[Interest] Issue with drawing an Qml Item with raw OpenGL calls

[Paweł Pietraszko]

Hi!

I want to draw a single item in QtQuick scene using raw OpenGL calls. It 's
important for me to do raw OpenGL calls because I want to integrate to
external library which will be drawing on my Item.

I have decided to take approach suggested in this stackoverflow question
<http://stackoverflow.com/questions/27948211/comparison-of-two-approaches-to-rendering-raw-opengl-into-a-qml-ui-in-qt>
.

I have created a Qt Quick item deriving from QQuickFramebufferObject and
exposed it to QML as Renderer (code is based on Qt example: Scene Graph -
Rendering FBOs
<http://doc.qt.io/qt-5/qtquick-scenegraph-textureinsgnode-example.html>):

class FboInSGRenderer : public QQuickFramebufferObject {
    Q_OBJECTpublic:
    Renderer *createRenderer() const;};

source file:

class LogoInFboRenderer : public QQuickFramebufferObject::Renderer {
    public:
        LogoInFboRenderer() { }

        void render() {
            int width = 1, height = 1;
            glEnable(GL_BLEND);
            glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
            glColor4f(0.0, 1.0, 0.0, 0.8);
            glBegin(GL_QUADS);
            glVertex2f(0, 0);
            glVertex2f(width, 0);
            glVertex2f(width, height);
            glVertex2f(0, height);
            glEnd();

            glLineWidth(2.5);
            glColor4f(0.0, 0.0, 0.0, 1.0);
            glBegin(GL_LINES);
            glVertex2f(0, 0);
            glVertex2f(width, height);
            glVertex2f(width, 0);
            glVertex2f(0, height);
            glEnd();

            update();
        }

        QOpenGLFramebufferObject *createFramebufferObject(const QSize &size) {
            QOpenGLFramebufferObjectFormat format;
            
format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
            format.setSamples(4);
            return new QOpenGLFramebufferObject(size, format);
        }};
QQuickFramebufferObject::Renderer *FboInSGRenderer::createRenderer() const {
    return new LogoInFboRenderer();}

In Qml I use it as follows:

import QtQuick 2.4import SceneGraphRendering 1.0
Rectangle {
    width: 400
    height: 400
    color: "purple"
    Renderer {
        id: renderer
        anchors.fill: parent
    }}

I was expecting to see that rendered "X" will fill entire scene, but
instead I get the result I included in attachment.

Other experiments seem to confirm that drew shape has always it's size
(width/height) divided by 2. I also checked that size parameter in
createFramebufferObject has correct value. Looking into docs led me to
property textureFollowsItemSize
<http://doc.qt.io/qt-5/qquickframebufferobject.html#textureFollowsItemSize-prop>
 in QQuickFramebufferObjectclass but it is by default set to true.

Am I doing something wrong or should I consider this behavior as Qt bug?

(I have also post this question on stackoverflow
<http://stackoverflow.com/questions/28070778/issue-with-drawing-an-qml-item-with-raw-opengl-calls>
)

Best regards,

Paweł

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