Draw smoothly scaled image using QPainter and OpenGLWidget?

[Taytoo]

I'm trying to smoothly scale down a QImage inside OpenGLWidget (being drawing with a QPainter) but it looks really bad i.e. text becomes unreadable etc. What's the most efficient way of scaling image in Qt?

[Taytoo]

Found a solution, basically created a 2D Texture with same image format. setData is working now without any issue.

[SGaist]

Hi,

Can you share the code you are using ?
What is the size of the original image ?
What is the target size ?

[Taytoo]

@SGaist Here's the paintEvent method:

QPainter painter;
painter.begin(this);
painter.setRenderHint(QPainter::Antialiasing);
	
if (image)
{
	QRect wndRect = this->rect();
        painter.drawImage(wndRect, *image, image->rect());
}

painter.end();

Original Image size: 1920 x 1080
Widget size: 960 x 540 (50% of original size)

[SGaist]

Did you check QImage::scaled ?

[Taytoo]

@SGaist Yes but its too slow and SmoothTransformation setting creates a blurry looking image - when there is text.

Was hoping for OpenGL to take care of scaling etc.

[SGaist]

Then you have to make a texture of your image and draw it directly with OpenGL.

[JoeCFD]

@Taytoo Always better to use svg as original image type and scale it from there.
And use Qt::KeepAspectRatio or try to make height for width or width for height.

[Taytoo]

@SGaist Just got it working via OpenGL using an example I found online. The problem now is that when I try to update the image by calling setData on texture, then I get following messages in debugger, however, the display is updated without any issues.

QOpenGLTexture::setFormat(): Cannot change format once storage has been allocated
Cannot resize a texture that already has storage allocated.
To do so, destroy() the texture and then create() and setSize()
Cannot set mip levels on a texture that already has storage allocated.
To do so, destroy() the texture and then create() and setMipLevels()

I'm updating Texture using code:

texture->setData(image, QOpenGLTexture::MipMapGeneration::DontGenerateMipMaps);

An image with same RGB32 format and same width/height was used to create the texture, so whats the reason for warnings above?

[Taytoo]

Found a solution, basically created a 2D Texture with same image format. setData is working now without any issue.