1704750 - (sw-wr) Rotating cube demo has misrendering on the top and bottom faces when rotated along the z axis

Status: RESOLVED FIXED

(Core :: Graphics: WebRender, defect)

Description

[Mayank Bansal]

1.enable SW-WR
2.go to https://3dtransforms.desandro.com/cube and scroll to the very bottom, where there is a demo you can play with.
3. Rotate the cube from front to right/ front to back .

ER: proper rendering
AR: There is some rendering artifact on the top face and the bottom face of the cube

Fun twist: If you open the demo in Codepen (https://codepen.io/desandro/pen/KRWjzm) the artifacts appear if you select the text on the front face of the cube and then rotating.

Comment 1

[Mayank Bansal]

Attachment: bandicam 2021-04-13 14-56-18-690.mp4

Comment 2

[Lee Salzman [:lsalzman]]

Attachment: Bug 1704750 - Use simpler check for triangle winding. r?aosmond

The original triangle winding check was designed to be run inside the span
rasterizer every time the edges changed. However, at present, this only ever
gets used once at the very top of the triangle to determine winding and then
assumes winding is constant thereafter.

This updates the check to work on these assumptions to be simpler and less
fragile where numerical precision is concerned. If we can assume the candidate
edges both start on the same row, then the check boils down to seeing if they
share the same starting point and doing a cross-product.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/097daf3a851d
Use simpler check for triangle winding. r=aosmond

Comment 4

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/097daf3a851d

Comment 5

[Mayank Bansal]

Attachment: testcase.html

Rotate the red square such that it intersects one of the edges of the window. If need be, increase the size of the red square, or just increase the zoom level.
Or, open the developer toolbar and dock it to the bottom of the page and increase its height till it start intersecting with the red square. Then try rotating the red square. The bug should repro.

Comment 6

[Mayank Bansal]

Attachment: bug.png

screenshot showing what all I did to repro the bug.

Comment 7

[Mayank Bansal]

Attachment: Simpler testcase.html

Hopefully a much simpler testcase.

STR: Increase the zoom of the page till the bottom point intersects the browser window. Then start resizing the browser horizontally with the mouse till you see the artifact
Edit: You can probably remove the preserve-3d and backface-visibility thingy

Comment 8

[Mayank Bansal]

Attachment: bandicam 2021-05-03 22-06-56-836.mp4

Comment 9

[Lee Salzman [:lsalzman]]

Attachment: Bug 1704750 - Simplify rasterizer edge termination condition. r?aosmond

Floating point imprecision can cause the existing test to be unreliable. Just simplify it so it loops around till it overlaps one edge with another. This does a tiny bit more work but will have less chance of exiting earlier than it should.

Comment 10

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/fb960d468a07
Simplify rasterizer edge termination condition. r=kvark

Comment 11

[Narcis Beleuzu [:NarcisB]]

https://hg.mozilla.org/mozilla-central/rev/fb960d468a07