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cogl/clip-stack-gl: Set glStencilMask correctly for clip regions

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Previously we were using a mask of 0x1 for the lifetime of the stencil.
This was wrong for two reasons:

  * The intersection algorithm needs to count up to a maximum 2, so a
    mask of 1 would clamp to 1 instead. Then decrementing all pixels
    resulted in all pixels being zero even though we want some to be 1.
    So the stencil then blocked some color buffer pixels being rendered.

  * The lifetime of the mask was too long. By leaving it non-zero at
    the end of the function we could accidentally end up modifying the
    stencil contents during our later color buffer paints.

This fixes faulty rendering of some actors seen in gnome-shell with
test case: `env COGL_DEBUG=stencilling`

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1873>
This commit is contained in:
Daniel van Vugt 2021-05-20 17:39:44 +08:00
parent 5991f53c84
commit 216bb7f960
1 changed files with 2 additions and 1 deletions
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3
cogl/cogl/driver/gl/cogl-clip-stack-gl.c
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@ -158,6 +158,7 @@ add_stencil_clip_region (CoglFramebuffer *framebuffer,
GE( ctx, glColorMask (FALSE, FALSE, FALSE, FALSE) );
GE( ctx, glDepthMask (FALSE) );
GE( ctx, glStencilMask (0x3) );
if (merge)
{
@ -167,7 +168,6 @@ add_stencil_clip_region (CoglFramebuffer *framebuffer,
else
{
GE( ctx, glEnable (GL_STENCIL_TEST) );
GE( ctx, glStencilMask (0x1) );
/* Initially disallow everything */
GE( ctx, glClearStencil (0) );
@ -240,6 +240,7 @@ add_stencil_clip_region (CoglFramebuffer *framebuffer,
/* Restore the stencil mode */
GE (ctx, glDepthMask (TRUE));
GE (ctx, glColorMask (TRUE, TRUE, TRUE, TRUE));
GE( ctx, glStencilMask (0x0) );
GE( ctx, glStencilFunc (GL_EQUAL, 0x1, 0x1) );
GE( ctx, glStencilOp (GL_KEEP, GL_KEEP, GL_KEEP) );
}