glsl = MakeTextureDrawShader(windowPtr, shadertype [, backgroundColorOffset = [0,0,0,0]]);

Create a special GLSL shader that can apply some special image processing
operations that modify the normal drawing of textures via
Screen(‘DrawTexture(s)’). Return a handle ‘glsl’ to the shader.

The created shader can be attached to specific textures via passing the
returned ‘glsl’ argument as the optional ‘textureShader’ argument for the
Screen(‘MakeTexture’, ….., textureShader); function. It can be also
applied on a per-draw basis by passing the ‘glsl’ handle as optional
‘textureShader’ argument to the Screen(‘DrawTexture(s)’, …); function.

Mandatory arguments:

‘windowPtr’ A handle to the onscreen window to which the shader should be

‘shadertype’ A name string that defines the type of shader / image
processing operation to apply. It can be one of the following:

  • ‘SeparateAlphaChannel’: Alpha values are looked up at the regular
    locations defined via the ‘srcRect’ parameter in a Screen(‘DrawTexture’)
    call. RGB color values are looked up at srcRect, offset by some (dx,dy)
    offset, and (dx2,dy2) offset, as provided by the ‘auxParameters’ vector
    [dx, dy, dx2, dy2].
    Providing non-zero offset values for dx and dy allows to “shift” or
    “scroll” the RGB or Luminance image inside a texture during drawing,
    while keeping the alpha image at a fixed location, solely defined by
    ‘srcRect’. This is mostly useful if you want to draw some drifting
    stimulus with a fixed alpha channel mask applied, ie., the color image
    should move, but the alpha mask should stay fixed. The (dx2,dy2) offsets
    allow to apply some offset before applying rotations, again only to the
    color channels, not the alpha channel.

  • ‘PremultipliedAlphaChannel’: Like ‘SeparateAlphaChannel’, but the alpha
    value is not written to the framebuffer, but premultiplied to the RGB
    color pixel before writeout.

‘backgroundColorOffset’ Optional, defaults to [0 0 0 0]. A RGBA offset
color to add to the final RGBA colors of the drawn texture, prior to
drawing it.

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