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Norman Barrows

fixed function blendops for snow tex on ground tex?

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anyone remember fixed function blend ops?


anyone any good with them?


in the past i resorted to writing my own 10 channel weighted texture blender because i couldn't seem to get the desired result (4 way weighted blending) with blendops. 


i'm now faced with the challenge of drawing snow on the ground in Caveman (openworld rpg/person sim - fps interface).


in the previous version, i used a pre-blended 50-50 snow texture for each regular ground texture, and a single 100% snow texture.  


but this time around, there are more types of terrain, and each type now has 4 seamless tiled textures (for the moment, may add more) instead of just one base texture that got mirrored in x and z to reduce moire`.  so lots more textures to make a "half snow" version of.  


obviously, variable alpha blending of a 100% snow texture would be ideal, but probably too much of a performance hit. basically drawing two complete ground meshes, the regular one, and a variably translucent snow one just above it.


on the other end of the scale, pre-blended textures would be fastest,but would be limited to the % blends done ahead of time, and memory could become a factor if a large number of textures was required. 


i need a low overhead way to do this.  so i was thinikng maybe a two stage blendop might not be so bad. and i'd only need one texture instead of pre-blending dozens.


any blendops come to mind?    are they fast? i really can't afford anything like a true multipass algo.

Edited by Norman Barrows

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default blendops are as follows:


stage 0:

colorop: modulate        arg1   texture               arg2: diffuse 

alphaop: select 1         arg1   texture/diffuse    arg2:  current        (diffuse is used if no texture is set)

uses uv coordinate set 0


stages 1 through 7:

colorop: disable           arg1:  texture               arg2: current

alphaop: disable          arg1: texture/diffuse     arg2: current        (diffuse is used if no texture is set)

stages 1-7 use uv coordinate sets 1-7 respectively.


the blendop desired is blend factor alpha, which blends between two textures based on the value of a d3d color (the alpha channel most likely) passed to the system.



1. stage 1 uses uv set 1. unless you have two sets of uv's in your vertex data, you need to set stage 1 to use uv set 0. otherwise d3d uses 0.0f for all uv coords for stage 1.

this results in a blend to the color of the UL pixel of the second texture.


2. setting the texture for stage 1 increases the reference count on the texture used. you must set the texture in stage 1 to NULL when done, to decrement the reference count again.






here's the code to turn on and off snow blending:


void turnon_snowblending()
int i;
float f;
D3DCOLOR color;
f=snow_accumulation;        // calc blend factor.....
if (f > 255.0f) f=255.0f;
Zd3d_device_ptr->SetRenderState(D3DRS_TEXTUREFACTOR,color);          // set blend factor.....
Zd3d_device_ptr->SetTexture(1,Ztex[341].tex);        // TEX 341          // set the snow texture.....
Zd3d_device_ptr->SetTextureStageState(1,D3DTSS_COLOROP,D3DTOP_BLENDFACTORALPHA);     // tell it to blend
//Zd3d_device_ptr->SetTextureStageState(1,D3DTSS_COLORARG1,D3DTA_TEXTURE);                        // these commented out lines aren't required
///Zd3d_device_ptr->SetTextureStageState(1,D3DTSS_COLORARG2,D3DTA_CURRENT);                      // the arg values are the same as the default current sttings
//Zd3d_device_ptr->SetTextureStageState(1,D3DTSS_ALPHAOP,D3DTOP_BLENDFACTORALPHA);   // this apparently doesn't need to be turned on.
Zd3d_device_ptr->SetTextureStageState(1,D3DTSS_TEXCOORDINDEX,0);              // use uv set 0, there is no uv set 1, so it uses 0.0 for all uv's!
void turnoff_snowblending()
//D3DCOLOR color;                                                  
 // again, commented out lines are not required. you know how d3d is - sometimes you have to try a few things until you get the right combo.
// texturefactor doesn't need to be reset, because snow blending is the only place its used.
Zd3d_device_ptr->SetTexture(1,NULL);            // set tex to null to decrement reference to snow tex
Zd3d_device_ptr->SetTextureStageState(1,D3DTSS_COLOROP,D3DTOP_DISABLE);     // turn off blending

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