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Bug 3982

Summary: Raspberry Pi window contents crooked (slightly rotated)
Product: SDL Reporter: Eric wing <ewmailing>
Component: renderAssignee: Sam Lantinga <slouken>
Status: NEW --- QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: bandock666
Version: 2.0.7   
Hardware: ARM   
OS: Linux   
Attachments: screenshot of crooked window
test program

Description Eric wing 2017-11-27 22:10:15 UTC 
Created attachment 3104 [details]
screenshot of crooked window

I think this is a Raspberry Pi and not really SDL's fault, but I'm not completely sure so I'm filing it just in case.

On a new installation of Raspbian Stretch on a Pi 2, I enabled the GL Full KMS driver.
I built SDL 2.0.7 and ran a few simple programs that use the SDL 2D renderer.

I discovered that the contents in the window are slightly crooked...rotated counterclockwise by maybe a degree or two. I am attaching a screenshot.


The background clear color is cyan. I draw a purple rectangle the size of the window first. You should not see the background, but you can see a little sliver of cyan on the bottom-right corner and the left-bottom corner.

I then draw a red rectangle on top and if you look closely at the edges, you can see the lines are not straight.

Then I draw some text, and if you look closely, you can also see that it is not straight.


I also built SDL with the OpenGL ES driver that works without X11 and takes up the full screen. It does not have this problem. So I believe the problem is with the experimental OpenGL Full KMS driver.

I am also attaching a simple SDL program in case you want to try it.


I filed this same bug at
https://github.com/raspberrypi/linux/issues/2291
hoping Pi people might be able to fix it.
Comment 1 Eric wing 2017-11-27 22:10:59 UTC 
Created attachment 3105 [details]
test program
Comment 2 Joshua Moss 2018-02-24 04:45:33 UTC 
I think I might've figured it out at least to a degree.  I have a staunch feeling there might be floating point errors going on.  I just learned about this because I started to use SDL_RenderSetIntegerScale on my personal project (which benefited greatly).  When I just ran a new test build on my Raspberry Pi 3 after compiling; I noticed that it wasn't as crooked (in fact, stopped being more crooked as more data got added).  Without integer scale enabled, it's a complete mess (which might be floating point errors).

I haven't looked at the code yet (though I recently got a hold of the latest source build for 2.0.8), but figured I might give a clue from my own experience.