rhi: Correct another scissor/viewport clamping problem

When x or y are >= the width or height of the render target, then the
width or height of the scissor/viewport rect is zero, no further logic
is needed.

This is different from the case of x or y being negative, because then
there is still a chance that there is an in-bounds area (if width or
height are large enough).

It is important to make this check based on the original value of x
and y, not the clamped ones. Otherwise we end up with a 1 pixel wide
region even when the expected result is a width or height of 0.

Previously the incorrect subtraction of 1 in the final clamping of w and h
masked this, but once that is fixed, the issue fixed here becomes visible
in the cubemap_scissor manual test.

Change-Id: I3d4b0a163a16aa1116b1e838fa95c0faf7b56a3d
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/gui/rhi/qrhi_p_p.h b/src/gui/rhi/qrhi_p_p.h
index 729b85c4d5..cf725632f4 100644
--- a/src/gui/rhi/qrhi_p_p.h
+++ b/src/gui/rhi/qrhi_p_p.h
@@ -260,11 +260,11 @@ bool qrhi_toTopLeftRenderTargetRect(const QSize &outputSize, const std::array<T,
 
     const T widthOffset = *x < 0 ? -*x : 0;
     const T heightOffset = *y < 0 ? -*y : 0;
+    *w = *x < outputWidth ? qMax<T>(0, inputWidth - widthOffset) : 0;
+    *h = *y < outputHeight ? qMax<T>(0, inputHeight - heightOffset) : 0;
 
     *x = qBound<T>(0, *x, outputWidth - 1);
     *y = qBound<T>(0, *y, outputHeight - 1);
-    *w = qMax<T>(0, inputWidth - widthOffset);
-    *h = qMax<T>(0, inputHeight - heightOffset);
 
     if (*x + *w > outputWidth)
         *w = qMax<T>(0, outputWidth - *x);