GetDIBits and loop through pixels using X, Y

Question

I'm grabbing a portion of the screen and scanning through the pixels for a certain color range.

I looked at MSDN's Capturing an Image example and know how to use the functions.

I can get the bits into an array, but I'm not sure how to do it in such a way that I can loop through it as I would an image. A pseudo-example (which I'm sure is way off):

for ( x = 1; x <= Image.Width; x += 3 )
{
    for ( y = 1; y <= Image.Height; y += 3 )
    {
        red = lpPixels[x];
        green = lpPixels[x + 1];
        blue = lpPixels[x + 2];
    }
}

That's basically what I want to do, so if red, blue, and green is a certain color, I'll know what coordinate it's at (x, y) in the image.

I just don't know how to use GetDIBits in such a way, and how to setup the array appropriately to be able to accomplish this.

Answer 1

Apart from the good answers already given, here's an example of how to get a simple array structure to walk on. (You can use e.g. Goz' code for the iteration.)

GetDIBits reference @ MSDN

You have to select DIB_RGB_COLORS as flag for uUsage and set up the BITMAPINFO structure and the BITMAPINFOHEADER structure it contains. When you set biClrUsed and biClrImportant to zero, there is "no" color table, so you can read the pixels of the bitmap you get from GetDIBits as a sequence of RGB values. Using 32 as bit count (biBitCount) sets up the data structure according to MSDN:

The bitmap has a maximum of 2^32 colors. If the biCompression member of the BITMAPINFOHEADER is BI_RGB, the bmiColors member of BITMAPINFO is NULL. Each DWORD in the bitmap array represents the relative intensities of blue, green, and red, respectively, for a pixel. The high byte in each DWORD is not used.

Since a MS LONG is exactly 32 bit long (the size of a DWORD), you do not have to pay attention to padding (as described in the Remarks section).

Code:

HDC hdcSource = NULL; // the source device context
HBITMAP hSource = NULL; // the bitmap selected into the device context

BITMAPINFO MyBMInfo = {0};
MyBMInfo.bmiHeader.biSize = sizeof(MyBMInfo.bmiHeader);

// Get the BITMAPINFO structure from the bitmap
if(0 == GetDIBits(hdcSource, hSource, 0, 0, NULL, &MyBMInfo, DIB_RGB_COLORS))
{
    // error handling
}

// create the pixel buffer
BYTE* lpPixels = new BYTE[MyBMInfo.bmiHeader.biSizeImage];

// We'll change the received BITMAPINFOHEADER to request the data in a
// 32 bit RGB format (and not upside-down) so that we can iterate over
// the pixels easily. 

// requesting a 32 bit image means that no stride/padding will be necessary,
// although it always contains an (possibly unused) alpha channel
MyBMInfo.bmiHeader.biBitCount = 32;
MyBMInfo.bmiHeader.biCompression = BI_RGB;  // no compression -> easier to use
// correct the bottom-up ordering of lines (abs is in cstdblib and stdlib.h)
MyBMInfo.bmiHeader.biHeight = abs(MyBMInfo.bmiHeader.biHeight);

// Call GetDIBits a second time, this time to (format and) store the actual
// bitmap data (the "pixels") in the buffer lpPixels
if(0 == GetDIBits(hdcSource, hSource, 0, MyBMInfo.bmiHeader.biHeight,
                  lpPixels, &MyBMInfo, DIB_RGB_COLORS))
{
    // error handling
}
// clean up: deselect bitmap from device context, close handles, delete buffer

Answer 2

GetDIBits returns a one-dimensional array of values. For a bitmap that's M pixels wide by N pixels tall and uses 24-bit color, the first (M*3) bytes will be the first row of pixels. That can be followed by some padding bytes. It depends on the BITMAPINFOHEADER. There is usually padding to make the width a multiple of 4 bytes. So if your bitmap is 33 pixels wide, there will actually be (36*3) bytes per row.

This "pixels plus padding" is called the "stride". For RGB bitmaps, you can calculate stride with: stride = (biWidth * (biBitCount / 8) + 3) & ~3, where biWidth and biBitCount are taken from the BITMAPINFOHEADER.

I'm not sure how you want to traverse the array. If you want to go pixel-by-pixel from top left to lower right (assuming this is a top-down bitmap):

for (row = 0; row < Image.Height; ++row)
{
    int rowBase = row*stride;
    for (col = 0; col < Image.Width; ++col)
    {
        red = lpPixels[rowBase + col];
        // etc.
    }
}

Answer 3

In the link you post you create a 32-bit bitmap so I'll assume you are reading from a 32-bit bitmap (This assumption may be incorrect).

Therefore changing your loop to the following should work:

char* pCurrPixel = (char*)lpPixels;
for ( y = 0; y < Image.Height; y++ )
{
    for ( x = 0; x < Image.Width; x++ )
    {
        red = pCurrPixel[0];
        green = pCurrPixel[1];
        blue = pCurrPixel[2];

        pCurrPixel += 4;
    }
}

Things to bear in mind:

1.Arrays are 0 based in C/C++
2. You were stepping 3 pixels horizontally and vertically each time. Which meant you aren't visiting every pixel.
3. A bitmap is usually organised such that there are "height" spans of "width" pixels. Therefore you should step through each pixel in a span and then move to the next span.
4. As already pointed out make sure you aare reading pixels correctly. in 16-bit mode its more complex