ProceduralGarboriumDemo([ngabors=200]) – An aquarium full of cute little procedural gabors!
This demo shows how to use the Screen(‘DrawTextures’) command to draw a
large number of similar images quickly - in this case, Gabor patches of
different position, size and orientation. It also shows how to achieve
fast, mathematically correct, linear superposition of image patches by
use of alpha blending and the Psychtoolbox imaging pipeline. While you
could always achieve proper superposition by precomputing large images in
Matlab, then drawing them as textures, the use of alpha blending allows
to offload the computations to your graphics hardware - this allows
speedups by factors of more than 100x in some cases!
Furthermore the demo demonstrates how to use procedural shading to
compute gabor patches on-the-fly instead of precomputing a prototypical
gabor patch image within Matlab. The command CreateProceduralGabor()
allows to generate such a procedural gabor and then draw it with the
normal ‘DrawTexture’ or ‘DrawTextures’ texture drawing commands. However,
this gabor patch is not a image like a normal texture. Instead it is
computed during each draw operation by a so called GLSL shader program –
the gabor formula is evaluated on the fly inside your fast graphics card.
This method has a few advantages over the standard texture based method
(as demonstrated in GarboriumDemo)):
GPU’s are extremely fast at this kind of jobs, so the method is
significantly faster on modern GPU’s, especially for large patches,
allowing for even higher redraw rates.
As the formula is evaluated on the fly for each output pixel, there are
no resampling artifacts, regardless of size of your gabor.
You can change all interesting stimulus parameters on the fly – change
contrast, aspect ratio, spatial constant, frequency, phase, orientation
etc. for each patch during each redraw cycle without the need to
recompute any matrices and without any speed penalty.
The downsides of this method are the need for recent graphics hardware
(GPU’s) and the fact that the stimulus definition formula needs to be
implemented as a shader program in the GLSL language, which is less easy
to use - and less forgiving of programming errors - than the simple and
easy Matlab language. This means that you’re either restricted to our
predifined set of primitives, or you’ll have some steep learning curve.
Currently PTB provides several such stimuli (see the CreateProcedural*
series of functions for details).
The demo shows “an aquarium” of many cute little gabor patches, each moving
into a random direction. Sometimes the gabors intersect, in that case
you’ll see a linear superposition of them. The gabors also shift phase
and pulse (aspect ratio modulation) to make them more life-like – and to
demonstrate runtime change of interesting stimulus parameters, of course.
Each frame of the animation contains ‘ngabors’ patches, ngabors defaults
to 200. Change the number as first optional parameter ‘ngabors’ if you want
to exercise your graphics hardware and cpu.
You can exit the demo by pressing any key on the keyboard.