Fix composition of render-to-texture widgets on big endian

Converting from QImage::Format_ARGB32 to QRhiTexture::RGBA8 requires
texture swizzling also on big endian.  But unlike on little endian it
is not a red/blue swap that is needed, but a rather rotation of the
alpha channel from the first component to the last.

Add a new swizzling mode to the backingstorecompose fragment shader,
and rename the uniform to reflect that it no longer covers only
red/blue swapping.

Pick-to: 6.4
Change-Id: I001d275abdc88faaadea16e396ebe032b34e83f0
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/painting/qbackingstoredefaultcompositor.cpp

@@ -71,9 +71,7 @@ QRhiTexture *QBackingStoreDefaultCompositor::toTexture(const QImage &sourceImage
         Q_FALLTHROUGH();
     case QImage::Format_RGB32:
     case QImage::Format_ARGB32:
-#if Q_BYTE_ORDER == Q_LITTLE_ENDIAN
         *flags |= QPlatformBackingStore::TextureSwizzle;
-#endif
         break;
     case QImage::Format_RGBA8888_Premultiplied:
         *flags |= QPlatformBackingStore::TexturePremultiplied;
@@ -378,14 +376,14 @@ void QBackingStoreDefaultCompositor::updatePerQuadData(PerQuadData *d, QRhiTextu
 }
 
 void QBackingStoreDefaultCompositor::updateUniforms(PerQuadData *d, QRhiResourceUpdateBatch *resourceUpdates,
-                                                    const QMatrix4x4 &target, const QMatrix3x3 &source, bool needsRedBlueSwap)
+                                                    const QMatrix4x4 &target, const QMatrix3x3 &source, UpdateUniformOption option)
 {
     resourceUpdates->updateDynamicBuffer(d->ubuf, 0, 64, target.constData());
     updateMatrix3x3(resourceUpdates, d->ubuf, source);
     float opacity = 1.0f;
     resourceUpdates->updateDynamicBuffer(d->ubuf, 112, 4, &opacity);
-    qint32 swapRedBlue = needsRedBlueSwap ? 1 : 0;
+    qint32 textureSwizzle = static_cast<qint32>(option);
-    resourceUpdates->updateDynamicBuffer(d->ubuf, 116, 4, &swapRedBlue);
+    resourceUpdates->updateDynamicBuffer(d->ubuf, 116, 4, &textureSwizzle);
 }
 
 void QBackingStoreDefaultCompositor::ensureResources(QRhiSwapChain *swapchain, QRhiResourceUpdateBatch *resourceUpdates)
@@ -491,7 +489,11 @@ QPlatformBackingStore::FlushResult QBackingStoreDefaultCompositor::flush(QPlatfo
 
     ensureResources(swapchain, resourceUpdates);
 
-    const bool needsRedBlueSwap = (flags & QPlatformBackingStore::TextureSwizzle) != 0;
+#if Q_BYTE_ORDER == Q_LITTLE_ENDIAN
+    const UpdateUniformOption uniformOption = (flags & QPlatformBackingStore::TextureSwizzle) != 0? NeedsRedBlueSwap : NoOption;
+#else
+    const UpdateUniformOption uniformOption = (flags & QPlatformBackingStore::TextureSwizzle) != 0? NeedsAlphaRotate : NoOption;
+#endif
     const bool premultiplied = (flags & QPlatformBackingStore::TexturePremultiplied) != 0;
     SourceTransformOrigin origin = SourceTransformOrigin::TopLeft;
     if (flags & QPlatformBackingStore::TextureFlip)
@@ -512,7 +514,7 @@ QPlatformBackingStore::FlushResult QBackingStoreDefaultCompositor::flush(QPlatfo
         QMatrix4x4 target; // identity
         if (invertTargetY)
             target.data()[5] = -1.0f;
-        updateUniforms(&m_widgetQuadData, resourceUpdates, target, source, needsRedBlueSwap);
+        updateUniforms(&m_widgetQuadData, resourceUpdates, target, source, uniformOption);
     }
 
     const int textureWidgetCount = textures->count();
@@ -538,7 +540,7 @@ QPlatformBackingStore::FlushResult QBackingStoreDefaultCompositor::flush(QPlatfo
                 m_textureQuadData[i] = createPerQuadData(t);
             else
                 updatePerQuadData(&m_textureQuadData[i], t);
-            updateUniforms(&m_textureQuadData[i], resourceUpdates, target, source, false);
+            updateUniforms(&m_textureQuadData[i], resourceUpdates, target, source, NoOption);
         } else {
             m_textureQuadData[i].reset();
         }

src/gui/painting/qbackingstoredefaultcompositor_p.h

@@ -44,6 +44,12 @@ public:
                                              bool translucentBackground);
 
 private:
+    enum UpdateUniformOption {
+      NoOption = 0x00,
+      NeedsRedBlueSwap = 0x01,
+      NeedsAlphaRotate = 0x02,
+    };
+
     void ensureResources(QRhiSwapChain *swapchain, QRhiResourceUpdateBatch *resourceUpdates);
     QRhiTexture *toTexture(const QImage &image,
                            QRhi *rhi,
@@ -80,7 +86,7 @@ private:
     PerQuadData createPerQuadData(QRhiTexture *texture);
     void updatePerQuadData(PerQuadData *d, QRhiTexture *texture);
     void updateUniforms(PerQuadData *d, QRhiResourceUpdateBatch *resourceUpdates,
-                        const QMatrix4x4 &target, const QMatrix3x3 &source, bool needsRedBlueSwap);
+                        const QMatrix4x4 &target, const QMatrix3x3 &source, UpdateUniformOption option);
 };
 
 QT_END_NAMESPACE

src/gui/painting/shaders/backingstorecompose.frag

@@ -7,7 +7,7 @@ layout(std140, binding = 0) uniform buf {
     mat4 vertexTransform;
     mat3 textureTransform;
     float opacity;
-    int swapRedBlue;
+    int textureSwizzle;
 };
 
 layout(binding = 1) uniform sampler2D textureSampler;
@@ -16,8 +16,10 @@ void main()
 {
     vec4 tmpFragColor = texture(textureSampler, v_texcoord);
     tmpFragColor.a *= opacity;
-    if (swapRedBlue == 0)
+    if (textureSwizzle == 0)
         fragColor = tmpFragColor;
+    else if(textureSwizzle == 2)
+        fragColor.argb = tmpFragColor;
     else
-        fragColor = tmpFragColor.bgra;
+        fragColor.bgra = tmpFragColor;
 }

src/gui/painting/shaders/backingstorecompose.vert

@@ -9,7 +9,7 @@ layout(std140, binding = 0) uniform buf {
     mat4 vertexTransform;
     mat3 textureTransform;
     float opacity;
-    int swapRedBlue;
+    int textureSwizzle;
 };
 
 void main()