How to scale glDrawPixels?

Question

I need to scale the result of glDrawPixels image.

I'm drawing a 640x480 pixels image buffer with glDrawPixels in a Qt QGLWidget.

I tryed to do the following in PaintGL:

glScalef(windowWidth/640, windowHeight/480, 0);
glDrawPixels(640,480,GL_RGB,GL_UNSIGNED_BYTE,frame);

But it doesn't work.

I am setting the OpenGL viewport and glOrtho with the size of the widget as:

void WdtRGB::paintGL() {

         glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

         // Setup the OpenGL viewpoint
         glMatrixMode(GL_PROJECTION);
         glLoadIdentity();
         glOrtho(0, windowWidth, windowHeight, 0, -1.0, 1.0);

    glDepthMask(0);
        //glRasterPos2i(0, 0);
        glScalef(windowWidth/640, windowHeight/480, 0);
        glDrawPixels(640,480,GL_RGB,GL_UNSIGNED_BYTE,frame);
    }

    //where windowWidth and windowHeight corresponds to the widget size.
    /the init functions are:

    void WdtRGB::initializeGL() {

        glClearColor ( 0.8, 0.8, 0.8, 0.0); // Background to a grey tone

        /* initialize viewing values  */
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();

        glOrtho(0, windowWidth, windowHeight, 0, -1.0, 1.0);

        glEnable (GL_DEPTH_TEST);

    }

    void WdtRGB::resizeGL(int w, int h) {
        float aspect=(float)w/(float)h;

        windowWidth = w;
        windowHeight = h;
        glViewport (0, 0, (GLsizei) w, (GLsizei) h);
        glMatrixMode (GL_PROJECTION);
        glLoadIdentity ();

        if( w <= h )
                glOrtho ( -5.0, 5.0, -5.0/aspect, 5.0/aspect, -5.0, 5.0);
        else
                glOrtho (-5.0*aspect, 5.0*aspect, -5.0, 5.0, -5.0, 5.0);

        //printf("\nresize");
        emit changeSize ( );
    }

Answer 1

It sounds like what you actually need to do instead of calling glDrawPixels () is to load your image data into a texture and draw a textured quad the size of the window. So something like this:

glGenTextures (1, &texID);
glBindTextures (GL_TEXTURE_RECTANGLE_EXT, texID);
glTexImage2D (GL_TEXTURE_RECTANGLE_EXT, 0, GL_RGBA, 640, 480, 0, GL_RGBA, GL_UNSIGNED_INT_8_8_8_8, frame);
glBegin (GL_QUADS);
glTexCoord2f (0, 0);
glVertex2f (0, 0);
glTexCoord2f (640, 0);
glVertex2f (windowWidth, 0);
glTexCoord2f (640, 480);
glVertex2f (windowWidth, windowHeight);
glTexCoord2f (0, 480);
glVertex2f (0, windowHeight);
glEnd();

Or if that's too much work, glPixelZoom (windowWidth / 640, windowHeight / 480), might do the trick, too.

Answer 2

I need to scale the result of glDrawPixels image.

glDrawPixels directly goes to the framebuffer. So every incoming pixel is mapped 1:1 to the output. There is a function (ah, why am I telling you this) glPixelZoom, which allows you to zoom glDrawPixels.

BUT I urge you, not to use glDrawPixels!

Use textures quads instead. glDrawPixels is a depreciated function, no longer supported by modern OpenGL-3. And even when it was not deprecated it still is a very slow function. Textured quads are better in every regard.

Answer 3

Just for further reference: Instead of doing the glScalef(windowWidth/640, windowHeight/480, 0); before the glDrawPixels you should do a pixel zoom:

glRasterPos2i(0,0);
GLint iViewport[4];
glGetIntegerv(GL_VIEWPORT, iViewport);
glPixelZoom(iViewport[2]/640, iViewport[3]/480);
glDrawPixels(640,480,GL_RGB,GL_UNSIGNED_BYTE,frame);