Annotating an existing PNG screenshot results in a file twice as big when saved to disk

[maelvls]

Hi. I am using ksnip v1.9.1 and have noticed that anytime I open a PNG file, add some annotation and save it back to disk, the PNG file increases.

For example, the following screenshot wights 88kB initially:

$ ls -al before.png
.rw-rw-r-- 88k mvalais 13 Oct 15:36 before.png

$ file before.png
before.png: PNG image data, 1492 x 942, 8-bit/color RGBA, non-interlaced

I opened it in ksnip v1.9.1, and annotated it (the red curve) with a width of 5.

$ ls -al after.png
.rw-rw-r-- 154k mvalais 13 Oct 15:40 after.png

$ file after.png
after.png: PNG image data, 2984 x 1884, 8-bit/color RGBA, non-interlaced

I then opened the annotated file and annotated it again:

$ ls -al everafter.png
.rw-rw-r-- 360k mvalais 13 Oct 15:44 everafter.png

$ file everafter.png
everafter.png: PNG image data, 5968 x 3768, 8-bit colormap, non-interlaced

The PNG size doubles every time I save the file to disk; as a consequence, annotating the same file multiple times in a row by opening it, annotating it, saving it, and closing it in ksnip leads to a file bigger and bigger.

I took a look at the code and I found that the scaleFactor is set to 2.0 on my machine. That happens in kImageAnnotator's AnnotationArea.cpp:

QImage AnnotationArea::image()
{
	auto scaleFactor = mDevicePixelRatioScaler->scaleFactor();
	auto scaledSceneSize = this->sceneRect().size().toSize() * scaleFactor;

	// ...

	return image;
}

I have tried hardcoding scaleFactor to 1.0, which fixes the issue:

QImage AnnotationArea::image()
{
	auto scaleFactor = 1.0
	auto scaledSceneSize = this->sceneRect().size().toSize() * scaleFactor;

	// ...

	return image;
}

With 1.0, annotating, saving, and then reopening the same PNG file does not lead to an increase in file size.

What is the purpose of devicePixelRatioScaler? Do I get 2.0 because I use a hidpi screen (4k scaled to 150%)?

[DamirPorobic]

What is the purpose of devicePixelRatioScaler? Do I get 2.0 because I use a hidpi screen (4k scaled to 150%)?

Yes, but then again it should be 1.5 and not 2.0, Qt is not doing a good job there. What Qt versions do you have? Wayland or X11?
We had the issue that sometimes images look small or are blurry because they are stretched with scaling enabled to this was an attempt to mitigate this issue.

Is in your case the image quality changing?

[maelvls]

I use X11, using Qt 5.15.2.

$ ldd $(which ksnip) | grep Qt
libQt5Core.so.5 => /lib/x86_64-linux-gnu/libQt5Core.so.5 (0x00007fa0478a0000)
...

$ sudo apt show libqt5core5a
Package: libqt5core5a
Version: 5.15.2+dfsg-5ubuntu1

We had the issue that sometimes images look small or are blurry because they are stretched with scaling enabled to this was an attempt to mitigate this issue.

Where did this issue happen? Was it in the saved file, or in the UI?

Is in your case the image quality changing?

By image quality changing, what do you mean?

When I set scaleFactor to 1.0, the UI image quality is the same; the only impact is the saved file. Depending on the resolution of the source image, the annotations lose their sharpness (since the image resolution is lower than what the UI displays).

I find it surprising that the image size before and after editing aren't the same.

[DamirPorobic]

I think that this was affecting more taking screenshots then opening existing files but yes, this sharpness was an issue if I recall correctly.

[DamirPorobic]

What could be the case that this was affecting older Qt versions but was fixed with in never.