[PATCH] Main-menu rear-most background layer shifts suddenly sideways

[s0600204]

To reproduce:

• Start 0AD, (windowed at minimum resolution shows it up well, but fullscreen/maximised will work)
• Watch the rear-most layer (the one with the hills)
• After about 26s, the hills 'jump' horizontally.
• If you wait a little longer, and the layers start scrolling the other way, the backdrop 'jumps' back.

The problem seems to be on line 62 of mainmenu.js. What's written there would only work if the image loops/tiles every screen.right pixels. If the line was changed to left -= w;, it should work slightly better... except... the value of w also appears to be incorrect.

w is, as far as I can tell, the calculated scaled width of the layer. Except it's not calculated properly. The line assumes that the layer's sprite has an aspect ratio of 16:9. But the native size of all three layers is 2048x1024, an aspect ratio of 2:1.

The attached patch proposes a fix. (Yes, the patch still makes an assumption about the aspect ratio, but as far as I know there's no way to determine a sprite's original texture size via JS.)

Refs. #2993

[s0600204]

Musn't forget these...

[Itms]

Sander, do you have some time to look into this?

[sanderd17]

I don't see why the change to 2:1 ratio is needed. w is just an estimated width of a single screen, so estimated at 16:9. It serves mainly to bring the characters on a certain screen, rather than putting a seam between them.

Apart from that, your patch is probably correct, and I didn't notice it because my screen was exactly 16:9, so screen.right matched w.

[s0600204]

w is also the amount of pixels subtracted from left whenever the code thinks the left edge of the layer is to the right of the left edge of the screen.

If w is too small or too large, then the jump when the layer edge crosses over and w is no longer subtracted from left is noticable.

[sanderd17]

I see. Using screen.right was indeed wrong there, but using w isn't right either, as w means something different there.

It would be better to just hard-code it to 2*h. Similar to line 67.

[s0600204]

Proposed fix, updated.

[leper]

In 16905:

Fix sudden main-menu background jumps. Patch by s0600204. Fixes #3312.

[leper]

Thanks for the patch. Would be nicer to have a patch from the top level directory (that is paths start with binaries/....).