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Lecture 12

Lecture 12. Blending, Anti-aliasing, Fog, Display Lists. Blending. Alpha value: It’s been ignored so far. We use alpha value for blending.

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Lecture 12

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  1. Lecture 12 Blending, Anti-aliasing, Fog, Display Lists

  2. Blending • Alpha value: It’s been ignored so far. • We use alpha value for blending. • Fragments: After the rasterization stage (including texturing and fog), the data are not yet pixels. At this stage, they are called fragments.(These are called source) • Pixels: Then each fragment undergoes a series of tests and operations after which, it is called a pixel.(These are called destination) • Blending: Specifies a blending function that combines color values from a source and a destination

  3. Blending Function • Blending Function: Specifies how source and destination are blended together. • Final result of combination for each pixel is calculated from the following formula: (Rs.Sr+Rd.Dr, Gs.Sg+Gd.Dg, Bs.Sb+Bd.Db, As.Sa+Ad.Da) • glBlendFunc(Glenumsfactor, Glenumdfactor)

  4. Blending Coefficients Table

  5. Samples of Blending • Draw two images blending together equally: • Source: GL_SRC_ALPHA • Destination: GL_ONE_MINUS_SRC_ALPHA • Draw the source with alpha=0.5 • Draw three images blending together equally: • Set the destination factor to GL_ONE • Set the source factor to GL_SRC_ALPHA • Draw each of the images with an alpha equal to 0.3333333 • Need more? Refer to the RedBook, Chapter 6, “Sample Uses of Blending”

  6. Anti-aliasing

  7. SolutionUse Coverage values and Alpha

  8. Enabling Anti-alias • glEnable(GL_POINT_SMOOTH) • glEnable(GL_LINE_SMOOTH) • You may want to optimize anti-aliasing: • Use glHint() along with the parameters. • Use blending when two lines cross each other • Use GL_SRC_ALPHA (source) and GL_ONE_MINUS_SRC_ALPHA (destination). • You can use GL_ONE for the destination factor to make lines a little brighter where they intersect

  9. Anti-aliasing Polygons

  10. Anti-aliasing Polygons

  11. Anti-aliasing Polygons • Use glEnable(GL_POLYGON_SMOOTH ): This will cause pixels on the edges of the polygon to be assigned fractional alpha values based on their coverage. • Turn off the depth buffer • set the blending factors to GL_SRC_ALPHA_SATURATE (source) and GL_ONE (destination) • Sort all the polygons in your scene so that they're ordered from front to back before drawing them

  12. Fog • Makes objects fade into the distance. • Fog is a general term that describes forms of atmospheric effects. • Fog can be used to simulate haze, mist, smoke, or pollution.

  13. Using Fog • glEnable(GL_FOG) • Use Fog equation • Use Fog color • glFog{if}(GLenumpname, TYPE param)

  14. Using Fog

  15. Display Lists • Advantages of using Display Lists: • Store OpenGL commands for later execution • Using display lists, you can define the geometry and/or state changes once and execute them multiple times. • Saves lots of resources when an OpenGL program is running over a network.(Saves lots of back and forth communications)

  16. Using Display Lists • Use glGenLists(size) to create names. • Use glNewList(name, GL_COMPILE/GL_EXECUTE/GL_COMPILE_ANDEXECUTE), glEndList() to define a list • Use glCallList(list_name) to recall a list.

  17. Limitations • Only the values for expressions are stored in the list Not the expressions. • Example: GLfloatcolor_vector[3] = {0.0, 0.0, 0.0}; glNewList(1, GL_COMPILE); glColor3fv(color_vector); glEndList(); color_vector[0] = 1.0; • The color is not changed in the list after the last line is executed.

  18. Limitations • The following commands are not stored in a display list: • Commands that set client state. • Commands that retrieve state values aren't stored in a display list.

  19. Hierarchical Display Lists • A hierarchical display list is allowed in OpenGL, which is a display list that executes another display list. • Example: glNewList(listIndex,GL_COMPILE); glCallList(handlebars); glCallList(frame); glTranslatef(1.0,0.0,0.0); glCallList(wheel); glTranslatef(3.0,0.0,0.0); glCallList(wheel); glEndList();

  20. END

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