Multi-Texture.Blending

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    http://www.codesampler.com/oglsrc/oglsrc_4.htm
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This technique is very popular in terrain rendering engines which use it to blend dramatically different textures such as rock and grass together with out creating a noticeable edge. For example, with three textures consisting of stone, grass, and sand you can render a mountain that blends in patches of grass and sand at its base.

Relevant Keywords: GL_ARB_multitexture, glActiveTextureARB, glMultiTexCoord2fARB, glClientActiveTextureARB, glTexEnvi, glVertex3f, wglGetProcAddress, glGetString, auxDIBImageLoad, glGenTextures, glBindTexture, glTexParameteri, glTexImage2D, glEnable, glDisable, glBindTexture, AUX_RGBImageRec, GL_INTERPOLATE_ARB, GL_SRC_COLOR, GL_SRC_ALPHA, GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE, GL_ADD, GL_TEXTURE0_ARB, GL_TEXTURE1_ARB, GL_TEXTURE2_ARB, GL_EXTENSIONS, GL_BLEND, GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_TEXTURE_MAG_FILTER, GL_LINEAR, GL_UNSIGNED_BYTE, PFNGLACTIVETEXTUREARBPROC, PFNGLMULTITEXCOORD2FARBPROC, and PFNGLCLIENTACTIVETEXTUREARBPROC.