Transparency & Translucency in OpenGL

Last Updated March 3, 2010

Screendoor Transparency in OpenGL

In OpenGL, the term stiplling refers to the process of drawing a polygon through a mask. The mask are 32x32 bits. See this page for how to create complex masks.

GLubyte stipple[128]; // create a 32x32 (1024) array of bits (128 bytes= 1024 bits)
	for(int i=0;i<128;){ // populate array with stipple pattern
		stipple[i++]=85;stipple[i++]=85;stipple[i++]=85;stipple[i++]=85;// evenrow, 85=01010101
		stipple[i++]=170;stipple[i++]=170;stipple[i++]=170;stipple[i++]=170;// odd row,170=10101010

	}

Switch on/off polygon stippling using glEnable(GL_POLYGON_STIPPLE)/glDisable(GL_POLYGON_STIPPLE). Select a stipple pattern with glPolygonStipple(stipple).

Alpha Mask in OpenGL

In OpenGL, the alpha mask is controlled by the functions glEnable(GL_ALPHATEST) & glAlphaFunc(). glAlphaFunc() is used to set the test for drawing or discarding a pixel.

Alpha Blending in OpenGL

OpenGL functions related to translucency;

glEnable(GL_BLEND), turn on blending
glBlendFunc(), specify the blending function to use. For best results use; glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDepthMask(), turn on/off depth buffer writing
For rendering unLit objects, set the alpha value of a vertex using glColour4f(red, green,blue, alpha)
For rendering lit objects set the alpha value with glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE, rgba);

Chapter 6 of the OpenGL programming guide discusses blending in more detail.

OpenGL.Org FAQs on blending