| ID: | 6282 |
| From: | rn...@igor2.repo.hu |
| Date: | Wed, 10 Jan 2024 12:11:03 +0100 (CET) |
| Subject: | [pcb-rnd] librnd opengl rendering fix: how I found the bug |
Hi all, I share this because I find it funny. I've found this bug while working on osm-rnd because librnd coordinates there are always large: I managed to fit the whole globe into 32 bit coords and get a ~centimeter resolution, but the cost is that editing around Budapest yields librnd coords as large as 604060000 decimal. The other day I was testing osm-rnd with opengl (and lesstif too, just to make sure it really works with all our HIDs). As I was looking at buildings, I saw a lot of them not having orthogonal walls - which is common when a novice user draws them free-hand and doesn't know about the tools to orthogonalize the result. So for a while I didn't care. Then I saw the same distortion on buildings I remember drawing a decade ago and I know I did use tools to make sure walls were orthogonal. First I thought someone tried to move or align things and that introduced the distortion... But then I figured too many of those old buildings of mine looked bad. So I restarted osm-rnd with the gdk render and saw the same building all rendering properly! The error this bug caused was much more visible in osm-rnd because, in pcb-rnd terms, I was editing 604mm away from 0;0 and was zoomed in so much to have only a few dozen nanometers per pixels. (In osm-rnd terms this really means editing near Budapest with zoom set to a fraction of a meter per pixel which is normal if you are tracing buildings) The other hint was that osm-rnd draws a little square for nodes (points); these looks perfect squares in gdk rendering but looked like narrow rectangles with random aspect ratio with the opengl rendering. That was the same bug: these squares are a few 10 "nanometers" in size when I zoom in, and with the original code using 32 bit floats on GPU side to represent numbers in the range of 604060000, it's trivial you get huge rounding errors. Best regards, Igor2
Reply subtree:
6282 [pcb-rnd] librnd opengl rendering fix: how I found the bug from rn...@igor2.repo.hu