Magnifying or Reducing an Image
Normally, each pixel in an image is written to a single pixel on the
screen. However, you can arbitrarily magnify or reduce an image by using
glPixelZoom(). void glPixelZoom(GLfloat zoomx, GLfloat zoomy);
Sets the magnification or reduction factors for pixel-write operations, in the
x- and y-dimensions. By default, zoomx and zoomy are 1.0.
If they're both 2.0,
each image pixel is drawn to 4 screen pixels. Note that fractional
magnification or reduction factors are allowed, as are negative factors.
You can use negative values to flip the image.