Avoid continuous texture alloc in Composition Modes example

There seems to be some confusion from back when the example were
mass-ported to the QOpenGL stuff in Qt 5 times.

Fixes: QTBUG-109119
Change-Id: Ic4bcd010df3fcf82e16385ce241b379f0c351788
Reviewed-by: Christian Strømme <christian.stromme@qt.io>
(cherry picked from commit 919664b29a874e8cdc3d74427654cbaa2b61be29)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/widgets/painting/composition/composition.cpp

@@ -266,6 +266,7 @@ CompositionRenderer::CompositionRenderer(QWidget *parent)
     setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
 #if QT_CONFIG(opengl)
     m_pbuffer_size = 1024;
+    m_base_tex = 0;
 #endif
 }
 
@@ -361,6 +362,7 @@ void CompositionRenderer::paint(QPainter *painter)
 {
 #if QT_CONFIG(opengl)
     if (usesOpenGL() && glWindow()->isValid()) {
+        auto *funcs = QOpenGLContext::currentContext()->functions();
 
         if (!m_blitter.isCreated())
             m_blitter.create();
@@ -385,10 +387,13 @@ void CompositionRenderer::paint(QPainter *painter)
             p.setCompositionMode(QPainter::CompositionMode_SourceOver);
             drawBase(p);
             p.end();
+            if (m_base_tex)
+                funcs->glDeleteTextures(1, &m_base_tex);
             m_base_tex = m_fbo->takeTexture();
         }
 
         painter->beginNativePainting();
+        uint compositingTex;
         {
             QPainter p(m_fbo.get());
             p.beginNativePainting();
@@ -400,19 +405,18 @@ void CompositionRenderer::paint(QPainter *painter)
             p.endNativePainting();
             drawSource(p);
             p.end();
-            m_compositing_tex = m_fbo->takeTexture();
+            compositingTex = m_fbo->texture();
         }
         painter->endNativePainting();
 
         painter->beginNativePainting();
-        auto *funcs = QOpenGLContext::currentContext()->functions();
         funcs->glEnable(GL_BLEND);
         funcs->glBlendEquation(GL_FUNC_ADD);
         funcs->glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
         m_blitter.bind();
         const QRect targetRect(QPoint(0, 0), m_fbo->size());
         const QMatrix4x4 target = QOpenGLTextureBlitter::targetTransform(targetRect, QRect(QPoint(0, 0), size()));
-        m_blitter.blit(m_compositing_tex, target, QOpenGLTextureBlitter::OriginBottomLeft);
+        m_blitter.blit(compositingTex, target, QOpenGLTextureBlitter::OriginBottomLeft);
         m_blitter.release();
         painter->endNativePainting();
     } else

examples/widgets/painting/composition/composition.h

@@ -195,7 +195,6 @@ private:
     std::unique_ptr<QFboPaintDevice> m_fbo;
     int m_pbuffer_size; // width==height==size of pbuffer
     uint m_base_tex;
-    uint m_compositing_tex;
     QSize m_previous_size;
     QOpenGLTextureBlitter m_blitter;
 #endif

examples/widgets/painting/shared/fbopaintdevice.cpp

@@ -71,11 +71,13 @@ QFboPaintDevice::QFboPaintDevice(const QSize &size, bool flipped, bool clearOnIn
         context()->functions()->glClearColor(0, 0, 0, 0);
         context()->functions()->glClear(GL_COLOR_BUFFER_BIT);
     }
+    m_resolvedFbo = new QOpenGLFramebufferObject(m_framebufferObject->size(), m_framebufferObject->attachment());
 }
 
 QFboPaintDevice::~QFboPaintDevice()
 {
     delete m_framebufferObject;
+    delete m_resolvedFbo;
     delete m_surface;
 }
 
@@ -87,12 +89,19 @@ void QFboPaintDevice::ensureActiveTarget()
     m_framebufferObject->bind();
 }
 
+GLuint QFboPaintDevice::texture()
+{
+    m_resolvedFbo->bind(); // to get the backing texture recreated if it was taken (in takeTexture) previously
+    QOpenGLFramebufferObject::blitFramebuffer(m_resolvedFbo, m_framebufferObject);
+    return m_resolvedFbo->texture();
+}
+
 GLuint QFboPaintDevice::takeTexture()
 {
-    // We have multisamples so we can't just forward takeTexture().
-    QOpenGLFramebufferObject resolvedFbo(m_framebufferObject->size(), m_framebufferObject->attachment());
-    QOpenGLFramebufferObject::blitFramebuffer(&resolvedFbo, m_framebufferObject);
-    return resolvedFbo.takeTexture();
+    m_resolvedFbo->bind(); // to get the backing texture recreated if it was taken (in takeTexture) previously
+    // We have multisamples so we can't just forward takeTexture(), have to resolve first.
+    QOpenGLFramebufferObject::blitFramebuffer(m_resolvedFbo, m_framebufferObject);
+    return m_resolvedFbo->takeTexture();
 }
 
 QImage QFboPaintDevice::toImage() const

examples/widgets/painting/shared/fbopaintdevice.h

@@ -69,6 +69,7 @@ public:
 
     bool isValid() const { return m_framebufferObject->isValid(); }
     GLuint handle() const { return m_framebufferObject->handle(); }
+    GLuint texture();
     GLuint takeTexture();
     QImage toImage() const;
 
@@ -83,6 +84,7 @@ public:
 
 private:
     QOpenGLFramebufferObject *m_framebufferObject;
+    QOpenGLFramebufferObject *m_resolvedFbo;
     QSurface *m_surface;
 };