Attributes

Earlier in this chapter you saw how to set up and query individual states from OpenGL. Now lets take a look at how to save and restore the values of a set of related state variables with a single command.

An attribute group is a set of related state variables that OpenGL classifies into a group. For example, the line group consists of all the line drawing attributes, such as width, stipple pattern and anti-aliasing. The polygon group consists of the same set of attributes, just for polygons. You can save and restore these values by using the GL.PushAttrib and GL.PopAttrib functions. These are the signatures

void GL.PushAttrib(AttribMask);
void GL.PopAttrib();

PushAttrib saves all of the attributes for an attribute group specified by it's argument (an AttribMask enum). To be more specific, it pushes all the attributes onto a stack known as the attribute stack. PopAttrib restores the previous attributes by poping the top of the attribute stack.

The argument to PushAttrib is a bit-msak. The arguments you pass in can be combined with a bitwise OR ( | )The following AttribMask values are valid:

• AllAttribBits All opengl state variables in all attribute groups
• EnableBit Enable state variables
• FogBit Fog state variables
• LightingBit Lighting state variables
• LineBit Line state variables
• PointBit Point state variables
• PolygonBit Polygon state variables
• TextureBit Texturing state variables

It's common to set a default state for everything in your initialize function. Then, in your render function you push and pop attribs in order to set custom states and restore the defaults. For example, something like this

void RenderModel() {
    // Push bits as we might go into wireframe mode, or into untextured mode. 
    // We don't know for sure, it's configured with a variable.
    // Some models might be wireframe, others might not!
    GL.PushAttrib(AttribMask.PolygonBit | AttribMask.TextureBit);

    if (renderWireframe) {
        GL.PolygonMode(MaterialFace.FrontAndBack, PolygonMode.Line);
    }

    // This function might change texturing paramaters!
    DoTexturing();

    GL.Begin(PrimitiveType.Triangles);
    foreach(Traignle t in triangles) {
        GL.Vertex3(t.v1.x, t.v1.y, t.v1.z);
        GL.Vertex3(t.v2.x, t.v2.y, t.v2.z);
        GL.Vertex3(t.v3.x, t.v3.y, t.v3.z);
    }
    GL.End();

    // Restore polygon and texture render modes to default!
    GL.PopAttrib();
}