel_color[3] = Pixels[RowLength * y * BitsPerPixel / 8 + x * BitsPerPixel / 8 + 3];
}

I know the data in bitmap are stored up side down, so I wanted to invert it using the y = height - y; but with this line I only get some values which even are not in the image data array. Without inverting the image I get some values which are in the array but they never correspond with the coords given. My bitmap can be 24-bit or 32-bit.

Answer 1

For bit depth = 24, 3 bytes are stored. The padding is not done per pixel, only on each row:

const int bytesPerPixel = BitsPerPixel / 8;
const int align = 4;
const int RowLength = (width * bytesPerPixel + (align - 1)) & ~(align - 1);
...
pixel_color[0] = Pixels[RowLength * y + x * bytesPerPixel];
...