All parameters, except for custom parameters, have some default interface that the host creates for them. Be it a numeric slider, colour swatch etc... Effects can override the default interface (or set an interface for a custom parameter) by setting the kOfxParamPropInteractV1. This will completely replace the parameters default user interface in the 'paged' and 'hierarchical' interfaces, but it will not replace the parameter's interface in any animation sheet.

Properties affecting custom interacts for parameters are...

The viewport for such interacts will be dependant upon the various properties above, and possibly a per host override in any XML resource file.

The GL_PROJECTION matrix will be an orthographic 2D view with -0.5,-0.5 at the bottom left and viewport width-0.5, viewport height-0.5 at the top right.

The GL_MODELVIEW matrix will be the identity matrix.

The bit depth will be double buffered 24 bit RGB.

A parameter's interact draw function will have full responsibility for drawing the interact, including clearing the background and swapping buffers.