Optimized clipping in the GL 2 engine for winding fills.

When no stencil clip is set we reduce the number of compositing passes
for rendering winding fill paths from four to two. When stencil clip is
set, the number of compositing passes is reduced from five to four.

For clipping with a winding fill path, the number of compositing passes
are reduced from five to four when stencil clipping is already enabled.

A performance improvement of up to 85 % was measured in certain cases.

Reviewed-by: Trond

2 files changed, 41 insertions, 29 deletions

diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
index 5875124..b3638ff 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
@@ -391,7 +391,6 @@ void QGL2PaintEngineExPrivate::setBrush(const QBrush* brush)
 }
 
 
-// Unless this gets used elsewhere, it's probably best to merge it into fillStencilWithVertexArray
 void QGL2PaintEngineExPrivate::useSimpleShader()
 {
     shaderManager->simpleProgram()->enable();
@@ -891,16 +890,26 @@ void QGL2PaintEngineExPrivate::fill(const QVectorPath& path)
 
         fillStencilWithVertexArray(vertexCoordinateArray, path.hasWindingFill());
 
-        // Stencil the brush onto the dest buffer
-        glStencilFunc(GL_EQUAL, GL_STENCIL_HIGH_BIT, GL_STENCIL_HIGH_BIT); // Pass if stencil buff value != 0
-        glStencilOp(GL_KEEP, GL_ZERO, GL_ZERO);
-        glStencilMask(GL_STENCIL_HIGH_BIT);
+        glStencilMask(0xff);
+        glStencilOp(GL_KEEP, GL_REPLACE, GL_REPLACE);
+
+        if (q->state()->clipTestEnabled) {
+            // Pass when high bit is set, replace stencil value with current clip
+            glStencilFunc(GL_NOTEQUAL, q->state()->currentClip, GL_STENCIL_HIGH_BIT);
+        } else if (path.hasWindingFill()) {
+            // Pass when any bit is set, replace stencil value with 0
+            glStencilFunc(GL_NOTEQUAL, 0, 0xff);
+        } else {
+            // Pass when high bit is set, replace stencil value with 0
+            glStencilFunc(GL_NOTEQUAL, 0, GL_STENCIL_HIGH_BIT);
+        }
 
         prepareForDraw(currentBrush->isOpaque());
 
         if (inRenderText)
             prepareDepthRangeForRenderText();
 
+        // Stencil the brush onto the dest buffer
         composite(vertexCoordinateArray.boundingRect());
 
         if (inRenderText)
@@ -916,7 +925,7 @@ void QGL2PaintEngineExPrivate::fill(const QVectorPath& path)
 void QGL2PaintEngineExPrivate::fillStencilWithVertexArray(QGL2PEXVertexArray& vertexArray, bool useWindingFill)
 {
 //     qDebug("QGL2PaintEngineExPrivate::fillStencilWithVertexArray()");
-    glStencilMask(0xffff); // Enable stencil writes
+    glStencilMask(0xff); // Enable stencil writes
 
     if (dirtyStencilRegion.intersects(currentScissorBounds)) {
         QVector<QRect> clearRegion = dirtyStencilRegion.intersected(currentScissorBounds).rects();
@@ -948,39 +957,30 @@ void QGL2PaintEngineExPrivate::fillStencilWithVertexArray(QGL2PEXVertexArray& ve
 
     if (useWindingFill) {
         if (q->state()->clipTestEnabled) {
+            // Flatten clip values higher than current clip, and set high bit to match current clip
             glStencilFunc(GL_LEQUAL, GL_STENCIL_HIGH_BIT | q->state()->currentClip, ~GL_STENCIL_HIGH_BIT);
             glStencilOp(GL_KEEP, GL_REPLACE, GL_REPLACE);
             composite(vertexArray.boundingRect());
 
             glStencilFunc(GL_EQUAL, GL_STENCIL_HIGH_BIT, GL_STENCIL_HIGH_BIT);
-        } else {
-            glStencilFunc(GL_ALWAYS, 0, 0xffff);
+        } else if (!stencilClean) {
+            // Clear stencil buffer within bounding rect
+            glStencilFunc(GL_ALWAYS, 0, 0xff);
             glStencilOp(GL_ZERO, GL_ZERO, GL_ZERO);
             composite(vertexArray.boundingRect());
         }
 
         // Inc. for front-facing triangle
         glStencilOpSeparate(GL_FRONT, GL_KEEP, GL_INCR_WRAP, GL_INCR_WRAP);
-        //Dec. for back-facing "holes"
+        // Dec. for back-facing "holes"
         glStencilOpSeparate(GL_BACK, GL_KEEP, GL_DECR_WRAP, GL_DECR_WRAP);
         glStencilMask(~GL_STENCIL_HIGH_BIT);
         drawVertexArrays(vertexArray, GL_TRIANGLE_FAN);
 
         if (q->state()->clipTestEnabled) {
-            // clear high bit of stencil outside of path
-            glStencilFunc(GL_EQUAL, GL_STENCIL_HIGH_BIT | q->state()->currentClip, ~GL_STENCIL_HIGH_BIT);
-            glStencilOp(GL_KEEP, GL_INVERT, GL_INVERT);
-            glStencilMask(GL_STENCIL_HIGH_BIT);
-            composite(vertexArray.boundingRect());
-            // reset lower bits of stencil inside path to current clip
-            glStencilFunc(GL_EQUAL, GL_STENCIL_HIGH_BIT | q->state()->currentClip, GL_STENCIL_HIGH_BIT);
+            // Clear high bit of stencil outside of path
+            glStencilFunc(GL_EQUAL, q->state()->currentClip, ~GL_STENCIL_HIGH_BIT);
             glStencilOp(GL_KEEP, GL_REPLACE, GL_REPLACE);
-            glStencilMask(~GL_STENCIL_HIGH_BIT);
-            composite(vertexArray.boundingRect());
-        } else {
-            // set high bit of stencil inside path
-            glStencilFunc(GL_NOTEQUAL, 0, 0xffff);
-            glStencilOp(GL_KEEP, GL_INVERT, GL_INVERT);
             glStencilMask(GL_STENCIL_HIGH_BIT);
             composite(vertexArray.boundingRect());
         }
@@ -1020,7 +1020,7 @@ void QGL2PaintEngineExPrivate::resetClipIfNeeded()
     QGLRect rect(bounds.left(), bounds.top(), bounds.right(), bounds.bottom());
 
     // Set high bit on clip region
-    glStencilFunc(GL_LEQUAL, q->state()->currentClip, 0xffff);
+    glStencilFunc(GL_LEQUAL, q->state()->currentClip, 0xff);
     glStencilOp(GL_KEEP, GL_INVERT, GL_INVERT);
     glStencilMask(GL_STENCIL_HIGH_BIT);
     composite(rect);
@@ -1028,7 +1028,7 @@ void QGL2PaintEngineExPrivate::resetClipIfNeeded()
     // Reset clipping to 1 and everything else to zero
     glStencilFunc(GL_NOTEQUAL, 0x01, GL_STENCIL_HIGH_BIT);
     glStencilOp(GL_ZERO, GL_REPLACE, GL_REPLACE);
-    glStencilMask(0xFFFF);
+    glStencilMask(0xff);
     composite(rect);
 
     q->state()->currentClip = 1;
@@ -1636,6 +1636,7 @@ bool QGL2PaintEngineEx::begin(QPaintDevice *pdev)
     d->use_system_clip = !systemClip().isEmpty();
 
     d->dirtyStencilRegion = QRect(0, 0, d->width, d->height);
+    d->stencilClean = true;
 
     // Calling begin paint should make the correct context current. So, any
     // code which calls into GL or otherwise needs a current context *must*
@@ -1731,7 +1732,7 @@ void QGL2PaintEngineExPrivate::updateClipScissorTest()
         glStencilFunc(GL_LEQUAL, q->state()->currentClip, ~GL_STENCIL_HIGH_BIT);
     } else {
         glDisable(GL_STENCIL_TEST);
-        glStencilFunc(GL_ALWAYS, 0, 0xffff);
+        glStencilFunc(GL_ALWAYS, 0, 0xff);
     }
 
 #ifdef QT_GL_NO_SCISSOR_TEST
@@ -1787,7 +1788,7 @@ void QGL2PaintEngineExPrivate::clearClip(uint value)
 {
     dirtyStencilRegion -= currentScissorBounds;
 
-    glStencilMask(0xffff);
+    glStencilMask(0xff);
     glClearStencil(value);
     glClear(GL_STENCIL_BUFFER_BIT);
     glStencilMask(0x0);
@@ -1802,6 +1803,8 @@ void QGL2PaintEngineExPrivate::writeClip(const QVectorPath &path, uint value)
     if (matrixDirty)
         updateMatrix();
 
+    stencilClean = false;
+
     const bool singlePass = !path.hasWindingFill()
         && (((q->state()->currentClip == maxClip - 1) && q->state()->clipTestEnabled)
             || q->state()->needsClipBufferClear);
@@ -1819,7 +1822,7 @@ void QGL2PaintEngineExPrivate::writeClip(const QVectorPath &path, uint value)
     if (q->state()->clipTestEnabled)
         glStencilFunc(GL_LEQUAL, q->state()->currentClip, ~GL_STENCIL_HIGH_BIT);
     else
-        glStencilFunc(GL_ALWAYS, 0, 0xffff);
+        glStencilFunc(GL_ALWAYS, 0, 0xff);
 
     vertexCoordinateArray.clear();
     vertexCoordinateArray.addPath(path, inverseScale);
@@ -1841,9 +1844,17 @@ void QGL2PaintEngineExPrivate::writeClip(const QVectorPath &path, uint value)
 
         drawVertexArrays(vertexCoordinateArray, GL_TRIANGLE_FAN);
     } else {
-        glStencilFunc(GL_NOTEQUAL, value, GL_STENCIL_HIGH_BIT);
         glStencilOp(GL_KEEP, GL_REPLACE, GL_REPLACE);
-        glStencilMask(0xffff);
+        glStencilMask(0xff);
+
+        if (!q->state()->clipTestEnabled && path.hasWindingFill()) {
+            // Pass when any clip bit is set, set high bit
+            glStencilFunc(GL_NOTEQUAL, GL_STENCIL_HIGH_BIT, ~GL_STENCIL_HIGH_BIT);
+            composite(vertexCoordinateArray.boundingRect());
+        }
+
+        // Pass when high bit is set, replace stencil value with new clip value
+        glStencilFunc(GL_NOTEQUAL, value, GL_STENCIL_HIGH_BIT);
 
         composite(vertexCoordinateArray.boundingRect());
     }
diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h b/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h
index 28c972e..46be398 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h
@@ -223,6 +223,7 @@ public:
     bool shaderMatrixUniformDirty;
     bool opacityUniformDirty;
 
+    bool stencilClean; // Has the stencil not been used for clipping so far?
     QRegion dirtyStencilRegion;
     QRect currentScissorBounds;
     uint maxClip;