Make multisampling more robust in QOpenGLFramebufferObject

Some drivers are reported to get confused when passing different
sample counts (requested vs. actual) to the color and depth/stencil
attachments. To overcome this, pass the requested sample count to all
the attachments.

Task-number: QTBUG-33406
Change-Id: I17b0e3dbbd78de2ab0f45e95164b4f326d47aeff
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Reviewed-by: Kimmo Leppälä <kimmo.leppala@digia.com>

2 files changed, 9 insertions, 2 deletions

diff --git a/src/gui/opengl/qopenglframebufferobject.cpp b/src/gui/opengl/qopenglframebufferobject.cpp
index 7d91d2c497..cd6468cccd 100644
--- a/src/gui/opengl/qopenglframebufferobject.cpp
+++ b/src/gui/opengl/qopenglframebufferobject.cpp
@@ -436,9 +436,9 @@ void QOpenGLFramebufferObjectPrivate::init(QOpenGLFramebufferObject *, const QSi
     samples = qBound(0, int(samples), int(maxSamples));
 #endif
 
+    requestedSamples = samples;
     size = sz;
     target = texture_target;
-    // texture dimensions
 
     QT_RESET_GLERROR(); // reset error state
     GLuint fbo = 0;
@@ -472,6 +472,9 @@ void QOpenGLFramebufferObjectPrivate::init(QOpenGLFramebufferObject *, const QSi
         valid = checkFramebufferStatus(ctx);
 
         if (valid) {
+            // Query the actual number of samples. This can be greater than the requested
+            // value since the typically supported values are 0, 4, 8, ..., and the
+            // requests are mapped to the next supported value.
             funcs.glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_SAMPLES, &samples);
             color_buffer_guard = new QOpenGLSharedResourceGuard(ctx, color_buffer, freeRenderbufferFunc);
         }
@@ -542,7 +545,10 @@ void QOpenGLFramebufferObjectPrivate::initTexture(GLenum target, GLenum internal
 
 void QOpenGLFramebufferObjectPrivate::initAttachments(QOpenGLContext *ctx, QOpenGLFramebufferObject::Attachment attachment)
 {
-    int samples = format.samples();
+    // Use the same sample count for all attachments. format.samples() already contains
+    // the actual number of samples for the color attachment and is not suitable. Use
+    // requestedSamples instead.
+    const int samples = requestedSamples;
 
     // free existing attachments
     if (depth_buffer_guard) {
diff --git a/src/gui/opengl/qopenglframebufferobject_p.h b/src/gui/opengl/qopenglframebufferobject_p.h
index 7a653bc16d..75348d1481 100644
--- a/src/gui/opengl/qopenglframebufferobject_p.h
+++ b/src/gui/opengl/qopenglframebufferobject_p.h
@@ -131,6 +131,7 @@ public:
     GLenum target;
     QSize size;
     QOpenGLFramebufferObjectFormat format;
+    int requestedSamples;
     uint valid : 1;
     QOpenGLFramebufferObject::Attachment fbo_attachment;
     QOpenGLExtensions funcs;