Support arbitrary strides in the QPlatformBackingStore compositor

Otherwise platform backingstores providing QImages with extra pixels
would break horribly. The only such platform is wayland for now.

Previously the need to support this case was masked by the RGBA8888
image conversion that happened always with wayland due to its ARGB32_Pre
format. With the recent improvements this is not done anymore and so
the problems became apparent.

Task-number: QTBUG-50894
Change-Id: I27d7a1c8e25d152ca1227af1e2c38f7d4b6acbab
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>

1 files changed, 21 insertions, 4 deletions

diff --git a/src/gui/painting/qplatformbackingstore.cpp b/src/gui/painting/qplatformbackingstore.cpp
index 5ac903c71d..33d5e76e52 100644
--- a/src/gui/painting/qplatformbackingstore.cpp
+++ b/src/gui/painting/qplatformbackingstore.cpp
@@ -516,7 +516,23 @@ GLuint QPlatformBackingStore::toTexture(const QRegion &dirtyRegion, QSize *textu
     if (needsConversion)
         image = image.convertToFormat(QImage::Format_RGBA8888);
 
+    // The image provided by the backingstore may have a stride larger than width * 4, for
+    // instance on platforms that manually implement client-side decorations.
+    static const int bytesPerPixel = 4;
+    const int strideInPixels = image.bytesPerLine() / bytesPerPixel;
+    const bool hasUnpackRowLength = !ctx->isOpenGLES() || ctx->format().majorVersion() >= 3;
+
     QOpenGLFunctions *funcs = ctx->functions();
+
+    if (hasUnpackRowLength) {
+        funcs->glPixelStorei(GL_UNPACK_ROW_LENGTH, strideInPixels);
+    } else if (strideInPixels != image.width()) {
+        // No UNPACK_ROW_LENGTH on ES 2.0 and yet we would need it. This case is typically
+        // hit with QtWayland which is rarely used in combination with a ES2.0-only GL
+        // implementation.  Therefore, accept the performance hit and do a copy.
+        image = image.copy();
+    }
+
     if (resized) {
         if (d_ptr->textureId)
             funcs->glDeleteTextures(1, &d_ptr->textureId);
@@ -538,11 +554,9 @@ GLuint QPlatformBackingStore::toTexture(const QRegion &dirtyRegion, QSize *textu
         QRect imageRect = image.rect();
         QRect rect = dirtyRegion.boundingRect() & imageRect;
 
-        if (!ctx->isOpenGLES() || ctx->format().majorVersion() >= 3) {
-            funcs->glPixelStorei(GL_UNPACK_ROW_LENGTH, image.width());
+        if (hasUnpackRowLength) {
             funcs->glTexSubImage2D(GL_TEXTURE_2D, 0, rect.x(), rect.y(), rect.width(), rect.height(), GL_RGBA, pixelType,
-                                   image.constScanLine(rect.y()) + rect.x() * 4);
-            funcs->glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);
+                                   image.constScanLine(rect.y()) + rect.x() * bytesPerPixel);
         } else {
             // if the rect is wide enough it's cheaper to just
             // extend it instead of doing an image copy
@@ -564,6 +578,9 @@ GLuint QPlatformBackingStore::toTexture(const QRegion &dirtyRegion, QSize *textu
         }
     }
 
+    if (hasUnpackRowLength)
+        funcs->glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);
+
     return d_ptr->textureId;
 }
 #endif // QT_NO_OPENGL