Why does the read-back pixel color differ from color drawn on a QImage?

rajeshma

I have a problem with QPainter on QImage in Qt 5.4.

The image has Format_ARGB32. I want to set a given RGBA value on pixels in the image using a QPainter draw function and later read the value back using QImage::pixel.

Yet the value painted and the value read back are different. What am I doing wrong?

Sample code:

#include "mainwindow.h"
#include <QApplication>
#include <QDebug>
#include <QPainter>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QImage image(100, 100, QImage::Format_ARGB32);
    image.fill(0);
    uint value = 0x44fa112b; //some value..
    QPainter painter(&image);
    painter.setCompositionMode(QPainter::CompositionMode_Source);
    QColor color(qRed(value), qGreen(value), qBlue(value), qAlpha(value));
    QBrush brush(color);
    painter.setBrush(brush);
    painter.drawRect(0,0,image.width(), image.height());

    painter.end();

    uint value1 = image.pixel(50,50);

    qDebug() << "RESULT : value = " << QString::number(value,16) << "value1 = " << QString::number(value1,16);

    return a.exec();
}

Result: RESULT : value = "44fa112b" value1 = "44fb1329"

matthew.kuiash

@rajeshma Hi, I just tried this exact example with Qt 5.8 on macOS and I do not see the problem. Both value and value1 equal 0x 44fa112b.

Is it possible to try this on a more up-to-date version of Qt e.g. 5.8?

Chris Kawa

Looks like a bug in 5.4. You should upgrade if you can.

rajeshma

**Problem solved!!
Working properly when I tried with Qt 5.8
Looks like a bug with Qt 5.4.

Thanks to all :)**

? Offline

@rajeshma Next time, please mark your thread as solved, see: https://forum.qt.io/topic/71830 .