Issue with drawing an Qml Item with raw OpenGL calls

Question

I want to draw a single item in QtQuick scene using raw OpenGL calls. I have decided to take approach suggested in this question.

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)

class FboInSGRenderer : public QQuickFramebufferObject {
    Q_OBJECT
public:
    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 2.4
import 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 presented below:

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 in QQuickFramebufferObject class but it is by default set to true.

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

Answer 1

The drawn rectangle is half the sizes you expect because the default coordinate range is [-1, 1], not [0, 1] as your code assumes. If you want to use [0, 1] scale, then you should appropriately set the projection matrix:

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);