Getting Pixel Colour Screen

This is a discussion on Getting Pixel Colour Screen within the C++ Programming forums, part of the General Programming Boards category; Hi, I知 trying to get the colour of a pixel on the screen. I have tried using getpixel(), however when ...

  1. #1
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    Getting Pixel Colour Screen

    Hi, I知 trying to get the colour of a pixel on the screen. I have tried using getpixel(), however when getting the colour of the whole screen, pixel by pixel, it takes a long time for what I want it for.
    For this reason I tried to find an alternative, and found information about using GetDIBits(). I understand the general idea behind using it I think, but can稚 get it to work, since the limited programming I know is more mathematical and to do with modelling physics.
    I知 programming in the windows console, if someone could link me to a good tutorial, or even better provide the code required to get a pixel colour on the screen using this method I would be much appreciated.

  2. #2
    Slave MadCow257's Avatar
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    Probably not what you want, but there are other ways to optimize that might be more effective.

    How are you looping through the pixels?
    Are you in 24 bit mode?
    Is using the console neccesary?
    Are you processing all the pixels or can some be ignored?

  3. #3
    Yes, my avatar is stolen anonytmouse's Avatar
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    Welcome to the forums!

    The best way to do this is to copy the screen into a dib section. Quick sample code showing this is provided below:
    Code:
    #include <windows.h>
    #include <stdio.h>
    
    struct tagScreen
    {
        DWORD*     pixels;  /* Pointer to raw bitmap bits. Access with: pixels[(y * Screen.cx) + x] */
        size_t     cx, cy;  /* Width and height of bitmap. */
        HBITMAP    hBmp;
        HDC        hdcMem;
        HDC        hdcScreen;
    } Screen;
    
    /* Call once to setup.
     */
    void xSetupScreenBitmap(void)
    {
        BITMAPINFO bmp   = { 0 };
    
        Screen.cx = GetSystemMetrics(SM_CXSCREEN);
        Screen.cy = GetSystemMetrics(SM_CYSCREEN);
    
        bmp.bmiHeader.biSize        = sizeof(BITMAPINFOHEADER);
        bmp.bmiHeader.biWidth       = Screen.cx;
        bmp.bmiHeader.biHeight      = -(int) Screen.cy;
        bmp.bmiHeader.biPlanes      = 1;
        bmp.bmiHeader.biBitCount    = 32;
        bmp.bmiHeader.biCompression = BI_RGB;
    
        Screen.hBmp = CreateDIBSection(NULL, &bmp, DIB_RGB_COLORS, (void**) &Screen.pixels, NULL, 0);
        Screen.hdcMem = CreateCompatibleDC(NULL);
        SelectObject(Screen.hdcMem, Screen.hBmp);
    
        Screen.hdcScreen = CreateDC(TEXT("DISPLAY"), NULL, NULL, NULL);
    }
    
    /* Call to copy the current screen into the screen bitmap.
     */
    void xCopyScreen(void)
    {
        BitBlt(Screen.hdcMem, 0, 0, Screen.cx, Screen.cy, Screen.hdcScreen, 0, 0, SRCCOPY);
    }
    
    /* Convert a RGB value to the BGR used by Windows COLORREF values.
     */
    COLORREF RGB2BGR(DWORD color)
    {
        return RGB(GetBValue(color), GetGValue(color), GetRValue(color));
    }
    
    
    int main(void)
    {
        size_t x, y;
    
        xSetupScreenBitmap(); /* Call once for setup. */
        xCopyScreen();        /* Call whenever you want a fresh copy of the screen. */
    
        /* Shows how to loop through all pixels. */
        for (x = 0; x < Screen.cx; x++)
        {
            for (y = 0; y < Screen.cy; y++)
            {
                /* Use Screen.pixels[(y * Screen.cx) + x] as required. */
    
                /* TEST CODE: Output slighlty altered pixels to the screen. */
                SetPixel(Screen.hdcScreen, x, y, RGB2BGR(Screen.pixels[(y * Screen.cx) + x]) + 10);
            }
        }
    
        return 0;
    }
    Last edited by anonytmouse; 03-14-2006 at 06:59 PM. Reason: Made correction for C++ as pointed out below. Thanks.

  4. #4
    Registered User
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    Anonytmouse thank you very much for the reply, it was exactly what I was looking for.
    I did get an error when I tried to run it, I had to replace the line:
    Code:
    Screen.hBmp = CreateDIBSection(NULL, &bmp, DIB_RGB_COLORS, &Screen.pixels, NULL, 0);
    With:
    Code:
    Screen.hBmp = CreateDIBSection(NULL, &bmp, DIB_RGB_COLORS, (void **)&Screen.pixels, NULL, 0);
    But other than that it worked great.

    For anyone interested in the speed difference between this method, and using getpixel(), I found the time to use each method to get all the pixels of the screen. Using getpixel() it took 5.594 seconds, compared with the method provided by Anonytmouse which took 0.32 seconds, which is about 1/17.5 of the time.

    MadCow257 thank you for the reply as well, I am going to try any optimize it to make it more efficient, however I have to get all the pixels of the screen for what I am trying to do, and for this reason other than getting each row one at a time I cant see a better way. Also you mention me using the console as if it might have an effect on the speed of the code, might this be the case? I only use the console to allow for easy printing of values during making a program, to the screen.

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