When scrolling image with mouse at 50% zoom, image artifacts appear in newly uncovered areas. Unsure if all images are affected or only pathological images such as the one linked. Reproducible: Always Steps to Reproduce: 1. Download the image (caution large image: 8127x8127) 2. Zoom to 50% 3. Click and drag image to the top left. 4. Drag image up and down and note the areas uncovered, they exhibit inconsistent striping with the rest of the image. Actual Results: Gwenview shows artifacts while scrolling while zoomed to 50% using the click and drag technique. Expected Results: I would expect no artifacts of this nature. It's very difficult to see in normal usage of the program. I've never noticed it for years until I ran across this, admittedly pathological image. Unfortunately, I've been unable to build gwenview, so I cannot test against latest git or try to figure out what the issue is. I ran into some issues with cmake (KDE4_DATA_DIR-NOTFOUND) and could not find an authoratative source for the proper way to fix it.
Because it's not easy to see, here's normal and bugged side by side: http://imgur.com/a/pHfpj
Able to reproduce on 4.13.0beta3 using the Trusty packages
I can reproduce that problem here with 4.12.2 - as the image is scrolled around and parts of it move outside the viewport bounds and back in, those areas are rasterized differently each time with artifacting parallel to the viewport edge. The artifacts are applied in intervals depending on the scroll speed, i.e. it probably happens at the frame rate of the canvas painting.
I reproduced it on gwenview 4.11.4 Seems to only work with the one linked PNG with zoom around 50%.
From a casual reading of the code it seems like Gwenview scales subrectangles of the image on demand as things scroll around, so the artifacting is probably a byproduct of the sampling not matching up as the rectangles get combined on the buffer.
*** This bug has been marked as a duplicate of bug 271671 ***