EXT_stencil_two_side on ATI

OpenGL OpenGL: Advanced Coding
1

Hi there

Just tried my engine on an ATI card (Radeon 4xxx), and it seems not to support EXT_stencil_two_side.

Can anyone tell me, if there is some other extension on ATI, that provides the same functionality?

The driver supports OpenGL 3.3 (though i’m using a 2.1 context atm), and i was under the impression that stencil_two_side should have been made core at some point, so i am a bit surprised that the EXT-extension is not supported.

Thanks,
Jan.

2

I think it became part of core OpenGL in version 2.0, but in a different form; with the extension it is:


        glEnable(GL_STENCIL_TEST_TWO_SIDE_EXT);

        glActiveStencilFaceEXT(GL_BACK);
        glStencilOp(GL_KEEP,            // stencil test fail
                    GL_KEEP,            // depth test fail
                    GL_DECR_WRAP_EXT);  // depth test pass
        glStencilMask(~0);
        glStencilFunc(GL_ALWAYS, 0, ~0);

but in the core 2.0+:


        glStencilOpSeparate(GL_BACK, GL_KEEP,            // stencil test fail
                    GL_KEEP,            // depth test fail
                    GL_DECR_WRAP_EXT);  // depth test pass
        glStencilMaskSeparate(GL_BACK, ~0);
        glStencilFuncSeparate(GL_BACK, GL_ALWAYS, 0, ~0);

In http://www.opengl.org/registry/api/gl.spec it mentions a ARB_stencil_two_side extension, but I don’t think this extension ever got added to the registry as a proper extension.

3

Actually it’s the ATI extension that have been promoted to core:
http://www.opengl.org/registry/specs/ATI/separate_stencil.txt
Just use the core feature, it works everywhere.

4

It’s slightly different to that extension also, because that extension doesn’t include glStencilMaskSeparate equivalent, and StencilFuncSeparateATI operates differently:


StencilFuncSeparateATI(GLenum frontfunc, GLenum backfunc, GLint ref, GLuint mask);

vs


StencilFuncSeparate( GLenum face, GLenum func, GLint ref, GLuint mask );

In the ATI version, it’s not possible to set ref + mask for each face separately.

The OpenGL registry http://www.opengl.org/registry/api/gl.spec lists StencilFuncSeparateATI as an alias for StencilFuncSeparate, but it’s not safe to use as one, although it’s easily done by mistake as they both have the same parameter layout.

5

Great, thanks for the detailed information!