>Psychtoolbox>PsychDemos

ContrastModulatedNoiseTheElegantStyleDemo([noisesize=512] [, staticnoise=0])

This demo shows how to render contrast modulated noise efficiently on
current state of the art graphics hardware by use of the Psychtoolbox
imaging pipeline and high precision framebuffers.

A rectangular image with random noise of size
‘noisesize’ by ‘noisesize’ pixels is created, then drawn to the display.

We use alpha-blending tricks to modulate the contrast of the noise in
realtime. In this demo you can move the mouse pointer to drag around a
“modulation disk” of 50 pixels diameter. Noise inside the disk has a
different contrast ‘fgcontrast’ from the rest of the image, which has a
contrast of ‘bgcontrast’ (see top of code for fgcontrast and bgcontrast).

You can press the cursor left and cursor right keys to decrement or
increment the contrast inside the disk in steps of 0.01 units. Press
cursor up key to set the inside disk contrast equal to outside disk
contrast. Press ESCAPE key to finish the demo.

The optional ‘staticnoise’ flag - if set to 1 - will render a static
noise image instead of one that changes at each frame. That’s faster
because one doesn’t need to recreate the noise texture each frame.

How this works? Basically we use standard Screen 2D drawing commands to
draw a “contrast values weight map” into the alpha-channel, so the
alpha-channel encodes contrast values between 0.0 and 1.0.
Then we draw a noise texture of fixed contrast into the framebuffer
and the alpha-blending hardware takes care of modulating the
contrast of the drawn noise texture with the values from the
alpha-channel.

Hardware requirements: ATI Radeon X-1000 or later, NVidia Geforce 6000 or
later. Recommended is Radeon HD2000/3000/… or Geforce-8000/9000/…
hardware for maximum fun!

The allowable contrast values for standard 2D drawing commands are
limited to the range 0.0 to 1.0 on MacOS/X, they are not limited on
MS-Windows or GNU/Linux. Precision is set to 16bpc float, ie. about 3
digits behind the decimal point or 1024 discriminable levels. On the most
recent ATI HD-2000 and NVidia Geforce 8000 hardware one can lift this limit
to 32bpc float – 6.5 digits or about 8 million discriminable levels by a
one-line code change in this script ;-)