1. Computer Graphics Practical Lesson 9
Stencil buffer and frame buffer

2. Stencil buffer
Stencil buffer is similar to the depth buffer, and stored on the same memory ( 24 bits for depth, and 8 for stencil).
Allow prevention of drawing of pixels of objects.
With the depth buffer, it allows more complex rendering such as shadowing.

3. Setup stencil buffer Clear the buffer : glClearStencil(0);
Like anything in OpenGL, we need to enable stencil glEnable(GL_STENCIL_TEST);
Setup the stencil function, which defines how to perform test between existing value(stencil) and new value on the buffer(ref) glStencilFunc(func, ref=1,mask=1); funcion that used for comparison. ref is value to be compared with, and mask is the mask that used to compare.

4. Setup stencil buffer(cont’)
glStencilOp(fail,zfail,zpass) – defines the what to do with stencil buffer on each case: fail – when stencil test fails zfail – when stencil fails but stencil pass. zpass – when both tests fails.

5. The reflected airplane
A reflected airplane is drawn in the stencil example.
The reflected plane is drawn first, before the floor and the regular airplane.
It Is drawn using clipping – from the button by frustom clipping and on the top using glClipping

6. The reflected airplane(cont’)
In the stencil example the reflected airplane appears only until the height of the floor.
It is done using plain clipping. In openGL: double clipping_plain[]={X,Y,Z,D} – a plain represented by vector X,Y,Z and offset D glEnable(GL_CLIP_PLANE0); glClipPlane(GL_CLIP_PLANE0,clipping_plain);
In OpenGL there are up to 6 plains.
The plains are added to the plains of frustum(see practical lesson3).

7. The Frame buffer
To read from frame buffer: glReadPixels(x,y,width,height,format,type,buffer);
Format = GL_RGBA for example
Type of values on source array = GL_UNSIGNED_INT

8. Frame buffer(cont’)
glRasterPos2i(int,int) – define position for writing
glDrawPixels(width,height,GL_RGBA, GL_UNSIGNED_INT,buffer) – writing buffer.