2. OpenGL Special Effects Jian-Liang Lin 2002

3. Blending: Alpha Channel Alpha value can be specify by glColor* When blending is enabled, the alpha value is often used to combine the color value of the fragment being processed with that of the pixel already stored in the framebuffer Note: –Alpha values aren't specified in color-index mode, so blending operations aren't performed in color- index mode.

4. Blending: The Source and Destination Factors -1 let the source and destination blending factors be (Sr, Sg, Sb, Sa) and (Dr, Dg, Db, Da), respectively, and the RGBA values of the source and destination be indicated with a subscript of s or d. Then the final, blended RGBA values are given by –(RsSr+RdDr, GsSg+GdDg, BsSb+BdDb, AsSa+AdDa) –Each component of this quadruplet is eventually clamped to [0,1]. –You can use glBlendFunc() to specify these two factors –glEnable( GL_BLEND );

5. Blending: The Source and Destination Factors -2 void glBlendFunc(GLenum sfactor, GLenum dfactor);

6. Blending: Example glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA ); –( R S, G S, B S )*A S + ( R D, G D, B D )*( 1- A S ) Billboarding REDBOOK Sample: –Alpha.cppAlpha.cpp GLUT Advanced97 Sample: –alphablend.cpp, alphablendnosort.cppalphablend.cppalphablendnosort.cpp

7. Antialiasing -1 Antialiasing point –glEnable( GL_POINT_SMOOTH ); Antialiasing line –glEnable( GL_LINE_SMOOTH ); Antialiasing polygon –glEnable( GL_POLYGON_SMOOTH ); –Ref. OpenGL Programming Guide, Chapter 6

8. Antialiasing -2 Using glHint(…); – void glHint (GLenum target, GLenum hint); –Controls certain aspects of OpenGL behavior –hint can be GL_FASTEST / GL_NICEST / GL_DONT_CARE

9. Antialiasing -3 And you must enable alpha-blending –glEnable (GL_BLEND); –glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); REDBOOK Sample with my modification: –aargb2.cppaargb2.cpp

10. Fog: Introduction Fog –Makes objects fade into the distance –A general term that describes similar forms of atmospheric effects –It can be used to simulate haze, mist, smoke, or pollution

11. Fog: Using Fog glFog{I,f}[v]( GLenum pname, TYPE param ); –If pname is GL_FOG_MODE, param can be GL_EXP(default), GL_EXP2, GL_LINEAR –pname also can be GL_FOG_DENSITY, GL_FOG_START, GL_FOG_END, GL_FOG_COLOR, GL_FOG_INDEX

12. Fog: Example Ex: glEnable(GL_FOG); GLfloat fogColor[4] = {0.5, 0.5, 0.5, 1.0}; glFogi(GL_FOG_MODE, GL_EXP); glFogfv(GL_FOG_COLOR, fogColor); glFogf(GL_FOG_DENSITY, 0.35); glClearColor(0.5, 0.5, 0.5, 1.0); REDBOOK Sample: –Fog.cppFog.cpp

13. Accumulation Buffer -1 The accumulation buffer can be used for such things as: – scene antialiasing, –motion blur, –simulating photographic depth of field, –calculating the soft shadows that result from multiple light sources.

14. Accumulation Buffer -2 OpenGL graphics operations don't write directly into the accumulation buffer. Typically, a series of images is generated in one of the standard color buffers, and these are accumulated, one at a time, into the accumulation buffer. When the accumulation is finished, the result is copied back into a color buffer for viewing

15. Accumulation Buffer -3 void glAccum (GLenum op, GLfloat value); –Op can be GL_ACCUM, GL_LOAD, GL_RETURN, GL_ADD, GL_MULT – GL_ACCUM reads each pixel from the buffer currently selected for reading with glReadBuffer(), multiplies the R, G, B, and alpha values by value, and add s the result to the accumulation buffer – GL_LOAD does the same thing, except that the values replace those in the accumulation buffer rather than being added to them.

16. Accumulation Buffer -4 – GL_RETURN takes values from the accumulation buffer, multiplies them by value, and places the result in the color buffer(s) enabled for writing. – GL_ADD and GL_MULT simply add or multiply the value of each pixel in the accumulation buffer by value and then return it to the accumulation buffer. For GL_MULT, value is clamped to be in the range [-1.0,1.0]. For GL_ADD, no clamping occurs. glClearAccum( … ); glClear( GL_ACCUM_BUFFER_BIT );

17. Accumulation Buffer -5 Scene antialiasing –To perform scene antialiasing, –first clear the accumulation buffer and enable the front buffer for reading and writing. –Then loop several times (say, n ) through code that jitters and draws the image (jittering is moving the image to a slightly different position), accumulating the data with glAccum(GL_ACCUM, 1.0/ n ); –and finally calling glAccum(GL_RETURN, 1.0); – GLUT Advanced97 sample: Accumaa.cpp

18. Accumulation Buffer -6 Motion Blur –Suppose your scene has some stationary and some moving objects in it –Set up the accumulation buffer in the same way, but instead of spatially jittering the images, jitter them temporally. –The entire scene can be made successively dimmer by calling glAccum (GL_MULT, decayFactor ); –as the scene is drawn into the accumulation buffer, where decayFactor is a number from 0.0 to 1.0. –Smaller numbers for decayFactor cause the object to appear to be moving faster.

19. Accumulation Buffer -7 –You can transfer the completed scene with the object's current position and "vapor trail" of previous positions from the accumulation buffer to the standard color buffer with glAccum (GL_RETURN, 1.0); –GLUT Advanced Sample: Motionblur.cpp

20. Accumulation Buffer -8 Depth Of Field –REDBOOK Sample: Dof.cpp Other applications – ref. OpenGL Programming Guide, Chapter 10

21. Stencil Buffer -1 One use for the stencil buffer is to restrict drawing to certain portions of the screen, just as a cardboard stencil can be used with a can of spray paint to make fairly precise painted images. The stencil buffer prevents anything that wouldn't be visible through the windshield from being drawn. Thus, if your application is a driving simulation, you can draw all the instruments and other items inside the automobile once, and as the car moves, only the outside scene need be updated.

22. Stencil Buffer -2 void glStencilFunc (GLenum func, GLint ref, GLuint mask); – func The test function. The following eight tokens are valid: GL_NEVER –Always fails. GL_LESS –Passes if ( ref & mask ) < ( stencil & mask ). GL_LEQUAL –Passes if ( ref & mask ) ≤ ( stencil & mask ). GL_GREATER –Passes if ( ref & mask ) > ( stencil & mask ). GL_GEQUAL –Passes if ( ref & mask ) ≥ ( stencil & mask ).

23. Stencil Buffer -3 GL_EQUAL –Passes if ( ref & mask ) = ( stencil & mask ). GL_NOTEQUAL –Passes if ( ref & mask ) ¹ ( stencil & mask ). GL_ALWAYS –Always passes. – ref The reference value for the stencil test. The ref parameter is clamped to the range [0,2 n – 1], where n is the number of bitplanes in the stencil buffer. – mask A mask that is AND ed with both the reference value and the stored stencil value when the test is done.

24. Stencil Buffer -4 void glStencilOp (GLenum fail, GLenum zfail, GLenum zpass); – fail The action to take when the stencil test fails. The following six symbolic constants are accepted: GL_KEEP –Keeps the current value. GL_ZERO –Sets the stencil buffer value to zero. GL_REPLACE –Sets the stencil buffer value to ref, as specified by glStencilFunc. GL_INCR –Increments the current stencil buffer value. Clamps to the maximum representable unsigned value.

25. Stencil Buffer -5 GL_DECR –Decrements the current stencil buffer value. Clamps to zero. GL_INVERT –Bitwise inverts the current stencil buffer value. – zfail Stencil action when the stencil test passes, but the depth test fails. Accepts the same symbolic constants as fail. – zpass Stencil action when both the stencil test and the depth test pass, or when the stencil test passes and either there is no depth buffer or depth testing is not enabled. Accepts the same symbolic constants as fail.

26. Stencil Buffer -6 Some sample: –REDBOOK Sample: Stencil.cpp –GLUT Advanced Sample: Dissolve.cpp Shadowvol.cpp –NVidia Sample: Nvshadow.cpp

27. Any Question? ?