Workaround to VideoCore III scissor bug.

Some versions of VideoCore III drivers seem
to pollute and use stencil buffer when using
glScissors. Workaround is to clear stencil buffer
before disabling scissoring.

Task-number: QT-5308
Reviewed-by: Samuel Rødal

4 files changed, 16 insertions, 1 deletions

diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
index 047d589..999a074 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
@@ -2059,6 +2059,8 @@ void QGL2PaintEngineExPrivate::updateClipScissorTest()
     currentScissorBounds = bounds;
 
     if (bounds == QRect(0, 0, width, height)) {
+        if (ctx->d_func()->workaround_brokenScissor)
+            clearClip(0);
         glDisable(GL_SCISSOR_TEST);
     } else {
         glEnable(GL_SCISSOR_TEST);
diff --git a/src/opengl/qgl.cpp b/src/opengl/qgl.cpp
index 3b3da43..af160d8 100644
--- a/src/opengl/qgl.cpp
+++ b/src/opengl/qgl.cpp
@@ -1716,6 +1716,8 @@ void QGLContextPrivate::init(QPaintDevice *dev, const QGLFormat &format)
     workaround_brokenTextureFromPixmap = false;
     workaround_brokenTextureFromPixmap_init = false;
 
+    workaround_brokenScissor = false;
+
     for (int i = 0; i < QT_GL_VERTEX_ARRAY_TRACKED_COUNT; ++i)
         vertexAttributeArraysEnabledState[i] = false;
 }
diff --git a/src/opengl/qgl_egl.cpp b/src/opengl/qgl_egl.cpp
index a589118..07134cf 100644
--- a/src/opengl/qgl_egl.cpp
+++ b/src/opengl/qgl_egl.cpp
@@ -192,7 +192,9 @@ void QGLContext::makeCurrent()
         if (!d->workaroundsCached) {
             d->workaroundsCached = true;
             const char *renderer = reinterpret_cast<const char *>(glGetString(GL_RENDERER));
-            if (renderer && (strstr(renderer, "SGX") || strstr(renderer, "MBX"))) {
+            if (!renderer)
+                return;
+            if ((strstr(renderer, "SGX") || strstr(renderer, "MBX"))) {
                 // PowerVR MBX/SGX chips needs to clear all buffers when starting to render
                 // a new frame, otherwise there will be a performance penalty to pay for
                 // each frame.
@@ -229,6 +231,13 @@ void QGLContext::makeCurrent()
                         d->workaround_brokenFBOReadBack = true;
                     }
                 }
+            } else if (strstr(renderer, "VideoCore III")) {
+                // Some versions of VideoCore III drivers seem to pollute and use
+                // stencil buffer when using glScissors even if stencil test is disabled.
+                // Workaround is to clear stencil buffer before disabling scissoring.
+
+                // qDebug() << "Found VideoCore III driver, enabling brokenDisableScissorTest"; 
+                d->workaround_brokenScissor = true;
             }
         }
     }
diff --git a/src/opengl/qgl_p.h b/src/opengl/qgl_p.h
index c56b2db..d76f0b0 100644
--- a/src/opengl/qgl_p.h
+++ b/src/opengl/qgl_p.h
@@ -415,6 +415,8 @@ public:
     uint workaround_brokenTextureFromPixmap : 1;
     uint workaround_brokenTextureFromPixmap_init : 1;
 
+    uint workaround_brokenScissor : 1;
+
     QPaintDevice *paintDevice;
     QColor transpColor;
     QGLContext *q_ptr;