Refactored the GL QPixmap backend to use an FBO pool class.

The FBO pool will be useful in other places as well, plus it makes it
easier to deal with graphics memory management issues.

2 files changed, 106 insertions, 41 deletions

diff --git a/src/opengl/qpixmapdata_gl.cpp b/src/opengl/qpixmapdata_gl.cpp
index f0c7e20..f6a9bf4 100644
--- a/src/opengl/qpixmapdata_gl.cpp
+++ b/src/opengl/qpixmapdata_gl.cpp
@@ -55,6 +55,85 @@ QT_BEGIN_NAMESPACE
 
 extern QGLWidget* qt_gl_share_widget();
 
+/*!
+    \class QGLFramebufferObjectPool
+    \since 4.6
+
+    \brief The QGLFramebufferObject class provides a pool of framebuffer
+    objects for offscreen rendering purposes.
+
+    When requesting an FBO of a given size and format, an FBO of the same
+    format and a size at least as big as the requested size will be returned.
+
+    \internal
+*/
+
+static inline int areaDiff(const QSize &size, const QGLFramebufferObject *fbo)
+{
+    return qAbs(size.width() * size.height() - fbo->width() * fbo->height());
+}
+
+QGLFramebufferObject *QGLFramebufferObjectPool::acquire(const QSize &requestSize, const QGLFramebufferObjectFormat &requestFormat)
+{
+    QGLFramebufferObject *chosen = 0;
+    QGLFramebufferObject *candidate = 0;
+    for (int i = 0; !chosen && i < m_fbos.size(); ++i) {
+        QGLFramebufferObject *fbo = m_fbos.at(i);
+
+        QGLFramebufferObjectFormat format = fbo->format();
+        if (format.samples() == requestFormat.samples()
+            && format.attachment() == requestFormat.attachment()
+            && format.textureTarget() == requestFormat.textureTarget()
+            && format.internalFormat() == requestFormat.internalFormat())
+        {
+            // choose the fbo with a matching format and the closest size
+            if (!candidate || areaDiff(requestSize, candidate) > areaDiff(requestSize, fbo))
+                candidate = fbo;
+        }
+
+        if (candidate) {
+            m_fbos.removeOne(candidate);
+
+            const QSize fboSize = candidate->size();
+            QSize sz = fboSize;
+
+            if (sz.width() < requestSize.width())
+                sz.setWidth(qMax(requestSize.width(), qRound(sz.width() * 1.5)));
+            if (sz.height() < requestSize.height())
+                sz.setHeight(qMax(requestSize.height(), qRound(sz.height() * 1.5)));
+
+            // wasting too much space?
+            if (sz.width() * sz.height() > requestSize.width() * requestSize.height() * 2.5)
+                sz = requestSize;
+
+            if (sz != fboSize) {
+                delete candidate;
+                qDebug() << "Resizing fbo in pool:" << sz;
+                candidate = new QGLFramebufferObject(sz, requestFormat);
+            }
+
+            chosen = candidate;
+        }
+    }
+
+    if (!chosen) {
+        qDebug() << "Creating new fbo in pool:" << requestSize;
+        chosen = new QGLFramebufferObject(requestSize, requestFormat);
+    }
+
+    if (!chosen->isValid()) {
+        delete chosen;
+        chosen = 0;
+    }
+
+    return chosen;
+}
+
+void QGLFramebufferObjectPool::release(QGLFramebufferObject *fbo)
+{
+    m_fbos << fbo;
+}
+
 class QGLShareContextScope
 {
 public:
@@ -330,8 +409,11 @@ struct TextureBuffer
     QGL2PaintEngineEx *engine;
 };
 
-static QVector<TextureBuffer> textureBufferStack;
-static int currentTextureBuffer = 0;
+Q_GLOBAL_STATIC(QGLFramebufferObjectPool, _qgl_fbo_pool)
+QGLFramebufferObjectPool* qgl_fbo_pool()
+{
+    return _qgl_fbo_pool();
+}
 
 void QGLPixmapData::copyBackFromRenderFbo(bool keepCurrentFboBound) const
 {
@@ -378,7 +460,8 @@ void QGLPixmapData::swapBuffers()
     copyBackFromRenderFbo(false);
     m_renderFbo->release();
 
-    --currentTextureBuffer;
+    qgl_fbo_pool()->release(m_renderFbo);
+    delete m_engine;
 
     m_renderFbo = 0;
     m_engine = 0;
@@ -396,19 +479,6 @@ void QGLPixmapData::doneCurrent()
         m_renderFbo->release();
 }
 
-static TextureBuffer createTextureBuffer(const QSize &size, QGL2PaintEngineEx *engine = 0)
-{
-    TextureBuffer buffer;
-    QGLFramebufferObjectFormat fmt;
-    fmt.setAttachment(QGLFramebufferObject::CombinedDepthStencil);
-    fmt.setSamples(4);
-
-    buffer.fbo = new QGLFramebufferObject(size, fmt);
-    buffer.engine = engine ? engine : new QGL2PaintEngineEx;
-
-    return buffer;
-}
-
 bool QGLPixmapData::useFramebufferObjects()
 {
     return QGLFramebufferObject::hasOpenGLFramebufferObjects()
@@ -431,33 +501,15 @@ QPaintEngine* QGLPixmapData::paintEngine() const
             qt_gl_share_widget()->makeCurrent();
         QGLShareContextScope ctx(qt_gl_share_widget()->context());
 
-        if (textureBufferStack.size() <= currentTextureBuffer) {
-            textureBufferStack << createTextureBuffer(size());
-        } else {
-            QSize sz = textureBufferStack.at(currentTextureBuffer).fbo->size();
-            if (sz.width() < w || sz.height() < h) {
-                if (sz.width() < w)
-                    sz.setWidth(qMax(w, qRound(sz.width() * 1.5)));
-                if (sz.height() < h)
-                    sz.setHeight(qMax(h, qRound(sz.height() * 1.5)));
-
-                // wasting too much space?
-                if (sz.width() * sz.height() > w * h * 2.5)
-                    sz = QSize(w, h);
-
-                delete textureBufferStack.at(currentTextureBuffer).fbo;
-                textureBufferStack[currentTextureBuffer] =
-                    createTextureBuffer(sz, textureBufferStack.at(currentTextureBuffer).engine);
-                qDebug() << "Creating new pixmap texture buffer:" << sz;
-            }
-        }
-
-        if (textureBufferStack.at(currentTextureBuffer).fbo->isValid()) {
-            m_renderFbo = textureBufferStack.at(currentTextureBuffer).fbo;
-            m_engine = textureBufferStack.at(currentTextureBuffer).engine;
+        QGLFramebufferObjectFormat format;
+        format.setAttachment(QGLFramebufferObject::CombinedDepthStencil);
+        format.setSamples(4);
+        format.setInternalFormat(m_hasAlpha ? GL_RGBA : GL_RGB);
 
-            ++currentTextureBuffer;
+        m_renderFbo = qgl_fbo_pool()->acquire(size(), format);
 
+        if (m_renderFbo) {
+            m_engine = new QGL2PaintEngineEx;
             return m_engine;
         }
 
diff --git a/src/opengl/qpixmapdata_gl_p.h b/src/opengl/qpixmapdata_gl_p.h
index a6aa22d..aaeb8ac 100644
--- a/src/opengl/qpixmapdata_gl_p.h
+++ b/src/opengl/qpixmapdata_gl_p.h
@@ -61,6 +61,19 @@ QT_BEGIN_NAMESPACE
 
 class QPaintEngine;
 class QGLFramebufferObject;
+class QGLFramebufferObjectFormat;
+
+class QGLFramebufferObjectPool
+{
+public:
+    QGLFramebufferObject *acquire(const QSize &size, const QGLFramebufferObjectFormat &format);
+    void release(QGLFramebufferObject *fbo);
+
+private:
+    QList<QGLFramebufferObject *> m_fbos;
+};
+
+QGLFramebufferObjectPool* qgl_fbo_pool();
 
 class QGLPixmapData : public QPixmapData
 {