[Qt-interest] Fast Scaling of Images

[Till Oliver Knoll]

Am 16.04.2011 um 07:05 schrieb Joshua Grauman <jnfo-c at grauman.com>:

> I've never used openGL, would it be possible to pass off scaling of 
> the image to openGL? If so, can anyone get me started in the right 
> direction for how to do that?
>> 
>> 

The "cheap" way would be to use a QGraphicsScene/View with an underlying QGLWidget, refer to QGraphicsView#setRenderWidget (or the like).

Like this many operations - with a little bit of luck scaling as well - are then passed onwards to OpenGL.

Use a QGraphicsPixmapItem for your bitmap.

The "expensive" but optimal way would be to implement your own GL solution in a QGLWidget based widget. Grab a book or check out the tutorials on the web ("textures" would be a good starting point).

But note that many "raw" GL commands have been deprecated, notably most of the "geometry" functions: glVertex and Co. The basic idea since OpenGL 3 is to upload the whole geometry "at once" into the the graphic card memory using "vertex buffers" or so.

"Quads" have also been deprecated, so your image would be two triangles (forming a rectangle) with a texture on top (the image).

Oh and yes, most matrix operations have also been deprecated in OpenGL 3 ;) (A good thing Qt came up with its own matrix classes to support viewport/world transformations ;)

So for instance glScale() is deprecated, you need to come up with your own "scaling matrix" now (that's where the Qt matrix classes jump in - or other matrix/"game" libraries).

Oh, and one more thing: "At least turn the light on when you point the camera into the wrong direction with regards to a black cube in front of a black background!" ;) (your first "Hello GL world!" so to say ;)

Good luck!
  Oliver
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