Tool implementation - drawing in screen space. Lifted from MODO's #coding channel on Slack.

toolPixelDraw.cpp

/*
You return LXfTMOD_DRAW_PIXEL from your ToolModel::Flags method.
Then you'll be called for drawing into screen space.
If you support multiple drawing modes; 3D and screen space,
you can test in your Draw callback by calling ILxView::Flags,
and testing the return type for LXiVIEWv_PIXEL.
You can query the StrokeDraw object passed to your draw method for an ILxView.
When you're in your drawing call, you can call ILxView::Dimensions to get the size of the viewport
to calculate the position to draw.
*/

// Untested code
	unsigned int
tmod_Flags(void)
{
	return (LXfTMOD_DRAW_3D|LXfTMOD_DRAW_PIXEL);
}
	void
tmod_Draw(
	ILxUnknownID vts,
	ILxUnknownID stroke,
	int flags)
{
	CLxUser_StrokeDraw strokeDraw(stroke);
	CLxUser_View view(stroke);
	static double sBlack[] = {0.0, 0.0, 0.0};
	if(LXiVIEWv_PIXEL == view.Type())
	{
		// Get the view dimensions.
		int dimensions[2];
		if(LXx_OK(view.Dimensions(&dimensions[0], &dimensions[1])))
		{
			// Map 0.5 x 0.5 to the dimensions.
			double pixelPositions[] = {0.5, 0.5}
			pixelPositions[0] = (pixelPositions[0] / ((double) dimensions[0]));
			pixelPositions[1] = (pixelPositions[1] / ((double) dimensions[1]));
			// Draw a dot at the centre of the screen.
			strokeDraw.Begin(LXiSTROKE_POINTS, sBlack, 1.0);
			strokeDraw.Vert(pixelPositions[0], pixelPositions[1], LXiSTROKE_ABSOLUTE);
		}
	}
	else
	{
		// Draw a dot at the centre of the world.
		strokeDraw.Begin(LXiSTROKE_POINTS, sBlack, 1.0);
		strokeDraw.Vert(0.0, 0.0, 0.0, LXiSTROKE_ABSOLUTE);
	}
}