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mattgodbolt / owlet-editor

A modern BBC BASIC editor inspired by the BBC Micro Bot (https://bbcmicrobot.com)
https://bbcmic.ro
54 stars 4 forks source link

Previous screen contents show through MODE3 stripes #69

Closed ojwb closed 3 years ago

ojwb commented 3 years ago

E.g. this example results in a mixture of red, black and white in the stripes between the character rows (seen with Firefox):

MODE2:C.129:CLS:MODE6:P."HELLO WORLD":V.279;0;0;0;0

link: https://bbcmic.ro/#%7B%22v%22%3A1%2C%22program%22%3A%22MODE2%3AC.129%3ACLS%3AMODE3%3AP.%5C%22HELLO%20WORLD%5C%22%3AV.279%3B0%3B0%3B0%3B0%22%7D

A similar effect can be seen in MODE6 and also MODE7 (in the bottom few scanlines) - just change the MODE3 in the example code. From hacking on the bot code these areas come out transparent when we grab the screen contents from jsbeeb, and I suspect that's related to what's happening here.

I also notice in MODE5 that there's a stripe of red down the left side of the screen and some other glitchy pixels along the bottom.

I couldn't seem to reproduce in a quick test in chromium, so this may be browser-dependent.

ojwb commented 3 years ago

I don't seem to be able to reproduce this with Firefox 88.0, so I guess it was a firefox bug and has been fixed in the meantime.

Closing - I'll reopen if I see it again.

ojwb commented 3 years ago

Oh, or it could be it was fixed in owlet itself or in jsbeeb or some other dependency. I've just had a quick look at owlet and jsbeeb, but didn't spot anything likely.

mattgodbolt commented 3 years ago

We've definitely got a jsbeeb gremlin of this shape somewhere...it's been reported a number of times but we've never been able to pin it down.