QQuickContext2DFBOTexture: use normalized sub rect when using FBO with POT texture

When creating a Canvas item with renderStrategy set to Canvas.FramebufferObject
we might end up creating an FBO with a POT texture (e.g on iOS) that doesn't match
the size of the canvas. The result will be that the image turn out scaled.

QSGTexture::normalizedTextureSubRect() is a function that needs to be
overridden to be used. So this patch will create a subclass of QSGPlainTexture
that overrides it, and returns the normalized subrect inside the
FBO that that matches the size of the canvas.

Task-number: QTBUG-59192
Change-Id: I0f0e57e117656df21a94b7015bd160c44485046d
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

1 files changed, 17 insertions, 2 deletions

diff --git a/src/quick/items/context2d/qquickcontext2dtexture.cpp b/src/quick/items/context2d/qquickcontext2dtexture.cpp
index a8bf14ba9f..75a8429e98 100644
--- a/src/quick/items/context2d/qquickcontext2dtexture.cpp
+++ b/src/quick/items/context2d/qquickcontext2dtexture.cpp
@@ -91,6 +91,17 @@ struct GLAcquireContext {
     }
     QOpenGLContext *ctx;
 };
+
+class QSGPlainTextureWithSubRect : public QSGPlainTexture
+{
+public:
+    QSGPlainTextureWithSubRect(const QRectF &subRect) : m_subRect(subRect) {}
+    QRectF normalizedTextureSubRect() const override { return m_subRect; }
+
+private:
+    QRectF m_subRect;
+};
+
 #endif
 QQuickContext2DTexture::QQuickContext2DTexture()
     : m_context(0)
@@ -454,14 +465,18 @@ QVector2D QQuickContext2DFBOTexture::scaleFactor() const
 
 QSGTexture *QQuickContext2DFBOTexture::textureForNextFrame(QSGTexture *lastTexture, QQuickWindow *)
 {
-    QSGPlainTexture *texture = static_cast<QSGPlainTexture *>(lastTexture);
+    QSGPlainTextureWithSubRect *texture = static_cast<QSGPlainTextureWithSubRect *>(lastTexture);
 
     if (m_onCustomThread)
         m_mutex.lock();
 
     if (m_fbo) {
         if (!texture) {
-            texture = new QSGPlainTexture();
+            // Since the FBO might be have a POT size, we need to set a sub rect
+            qreal normalizedCanvasWidth = m_canvasDevicePixelRatio * qreal(m_canvasSize.width()) / m_fbo->size().width();
+            qreal normalizedCanvasHeight = m_canvasDevicePixelRatio * qreal(m_canvasSize.height()) / m_fbo->size().height();
+
+            texture = new QSGPlainTextureWithSubRect(QRectF(0, 0, normalizedCanvasWidth, normalizedCanvasHeight));
             texture->setHasAlphaChannel(true);
             texture->setOwnsTexture(false);
             m_dirtyTexture = true;