Get RGB pixel values from QImage buffer

  • Hi all,

    I'm currently trying to make some operations over the RGB values of a QImage region. To go over the buffer I use some char* variable but, the values I get are not the same as if I would try to acces with QImage::pixelColor(QPoint). Here is the code:

    QImage img;
    bool res = img.load("./scattering.jpg");
    QImage img2 = img.convertToFormat(QImage::Format_RGB888);
    x_or = 1;
    y_or = 254;
    wdth_r = 1278;
    height_r = 508;
    //Get gray values
    char* in = (char*)img2->bits();
    *out = (float*) malloc(wdth_r * height_r*sizeof(float));
    char R;
    char G;
    char B;
    int w_step = ((wdth_r) * 3 + 2); //RGB channels. This + 2 is only in Qt RGB24 buffers ?
    float * auxPt = *out;
    char* auxInPt = in + (y_or - 1) * w_step;
    char* auxInPtCol;
    for (int i = y_or - 1; i < (y_or + height_r - 1); i++)
    	char* line = (char*)inn->scanLine(i); 
    	auxInPtCol = auxInPt; //Equal to line. 
    	for (int j = x_or - 1; j < (x_or + wdth_r - 1); j++)
    		R = *auxInPtCol++;
    		G = *auxInPtCol++;
    		B = *auxInPtCol++;
    		Color aux = inn->pixelColor(QPoint(i, j));
    		char R_R =;
    		char G_R =;
    		char B_R =;
                    //Debugger cond
    		if (R_R != R)
    			int a = 2;
    		if (G_R != G)
    			int b = 2;
    		if (B_R != B)
    			int c = 3;
    		*auxPt = (R+G+B) / 3.f;
    	auxInPt += w_step;

  • Hi,
    why don't you use QColor::getRGB() instead? This will give you a nice set if int values.

    Also docs for QImage::pixelColor have a note:
    Warning: This function is expensive when used for massive pixel manipulations. Use constBits() or constScanLine() when many pixels needs to be read.

  • @artwaw

    Hi artwaw,

    Thanks for your reply. The thing is that those values are just to test if what I'm getting is right.

    The code part with the for's is actually a wrapped function where I pass as a parameter the char* buffer of the image. About passing the QImage as a param it is not an option as I want that this function work with all kind of generical RGB24 buffers


  • @artwaw

    I found what was the problem. Qt saves the image pixels in BGR order so, to get the right values it would be necccesary to catch them this way:

    B = *auxInPtCol++;
    G = *auxInPtCol++;
    R = *auxInPtCol++;

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