How to change QPixmap color from black to red

[oogolov]

I have png-file in black/white color.

QPixmap bearingPixmap;
bearingPixmap.load(":/Hinge.png");

How to change QPixmap black color to red?
In which way can I use createMaskFromColor and setMask?

[wysota]

If the image is index coloured then the easiest option is to change the colour map.

[oogolov]

I don't know what is index coloured image.
I'd like set this pixmap
Hinge.png
to my QGraphicsPixmapItem.
Then I want to change black color to red after picking the item.
I don't like to create one more red png-file.

[wysota]

I don't know what is index coloured image.

It's an image that has at most 8 bits per colour (meaning a maximum of 256 colours) as opposed to 24 or 32bit.

I'd like set this pixmap
Hinge.png
to my QGraphicsPixmapItem.

It's not an indexed image but you can convert it to one using gimp, imagemagick or any other graphics tool available.

Then you can use QImage::setColorTable() to set a new colour map for the image.

[Spitfire]

try

QPixmap px( "C:\\Hinge.png" );
QPixmap pxr( px.size() );
pxr.fill( Qt::red );
pxr.setMask( px.createMaskFromColor( Qt::transparent ) );

QPixmap px( "C:\\Hinge.png" );
QPixmap pxr( px.size() );
pxr.fill( Qt::red );
pxr.setMask( px.createMaskFromColor( Qt::transparent ) );

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You will get: test.png

Maybe not the most elengant way (and definitely not the fastest) but does the job.

[ehopperdietzel]

@Spitfire's code works good for flat images.
If your image has varying alpha values you can use:

QImage tmp = pixmap.toImage();
 
for(int y = 0; y < tmp.height(); y++)
  for(int x= 0; x < tmp.width(); x++)
    color.setAlpha(tmp.pixelColor(x,y).alpha());
    tmp.setPixelColor(x,y,color);
 
pixmap = QPixmap::fromImage(tmp);

QImage tmp = pixmap.toImage();

for(int y = 0; y < tmp.height(); y++)
  for(int x= 0; x < tmp.width(); x++)
    color.setAlpha(tmp.pixelColor(x,y).alpha());
    tmp.setPixelColor(x,y,color);

pixmap = QPixmap::fromImage(tmp);

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Commented:

// "pixmap" is your QPixmap
// "color" is your QColor
 
// Convert the pixmap to QImage
QImage tmp = pixmap.toImage();
 
// Loop all the pixels
for(int y = 0; y < tmp.height(); y++)
{
  for(int x= 0; x < tmp.width(); x++)
  {    
    // Read the alpha value each pixel, keeping the RGB values of your color
    color.setAlpha(tmp.pixelColor(x,y).alpha());
 
    // Apply the pixel color
    tmp.setPixelColor(x,y,color);
  }
}
 
// Get the coloured pixmap
pixmap = QPixmap::fromImage(tmp);

// "pixmap" is your QPixmap
// "color" is your QColor

// Convert the pixmap to QImage
QImage tmp = pixmap.toImage();

// Loop all the pixels
for(int y = 0; y < tmp.height(); y++)
{
  for(int x= 0; x < tmp.width(); x++)
  {    
    // Read the alpha value each pixel, keeping the RGB values of your color
    color.setAlpha(tmp.pixelColor(x,y).alpha());

    // Apply the pixel color
    tmp.setPixelColor(x,y,color);
  }
}

// Get the coloured pixmap
pixmap = QPixmap::fromImage(tmp);

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[asgawa]

This works since 5.6
http://doc.qt.io/qt-5/qimage.html#pixelColor