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

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

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(Glenum sfactor, Glenum dfactor)

Blending Coefficients Table

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 Need more? Refer to the RedBook, Chapter 6, “Sample Uses of Blending”

Anti-aliasing

Solution Use Coverage values and Alpha

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

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

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.

Using Fog glEnable(GL_FOG) Use Fog equation Use Fog color glFog{if}(GLenum pname, TYPE param)

Using Fog

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)

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

Limitations Only the values for expressions are stored in the list Not the expressions. Example: GLfloat color_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.

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.

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();

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