Flip or rotate viewport within QOpenGLWidget in Qt 5.8

CamelFrog

Hi,

In Qt 5.8.0 (and msvc 2015), I'm using a subclass of QOpenGLWidget with opengl 3.3 core profile to display images as textures and sometimes I need to rotate the image by multiples of 90 degrees.
I read that we can use either glPixelZoom or glScalef or other things to rotate, although it's not clear to me to which openGL version that applies. I tried both of them but the app does not compile.

This compiles and works fine when I do not try to rotate:

QOpenGLWidget::resizeGL(int w, int h) :
{
   glViewport( 0, 0, w, h );
}

But this does not work, i get a LNK2019 unresolved external symbol:

QOpenGLWidget::resizeGL(int w, int h) :
{
   glScalef(1, -1, 1); // Or glPixelZoom(1, -1) 
   glViewport( 0, 0, w, h );
}

Maybe glScalef() and glPixelZoom() are not for openGL 3.3? If that's true, how may I simply display a rotated or flipped texture in resizeGL() with OpenGL 3.3 core profile? In addition, should I setup the flip / rotation in paintGL() in addition to / instead of resizeGL()?

If that's relevant, here's my list of includes:

#include <QApplication>
#include <QOpenGLWidget>
#include <QtGui/QWindow>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QOpenGLShaderProgram>
#include <QOpenGLVertexArrayObject>
#include <QOpenGLBuffer>
#include <QOpenGLTexture>
#include <QScreen>

Thank you

johngod

Hi there

You cannot use glScalef() in the core profile. You now rotate the images using a rotation matrix in the paintGL() method (not in resizeGL(...) ) and then pass it to the vertex shader where the rotation is applied. Something like this:

void MyGL::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glUseProgram(program);

    QMatrix4x4 view;
    view.setToIdentity();
    view.rotate(90, 1.0f, 0.0f, 0.0f ); //90 degree around x axis
    view.scale(scaleX, scaleY, scaleZ);// also do some scaling if you want

    glUniformMatrix4fv(glGetUniformLocation(program , "matrixRotation"), 1, false, view.constData());// send the matrix to the shader
.....

Now in the vertex shader:

uniform mat4 matrixRotation;
layout (location = 0) in vec4 position;

void main()
{
    gl_Position = matrixRotation * position;
    ...