GLART week 9: Shadows


Examples:
        GLART_9_Shadow.java -- Use a custom matrix to make shadows.
        week4/GLART_4_matrix_example.java -- Demonstrate how a matrix can project geometry onto a plane

        also needed:
        spotlight_200.jpg  -- spotlight image
        Flag_of_the_United_States.png  -- flag image

Resources:
        Wikipedia Shadow Mapping Page


NOTES:
=================================================================

A simple shadow technique
	Project a 3D shape onto a flat surface
	things we need:
		a light position
		an object
		a plane
		a matrix to draw the object projected onto the plane


How to define a plane
	We need to define what plane the shadow will be cast on
	The Equation for a plane is four numbers:
		the normal of the plane (XYZ) and the distance to the orgin (at the closest point to the plane)
	normal of plane:
		an XYZ vector perpendicular to the plane
		pointing towards origin
	the length of the normal
		distance of the plane from the origin at it's closest point

	Example:
		A floor plane faces up the Y axis and is at the origin:
			0,1,0,0

		A wall that is five units to the right of the origin:
			-1,0,0,5
			the plane faces towards negative X, so it's facing left

		A wall that is five units to the left of the origin:
			1,0,0,5
			the plane faces towards positive X


How to define the shadow matrix
	make a matrix that projects the geometry onto a plane
	as if viewed from the light position
	the fourth column in matrix is the xyz direction of the plane

	see: glart_4_matrix_example.java
	change a number in the fourth column (try changing the first number to .4f)
	this column defines how the scene is projected
	change these numbers to project the scene onto a plane

	with a little math it's possible to create a matrix that
	renders geometry as if it is squashed against a plane (projected
	onto the plane) from the lights point of view.
	see: GLART_11_Shadow.java - makeShadowMatrix()


Make a shadow
	see: GLART_11_Shadow.java

	Make a matrix that projects the model onto the floor plane from the light
	position (makeShadowMatrix())

	In drawShadow() multiply the Modelview matrix by the shadow matrix, then
	draw the model. The matrix makes the model appear to be squashed against
	the floor plane.

	The "shadow" is actually the transformed model.  In drawShadow() if you comment
	out the glDisable() commands you can see that the shadow is actually the model
	rendered with texturing and lighting.  We disable lighting and texture to make
	the shadow look completely flat.  We disable depth test to insure
	that the shadow draws on top of the floor itself (in this case we're not drawing
	a floor, but in other cases where a floor exists it is more important).


Make shadow transparent
	turn on transparency with glcolor(r,g,b,.5f)
	but now geometry overlaps itself and looks wrong
	have to limit drawing within the shadow shape
	use stencil to prevent drawing twice in the shadow area
	see DEMO demoCamera:


DEMO demoCamera
	JOShadowOnPlane.java in jocode folder
	wrap functionality to draw a shadow
	note in drawShadow() the stencilOp() function increments the stencil value:

			gl.glStencilOp(GL.GL_KEEP, GL.GL_KEEP, GL.GL_INCR);

		and tests for stencil == 1 only
		thus eliminating stencil values 2,3,4 etc.
		which eliminates overlapping rendering.