Turning picture upside down when using glDrawPixels - c++

Question

I'm getting really strange results when using glDrawPixels() in combination with glRasterPos2*() and glPixelZoom(). I've got a picture and I'd like to show it like old framebuffers used to, i.e. (0,0) being in the topleft corner. Here's the code:
void GLWidget::resizeGL( int w, int h )
{
glViewport( 0, 0, w, h );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
glOrtho( 0, w, 0, h, -1, 1 );
}
void GLWidget::paintGL()
{
for( int i = 0; i < SCREEN_WIDTH; ++i )
{
displayBitmap[i] = 0xf81f;
}
for( int i = 239 * SCREEN_WIDTH; i < 239 * SCREEN_WIDTH + SCREEN_WIDTH; ++i )
{
displayBitmap[i] = 0xf800;
}
//glRasterPos2f( 0, SCREEN_HEIGHT - 0.1 );
glRasterPos2i( 0, SCREEN_HEIGHT - 1 );
glPixelZoom( 1, -1 );
glPixelStorei( GL_UNPACK_ALIGNMENT, 1 );
glDrawPixels( SCREEN_WIDTH, SCREEN_HEIGHT, GL_RGB, GL_UNSIGNED_SHORT_5_6_5, displayBitmap );
}
If executed as it is now, I'm getting the violet line at (0,1), i.e. one black line, then the violet line and then, in the invisible bottom area, is the red line.
OK, so I change
glRasterPos2i( 0, SCREEN_HEIGHT - 1 );
to
glRasterPos2i( 0, SCREEN_HEIGHT );
Nope, output is corrupted. Strangely, if I change it to:
glRasterPos2f( 0, SCREEN_HEIGHT - 0.1 );
it works, both lines are drawn, in the correct order (SCREEN_HEIGHT and SCREEN_HEIGHT - 1.0 lead to the same result as in the integer version).
What I'm doing wrong here? SCREEN_WIDTH = 320, SCREEN_HEIGHT = 240.

Answer 1

This works for me:
void display() {
unsigned char *pixels=captureScreenRegion(0,0,window_width,window_height);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glRasterPos2f(-1,1);
glPixelZoom( 1, -1 );
glDrawPixels(window_width,window_height,GL_RGB,GL_BYTE,pixels);
free(pixels);
glutSwapBuffers();
}

Answer 2

glDrawPixels is only poorly supported on consumer grade hardware. And in newer OpenGL versions it has been removed entirely. Instead of spending time trying to get this to work, just load your image into a texture and draw a textured quad with a trivial shader with it.
Trust me, messing around with glDrawPixels is not worth the effort.

Answer 3

I meet the same problem as you do. I solved it this way:
void VideoRenderWidget::resizeGL(int w, int h)
{
glViewport( 0, 0, w, h );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
glOrtho( 0, w, 0, h, 0.1, 1 );
glPixelZoom( 1, -1 );
glRasterPos3f(0, h - 1, -0.3);
}
void VideoRenderWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glDrawPixels(704, 576, GL_RGB, GL_UNSIGNED_BYTE, buffer_.data_);
}
This worked for me.. I am still developing this.. here 704 is video width and 576 is height.