Table of Contents Glide Programming Guide
Modulation Multi-Pass Fog
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- Example Special Effects.6 Three-pass modulation fogging.
3Modulation Multi-Pass FogThe modulation case assumes that the results of each pass are being multiplied together, and we wish to fog the final result: cdst = Fog(Pci) where ci is the color entering the fog unit in pass i This case occurs most commonly when applying light maps to a scene, and is more complex to implement than the additive case. Here is the procedure for the three-pass case; it can be generalized by induction. We wish to obtain: cdst = Fog(c1c2c3) = fcfog + (1-f)( c1c2c3) For the first pass, choose either GR_FOG_WITH_TABLE or GR_FOG_WITH_ITERATED_ALPHA as the fog mode and OR in GR_FOG_ADD2, as shown in Table Special Effects.2 and demonstrated in Example Special Effects.6. Set the source and destination alpha blending factors to GR_BLEND_ONE and GR_BLEND_ZERO, respectively. After the first pass, cdst = 1·Fog(c1) + 0·cdst = Fog(c1) = (1-f)c1 For the second pass (and all intermediate passes in the general case), disable fogging (grFogMode(GR_FOG_DISABLE)) and set the source and destination alpha blending factors to GR_BLEND_DST_COLOR and GR_BLEND_ZERO, respectively. (Using source and destination factors of GR_BLEND_ZERO and GR_BLEND_SRC_COLOR, respectively, will work as well.) After the second pass we have: cdst = cdst·cin + 0·cdst = cdst·c2 = (1-f)c1c2 For the final pass, enable fogging again, choosing either GR_FOG_WITH_TABLE or GR_FOG_WITH_ITERATED_ALPHA , and OR in GR_FOG_MULT2, which causes the blended pixel term to be suppressed. Set the source and destination alpha blending factors to GR_BLEND_ONE and GR_BLEND_PREFOG_COLOR, respectively. The result is: Fog(c3) = fcfog cdst = 1·Fog(c3) + c3·cdst = fcfog + c3·(1-f)c1c2 = fcfog + (1-f)c1c2c3 Example Special Effects.6 Three-pass modulation fogging. The code segment below assumes that a fog table has been defined. const GrFog_t fog[GR_TABLE_SIZE]; int i; /* load the fog table */ grFogTable(fog); /* set a fog color - how about smoke? */ grFogColorValue(0); /* set fog mode and alpha blending function for pass 1*/ grFogMode(GR_FOG_WITH_TABLE | GR_FOG_ADD2); grAlphaBlendFunction(GR_BLEND_ONE, GR_BLEND_ZERO, GR_BLEND_ONE, GR_BLEND_ZERO); /* draw pass 1 */ … /* set fog mode and alpha blending function for pass 2*/ grFogMode(GR_FOG_DISABLE); grAlphaBlendFunction(GR_BLEND_DST_COLOR, GR_BLEND_ZERO, GR_BLEND_ONE, GR_BLEND_ZERO); /* draw pass 2 */ … /* set fog mode and alpha blending function for final pass */ grFogMode(GR_FOG_WITH_TABLE | GR_FOG_MULT2); grAlphaBlendFunction(GR_BLEND_ONE, GR_BLEND_PREFOG_COLOR, GR_BLEND_ONE, GR_BLEND_ZERO); /* draw pass 3 */ … Download 6.22 Mb. Share with your friends:
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