How to print current pixel of an QImage?

Question

I have an RGB888 format qImage defined as follows:

int sizeX = 300; int sizeY = 300;
QImage img = QImage(sizeX, sizeY, QImage::Format_RGB888);

I wish to print current pixel of img one by one. So, I followed the example here:`

 for(int i=0; i<sizeX; i++){
 for(int j=0; j<sizeY; j++){
       img.setPixel(i, j, qRgb(rand()%256, rand()%256, rand()%256));
    }
}
QGraphicsScene *graphic = new QGraphicsScene(this);
graphic->addPixmap(QPixmap::fromImage(img));
ui->graphicsView->setScene(graphic);

But it prints the whole QImage. I want to print one by one. Thanks. Best regards.

Answer 1

As JarMan commented, you are looking for "animation".

Drawing pixel by pixel, and updating the UI for every pixel, may use animation solution as described in the following post.

In Qt5 we have to execute QtWidgets.QApplication.processEvents() for forcing Qt to redraw.

Note:
I implemented the code in Python (with PyQt5), the syntax is different from C++, but the concept is the same.

Initialization stage:

• The main class is inherited from QGraphicsView class.
• Create QGraphicsScene object:
  graphic = QGraphicsScene(0, 0, sizeX, sizeY)
• Create QPixmap object from image:
  pixmap = QPixmap.fromImage(img)
• add addPixmap to the graphic object, and keep the returned reference:
  pixmap_item = graphic.addPixmap(pixmap)
• Execute setScene(graphic)

Animation:
In the animation loop, we have to execute setPixmap on every iteration for updating the entire image.
(The is a way for drawing pixel by pixel, but it's a deviation from your question).

Simplified code for updating the image (executed each iteration):

img.setPixel(i, j, qRgb(randrange(256), randrange(256), randrange(256)));
pixmap = QPixmap.fromImage(img)
pixmap_item.setPixmap(pixmap)
QtWidgets.QApplication.processEvents()

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Complete Python code sample:

import sys
from PyQt5.QtWidgets import QGraphicsScene, QGraphicsView, QApplication
from PyQt5 import QtWidgets
from PyQt5.QtGui import QImage, QPixmap, QPainter, QColor, qRgb
from random import randrange
import time

class Example(QGraphicsView):
    def __init__(self):
        super().__init__()
        self.mPixmap = QPixmap()
        self.sizeX = 300
        self.sizeY = 300
        self.img = QImage(self.sizeX, self.sizeY, QImage.Format_RGB888)
        self.img.fill(QColor(60, 60, 60))  # Fill image with gray color
        self.initUI()

    def initUI(self):
        self.graphic = QGraphicsScene(0, 0, self.sizeX, self.sizeY)

        self.pixmap = QPixmap.fromImage(self.img)
        self.pixmap_item = self.graphic.addPixmap(self.pixmap)

        self.setScene(self.graphic)
        self.show()

    def doAnimation(self):
        for i in range(self.sizeX):
            for j in range(self.sizeY):
                self.img.setPixel(i, j, qRgb(randrange(256), randrange(256), randrange(256)));
                pixmap = QPixmap.fromImage(self.img)
                self.pixmap_item.setPixmap(pixmap)

                QtWidgets.QApplication.processEvents()
                time.sleep(0.001)  # Sleep 1msec (In Python it's slow enough without the sleep...)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    example = Example()
    example.doAnimation()
    app.exec_()

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I hope it's not too much Python for you...

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Sample output: