Improve QOpenGLFramebufferObject::toImage()

This patch makes it possible to use QOpenGLFramebufferObject::toImage()
together with QOpenGLPaintDevice::setPaintFlipped where the FBO is
already mirrored.

The patch also makes checks for proper BGRA support before trying to use
it, and fixes the rare case of OpenGLES on big endian.

[ChangeLog][QtGui][QOpenGLFramebufferObject] Introduce an argument
to QOpenGLFramebufferObject::toImage() to save mirroring the result.

Change-Id: I163d802736b7059411f7dda96a31385d772823cc
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/gui/opengl/qopenglframebufferobject.cpp

@@ -100,6 +100,23 @@ QT_BEGIN_NAMESPACE
 #define GL_DRAW_FRAMEBUFFER 0x8CA9
 #endif
 
+#ifndef GL_RGB8
+#define GL_RGB8                           0x8051
+#endif
+
+#ifndef GL_RGBA8
+#define GL_RGBA8                          0x8058
+#endif
+
+#ifndef GL_BGRA
+#define GL_BGRA                           0x80E1
+#endif
+
+#ifndef GL_UNSIGNED_INT_8_8_8_8_REV
+#define GL_UNSIGNED_INT_8_8_8_8_REV       0x8367
+#endif
+
+
 /*!
     \class QOpenGLFramebufferObjectFormat
     \brief The QOpenGLFramebufferObjectFormat class specifies the format of an OpenGL
@@ -1097,38 +1114,68 @@ QOpenGLFramebufferObjectFormat QOpenGLFramebufferObject::format() const
     return d->format;
 }
 
-Q_GUI_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format, bool include_alpha)
+static inline QImage qt_gl_read_framebuffer_rgba8(const QSize &size, bool include_alpha, QOpenGLContext *context)
 {
-    int w = size.width();
-    int h = size.height();
-
-    QOpenGLFunctions *funcs = QOpenGLContext::currentContext()->functions();
-    while (funcs->glGetError());
+    QOpenGLFunctions *funcs = context->functions();
+    const int w = size.width();
+    const int h = size.height();
+    bool isOpenGL12orBetter = !context->isOpenGLES() && (context->format().majorVersion() >= 2 || context->format().minorVersion() >= 2);
+    if (isOpenGL12orBetter) {
+        QImage img(size, include_alpha ? QImage::Format_ARGB32_Premultiplied : QImage::Format_RGB32);
+        funcs->glReadPixels(0, 0, w, h, GL_BGRA, GL_UNSIGNED_INT_8_8_8_8_REV, img.bits());
+        return img;
+    }
 
 #if Q_BYTE_ORDER == Q_LITTLE_ENDIAN
-    QImage img(size, (alpha_format && include_alpha) ? QImage::Format_ARGB32_Premultiplied
-                                                     : QImage::Format_RGB32);
-#ifdef QT_OPENGL_ES
-    GLint fmt = GL_BGRA_EXT;
-#else
-    GLint fmt = GL_BGRA;
+    // Without GL_UNSIGNED_INT_8_8_8_8_REV, GL_BGRA only makes sense on little endian.
+    const bool supports_bgra = context->isOpenGLES()
+            ? context->hasExtension(QByteArrayLiteral("GL_EXT_read_format_bgra"))
+            : context->hasExtension(QByteArrayLiteral("GL_EXT_bgra"));
+    if (supports_bgra) {
+        QImage img(size, include_alpha ? QImage::Format_ARGB32_Premultiplied : QImage::Format_RGB32);
+        funcs->glReadPixels(0, 0, w, h, GL_BGRA, GL_UNSIGNED_BYTE, img.bits());
+        return img;
+    }
 #endif
-    funcs->glReadPixels(0, 0, w, h, fmt, GL_UNSIGNED_BYTE, img.bits());
-    if (!funcs->glGetError())
-        return img.mirrored();
-#endif
-
-    QImage rgbaImage(size, (alpha_format && include_alpha) ? QImage::Format_RGBA8888_Premultiplied
-                                                           : QImage::Format_RGBX8888);
+    QImage rgbaImage(size, include_alpha ? QImage::Format_RGBA8888_Premultiplied : QImage::Format_RGBX8888);
     funcs->glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, rgbaImage.bits());
-    return rgbaImage.mirrored();
+    return rgbaImage;
+}
+
+static QImage qt_gl_read_framebuffer(const QSize &size, GLenum internal_format, bool include_alpha, bool flip)
+{
+    QOpenGLContext *ctx = QOpenGLContext::currentContext();
+    QOpenGLFunctions *funcs = ctx->functions();
+    while (funcs->glGetError());
+
+    switch (internal_format) {
+    case GL_RGB:
+    case GL_RGB8:
+        return qt_gl_read_framebuffer_rgba8(size, false, ctx).mirrored(false, flip);
+    case GL_RGBA:
+    case GL_RGBA8:
+    default:
+        return qt_gl_read_framebuffer_rgba8(size, include_alpha, ctx).mirrored(false, flip);
+    }
+
+    Q_UNREACHABLE();
+    return QImage();
+}
+
+Q_GUI_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format, bool include_alpha)
+{
+    return qt_gl_read_framebuffer(size, alpha_format ? GL_RGBA : GL_RGB, include_alpha, true);
 }
 
 /*!
-    \fn QImage QOpenGLFramebufferObject::toImage() const
+    \fn QImage QOpenGLFramebufferObject::toImage(bool flipped) const
 
     Returns the contents of this framebuffer object as a QImage.
 
+    If \a flipped is true the image is flipped from OpenGL coordinates to raster coordinates.
+    If used together with QOpenGLPaintDevice, \a flipped should be the opposite of the value
+    of QOpenGLPaintDevice::paintFlipped().
+
     Will try to return a premultiplied ARBG32 or RGB32 image. Since 5.2 it will fall back to
     a premultiplied RGBA8888 or RGBx8888 image when reading to ARGB32 is not supported.
 
@@ -1139,8 +1186,11 @@ Q_GUI_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format,
     For singlesampled framebuffers the contents is retrieved via \c glReadPixels. This is
     a potentially expensive and inefficient operation. Therefore it is recommended that
     this function is used as seldom as possible.
+
+    \sa QOpenGLPaintDevice::paintFlipped()
 */
-QImage QOpenGLFramebufferObject::toImage() const
+
+QImage QOpenGLFramebufferObject::toImage(bool flipped) const
 {
     Q_D(const QOpenGLFramebufferObject);
     if (!d->valid)
@@ -1166,9 +1216,9 @@ QImage QOpenGLFramebufferObject::toImage() const
         QRect rect(QPoint(0, 0), size());
         blitFramebuffer(&temp, rect, const_cast<QOpenGLFramebufferObject *>(this), rect);
 
-        image = temp.toImage();
+        image = temp.toImage(flipped);
     } else {
-        image = qt_gl_read_framebuffer(d->size, format().internalTextureFormat() != GL_RGB, true);
+        image = qt_gl_read_framebuffer(d->size, format().internalTextureFormat(), true, flipped);
     }
 
     if (prevFbo != d->fbo())
@@ -1177,6 +1227,19 @@ QImage QOpenGLFramebufferObject::toImage() const
     return image;
 }
 
+/*!
+    \fn QImage QOpenGLFramebufferObject::toImage() const
+    \overload
+
+    Returns the contents of this framebuffer object as a QImage. This method flips
+    the image from OpenGL coordinates to raster coordinates.
+*/
+// ### Qt 6: Remove this method and make it a default argument instead.
+QImage QOpenGLFramebufferObject::toImage() const
+{
+    return toImage(true);
+}
+
 /*!
     \fn bool QOpenGLFramebufferObject::bindDefault()
 

src/gui/opengl/qopenglframebufferobject.h

@@ -94,6 +94,7 @@ public:
     GLuint takeTexture();
     QSize size() const;
     QImage toImage() const;
+    QImage toImage(bool flipped) const;
     Attachment attachment() const;
 
     void setAttachment(Attachment attachment);