set_blend

Description:

public bool set_blend (string blend_string) throws BlendStringError

If not already familiar; please refer here for an overview of what blend strings are, and their syntax.

Note:

Use cogl_pipeline_set_blend instead

Blending occurs after the alpha test function, and combines fragments with the framebuffer.

Currently the only blend function Cogl exposes is ADD. So any valid blend statements will be of the form:

  <channel-mask>=ADD(SRC_COLOR*(<factor>), DST_COLOR*(<factor>))

Warning:

The brackets around blend factors are currently not optional!

This is the list of source-names usable as blend factors:

  • SRC_COLOR: The color of the in comming fragment
  • DST_COLOR: The color of the framebuffer
  • CONSTANT: The constant set via set_blend_constant

The source names can be used according to the color-source and factor syntax, so for example "(1-SRC_COLOR[A])" would be a valid factor, as would "(CONSTANT[RGB])"

These can also be used as factors:

  • 0: (0, 0, 0, 0)
  • 1: (1, 1, 1, 1)
  • SRC_ALPHA_SATURATE_FACTOR: (f,f,f,1) where f = MIN(SRC_COLOR[A],1-DST_COLOR[A])

Note:

Remember; all color components are normalized to the range [0, 1] before computing the result of blending.

Blend Strings/1:

Blend a non-premultiplied source over a destination with premultiplied alpha:

"RGB = ADD(SRC_COLOR*(SRC_COLOR[A]), DST_COLOR*(1-SRC_COLOR[A]))"
"A   = ADD(SRC_COLOR, DST_COLOR*(1-SRC_COLOR[A]))"

Blend Strings/2:

Blend a premultiplied source over a destination with premultiplied alpha

"RGBA = ADD(SRC_COLOR, DST_COLOR*(1-SRC_COLOR[A]))"

The default blend string is: 

   RGBA = ADD (SRC_COLOR, DST_COLOR*(1-SRC_COLOR[A]))

That gives normal alpha-blending when the calculated color for the material is in premultiplied form.

Parameters:

this

A CoglMaterial object
blend_string

A Cogl blend string describing the desired blend function.

Returns:

true if the blend string was successfully parsed, and the described blending is supported by the underlying driver/hardware. If there was an error, false is returned and error is set accordingly (if present).