>Psychtoolbox>PsychGLImageProcessing

Create a single-pass image processing shader for direct use with Screen(‘DrawTexture’)

Usage:

[shader, varargout] = CreateSinglePassImageProcessingShader(windowPtr, shaderType, varargin)

Creates a shader for window ‘windowPtr’ of type ‘shaderType’ with
optional, type specific, parameters. Returns a handle ‘shader’ and
optional other properties.

The following types are currently supported:

‘BackgroundMaskOut’

shader = CreateSinglePassImageProcessingShader(windowPtr, ‘BackgroundMaskOut’, backgroundColor [, tolerance]);

• This shader draws a texture, but removes all “backgroundColor” pixels
  during drawing, effectively masking them out. ‘backgroundColor’ is a 3
  component [R, G, B] vector with the RGB color values of the color to be
  considered a background to remove. All color values around an euclidean
  distance of ‘tolerance’ in 3D color space around the backgroundColor are
  considered background color. The default tolerance is 0.001 units, which
  for 8 bit colors means a perfect match – zero tolerance.

‘WeightedColorComponentSum’

shader = CreateSinglePassImageProcessingShader(windowPtr, ‘WeightedColorComponentSum’);

• This shader draws a texture by computing a weighted sum of the color values of all
  four color components (Red, Green, Blue, Alpha) for each input texel location (x,y),
  then outputs the resulting scalar value into all four components (Red, Green, Blue, Alpha)
  at location (x,y). The optional ‘modulateColor’ parameter of Screen(‘DrawTexture’, …);
  is used to define the weights used for the weighted sum:

For location (x,y) and color channels R,G,B,A, and Rin, Gin, Bin, Ain = input color values
from texture, and Rout, Bout, Gout, Aout the drawn output colors:

sum = Rin(x,y) * modulateColor(1) + Gin(x,y) * modulateColor(2) + Bin(x,y) * modulateColor(3) + Ain(x,y) * modulateColor(4);
Rout(x,y) = Gout(x,y) = Bout(x,y) = Aout(x,y) = sum;

One interesting application is computing linear combinations of up to four
“grayscale” images or “alpha masks” by storing the four different grayscale
or alpha images into the four color channels of a texture, then drawing that
texture, using the four components of ‘modulateColor’ as linear combination
weights to compute a “composite” output image which is mixed from the four
input layers, according to their weights. The optional ‘colorMaskNew’ parameter
of Screen(‘Blendfunction’) can be used to select which output color channels
should then be actually written to by the result of this linear combination, e.g.,
only RGB channels for a grayscale image, or only Alpha channel for a “morphed”
alpha mask image.
The moglmorpher(‘morphTexture’) function allows more general image morphing with
an arbitrary number of input image textures, but as long as no more than four
“monochromatic” images are required, this shader is especially efficient for this task.

History:
17.07.2011 mk Written.
24.11.2014 mk Add ‘WeightedColorComponentSum’ shader support.