King's Quest VI has a pixel rounding error

[shadyparadox]

There appears to be a rounding error when ScummVM tries to compute each pixel. It happens with the backgrounds, but is even more pronounced with the sprites.

I've attached screenshots of KQ6 running in ScummVM compared to the correct display. You can see the pixel rounding error results in a lot of inaccuracies, such as flattening the top of Alexander's head and dropping one of Saladin's eyes. Everything seems shifted upward and perhaps just slightly to the left.

Since it is a pervasive problem that continually yields weird results, I find the game to be no longer enjoyable, and would like to find a fix.

Like many SCI games, King's Quest VI was originally released at a 320x200 resolution, with the backgrounds taking up a 320x190 area. However, the game also has an upscaled Windows version with higher quality icons and portraits. I couldn't figure out the exact resolution of this upscaled version, but it results in the backgrounds having a 640x418 resolution. The error could occur during this part of the process, because I haven't observed this issue in other SCI games.

[shadyparadox]

I could probably figure out the algorithms, but I don't know SCI/ScummVM coding or how to set up a workspace. I'm a mathematician, not a coder.

[sluicebox]

Thanks! Fixed in: ​https://github.com/scummvm/scummvm/commit/4ecfa0c2170d8376fe09bdcdb698fb2ee919430f