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[uwac] fix rectangular glitch around surface damage regions

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Commit 09c12699 ("[uwac] window: fix damage region dimensions (rounding
errors)") added a 1-pixel expansion to the damage rect passed to
wl_surface_damage() (i.e. x-=1, y-=1, w+=2, h+=2) to guard against
rounding errors when a display scale factor is in use.

This causes visible rectangular glitches around every updated region.
The Wayland compositor re-composites exactly the declared damage region
from the attached buffer onto the screen. Because wlf_copy_image() only
writes pixels for the exact update rectangle, the 1-pixel border in the
buffer is never updated. With double-buffering the border pixels contain
stale data from the last time that buffer was used, and the compositor
faithfully composites those stale pixels, producing the artifact.

The floor/ceil rounding applied before the expansion is already
sufficient to cover any sub-pixel precision lost when converting from
buffer coordinates to surface coordinates. Remove the extra ±1 padding.
This commit is contained in:
Lars Christensen
2026-03-02 19:37:18 +01:00
parent 98eb11b253
commit 6bf6ac26ab
1 changed files with 8 additions and 8 deletions
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16
uwac/libuwac/uwac-window.c
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@@ -717,10 +717,10 @@ static void damage_surface(UwacWindow* window, UwacBuffer* buffer, int scale)
for (int i = 0; i < nrects; i++, box++) for (int i = 0; i < nrects; i++, box++)
{ {
const int x = ((int)floor(box->x1 / scale)) - 1; const int x = (int)floor(box->x1 / scale);
const int y = ((int)floor(box->y1 / scale)) - 1; const int y = (int)floor(box->y1 / scale);
const int w = ((int)ceil((box->x2 - box->x1) / scale)) + 2; const int w = (int)ceil((box->x2 - box->x1) / scale);
const int h = ((int)ceil((box->y2 - box->y1) / scale)) + 2; const int h = (int)ceil((box->y2 - box->y1) / scale);
wl_surface_damage(window->surface, x, y, w, h); wl_surface_damage(window->surface, x, y, w, h);
} }
@@ -739,10 +739,10 @@ static void damage_surface(UwacWindow* window, UwacBuffer* buffer, int scale)
const double dy = floor(1.0 * box->top / scale); const double dy = floor(1.0 * box->top / scale);
const double dw = ceil(1.0 * (box->right - box->left) / scale); const double dw = ceil(1.0 * (box->right - box->left) / scale);
const double dh = ceil(1.0 * (box->bottom - box->top) / scale); const double dh = ceil(1.0 * (box->bottom - box->top) / scale);
const int x = ((int)dx) - 1; const int x = (int)dx;
const int y = ((int)dy) - 1; const int y = (int)dy;
const int w = ((int)dw) + 2; const int w = (int)dw;
const int h = ((int)dh) + 2; const int h = (int)dh;
wl_surface_damage(window->surface, x, y, w, h); wl_surface_damage(window->surface, x, y, w, h);
} }