QSGGeometry. Batch 2 triangles to draw a rectangle with QSGGeometry::DrawTriangleStrip.

[ALKMAN]

I am trying to separate the triangles that I draw with QSGGeometry::DrawTriangleStrip. into batches of 2 in order to draw seperate rectangles with a texture(eventually).

I cannot figure out how I can do this.
It seems that passing indices do not affect how OpenGL handles triangle strips.

Am I missing something important here?

My end result should be 2 rectangles but not joined together like in the pic below.

QSGNode *pianoroll_preview::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *)
{
    pianoroll_preview_instance *n= static_cast<pianoroll_preview_instance *>(oldNode);
    QRectF rect = boundingRect();

    QSGGeometryNode *node = nullptr;
    QSGGeometry *geometry = nullptr;

    int vCount = 4;
    if (!n) {
        n = new pianoroll_preview_instance();

        node = new QSGGeometryNode;
        geometry = new QSGGeometry(QSGGeometry::defaultAttributes_ColoredPoint2D(), vCount);
        geometry->setDrawingMode(QSGGeometry::DrawTriangleStrip);
        //geometry = node->geometry();
        geometry->allocate( vCount * 2 );
        node->setGeometry(geometry);
        node->setFlag(QSGNode::OwnsGeometry);
        QSGVertexColorMaterial *material = new QSGVertexColorMaterial;
        node->setMaterial(material);
        node->setFlag(QSGNode::OwnsMaterial);
        n->appendChildNode( node );
    }

    QSGGeometry::ColoredPoint2D *vertex = geometry-> vertexDataAsColoredPoint2D();

    quint16* indices = geometry->indexDataAsUShort();
    indices[0] = 0; indices[1] = 1; indices[2] = 2;
    indices[3] = 0; indices[4] = 2; indices[5] = 3;

    indices[6] = 0; indices[7] = 1; indices[8] = 2;
    indices[9] = 0; indices[10] = 2; indices[11] = 3;

    int w = rect.width();
    int h = rect.height();

    vertex[0].set( 0, 0, RED );
    vertex[1].set( w/4, 0, GREEN );
    vertex[2].set( 0, w/4, YELLOW );
    vertex[3].set( w/4, w/4, 0,0,255, 0 );


    vertex[4].set( w/2+w/4, h/2+h/4, RED );
    vertex[5].set( w,       h/2+h/4, GREEN  );
    vertex[6].set( w/2+w/4, h, YELLOW );
    vertex[7].set( w, h, BLUE );


    return n;
}

I am on QT 5.15.2 and OpenGL ES 2.0

[johngod]

Hi @ALKMAN

Note that I never used QSGGeometry, but the primitive DrawTriangleStrip should be similar to OpenGL primitives, so you need to use degenerated triangles to break apart the 2 rectangles. You should only need a indice list of size 10 with the following vertices:
0,1,2,3,3,4,45,6,7

indices[0] = 0; indices[1] = 1; indices[2] = 2;
indices[3] = 3; indices[4] = 3; indices[5] = 4;
indices[6] = 4; indices[7] = 5; indices[8] = 6;
indices[9] = 7;