CS 354 Computer Graphics Lecture 2: Viewing January 19, 2012 http://www.cs.utexas.edu/~mjk/teaching/cs354_s12/

1. CS 354 Viewing Stuff Mark Kilgard University of Texas January 19, 2012

10. A Simplified Graphics Pipeline Application Vertex batching & assembly Triangle assembly Triangle clipping Triangle rasterization Fragment shading Depth testing Color update Application- OpenGL API boundary Framebuffer NDC to window space NDC = Normalized Device Coordinates, this is a [-1,+1] 3 cube Really lots more steps than this but these are the non-trivial operations in our simple triangle example Depth buffer

11. Full OpenGL state machine Lots more operations are supported Just most can be disable or made trivial (pass through modes)

15. Pieces of the display Callback glShadeModel ( GL_SMOOTH ); // smooth color interpolation glEnable ( GL_DEPTH_TEST ); // enable hidden surface removal glClear (GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT); glBegin (GL_TRIANGLES); { // every 3 vertexes makes a triangle glColor4ub (255, 0, 0, 255); // RGBA=(1,0,0,100%) glVertex3f (-0.8, 0.8, 0.3); // XYZ=(-8/10,8/10,3/10) glColor4ub (0, 255, 0, 255); // RGBA=(0,1,0,100%) glVertex3f ( 0.8, 0.8, -0.2); // XYZ=(8/10,8/10,-2/10) glColor4ub (0, 0, 255, 255); // RGBA=(0,0,1,100%) glVertex3f ( 0.0, -0.8, -0.2); // XYZ=(0,-8/10,-2/10) } glEnd (); Graphics state setting Framebuffer buffer clearing Triangle rendering

23. Assembling a Vertex glColor4f glColor3f glColor4ub, etc. glTexCoord2f glTexCoord3s glTexCoord4i, etc. glNormal3f glNormal3s glNormal3b, etc. the “latch” behavior glVertex2s glVertex3f glVertex4d assemble a vertex with all its attributes to triangle assembly glVertex* commands trigger a latch that assembles a complete vertex R G B A S T R Q Nx Ny Nz X Y Z W Nx Ny Nz S T R Q R G B A X Y Z W

27. glBegin Primitive Batch Types

29. Other Begin Mode State Machines: GL_TRIANGLE_STRIP initial no vertex one vertex two vertexes Begin(TRIANGLE_ STRIP) Vertex Vertex Vertex / Emit Triangle End End End two vertexes Vertex / Emit Reverse Triangle End

30. Other Begin Mode State Machines: GL_POINTS and GL_LINES initial no vertex one vertex Begin(LINES) Vertex / Emit Line End End initial no vertex Begin(POINTS) Vertex / Emit Point End Actual hardware state machine handles all OpenGL begin modes, so rather complex

36. Clipped Triangle Visualized Clipped and Rasterized Normally Visualization of NDC space Notice triangle is “poking out” of the cube; this is the reason that should be clipped

37. Break Clipped Triangle into Two Triangles But how do we find these “new” vertices? The edge clipping the triangle is the line at X = -1 so we know X = -1 at these points—but what about Y?

38. Use Ratios to Interpolate Clipped Positions (-1.8, 0.8, 0.3) (-0.8, 0.8, -0.2) (0, -0.8, -0.2) origin at (0,0,0) X = -1 Y = (1.8/2.6)×0.8 + (0.8/2.6)×0.8 = 0.8 Z = (1.8/2.6)×0.3 + (0.8/2.6)×-0.2 = 0.1461538 -1-(-1.8)=0.8 0.8-(-1)=1.8 0.8-(-1.8)=2.6 (-1,0.8,0.146153) Straightforward because all the edges are orthogonal Weights: 1.8/2.6 0.8/2.6, sum to 1

39. Use Ratios to Interpolate Clipped Positions (-1.8, 0.8, 0.3) (-0.8, 0.8, -0.2) (0, -0.8, -0.2) origin at (0,0,0) 0-(-1.8) = 1.8 0-(-1) = 1 X = -1 Y = (1/1.8)×0.8 + (0.8/1.8)×-0.8 = 0.08888… Z = (1/1.8)×0.3 + (0.8/1.8)×-0.2 = 0.07777… (-1,0.0888,0.0777) -1-(-1.8) = 0.8 Weights: 1/1.8 0.8/1.8, sum to 1

43. Triangle clipped by Two Planes Visualization Recursive process can make 4 triangles And it gets worse with more non-trivial clipping

54. Today’s GPU Graphics Pipeline is More Complex and Programmable Geometry Program 3D Application or Game OpenGL API GPU Front End Vertex Assembly Vertex Shader Clipping, Setup, and Rasterization Fragment Shader Texture Fetch Raster Operations Framebuffer Access Memory Interface CPU – GPU Boundary OpenGL 3.3 Attribute Fetch Primitive Assembly Parameter Buffer Read programmable fixed-function Legend For future lectures…