Adding a "stamp" as a texture layer

[Benjamin Drucker]

I am making a 3d drawing program where I want to be able to "stamp" a small image on to a geometry.  I don't know how to do this yet but I have outlined the problem and I have solved a few sub-problems.  I could use some help filling in the holes.  As I see it, the steps are:

- [done] use ray-trace picking to determine the object and the world position

- [done] add a new layer to that object's texture [done]

- [done] neither "multiply" nor "add" work here as a blend method. I need the alpha of the "stamp" to control how it's blended in with the base color and previous image texture layers.  Locally I have added a new blend method to scenejs.js, "alpha".  My patch is probably too simplistic but it works AFAIK.  (Patch at end of post).

- current wrapS/T methods don't work for me either.  I want a single copy of the stamp image.  I am looking in to this now.

- how to convert picked object and world position to a texture layer offset?  No idea.

Any help with the last two points and/or suggestions on how to achieve my custom blend mode with a custom shader would be appreciated.  (Or, if an "alpha" blend mode makes general sense, maybe this should be added to scenejs?  I don't care what you call it)

--------------------------------

"Alpha" blend method patch:

index 0210c21..baf0b77 100644

--- a/lib/scenejs.js

+++ b/lib/scenejs.js

@@ -9575,6 +9575,10 @@ var SceneJS_DrawList = new (function() {

                 if (layer.applyTo == "baseColor") {

                     if (layer.blendMode == "multiply") {

                         src.push("color = color * (SCENEJS_uLayer" + i + "BlendFactor * texture2D(SCENEJS_uSampler" + i + ", vec

+                    } else if (layer.blendMode == "alpha") {

+                        src.push("vec3 tColor = texture2D(SCENEJS_uSampler" + i + ", vec2(textureCoord.x, 1.0 - textureCoord.y))

+                        src.push("float tAlpha = texture2D(SCENEJS_uSampler" + i + ", vec2(textureCoord.x, 1.0 - textureCoord.y)

+                        src.push("color = ((1.0 - tAlpha) * color) + (tAlpha * tColor);");

                     } else {

                         src.push("color = ((1.0 - SCENEJS_uLayer" + i + "BlendFactor) * color) + (SCENEJS_uLayer" + i + "BlendFa

                     }

@@ -22837,7 +22841,7 @@ var SceneJS_textureModule = new (function() {

                     }

                 }

                 if (layerParam.blendMode) {

-                    if (layerParam.blendMode != "add" && layerParam.blendMode != "multiply") {

+                    if (layerParam.blendMode != "add" && layerParam.blendMode != "multiply" && layerParam.blendMode != "alpha") 

                         throw SceneJS_errorModule.fatalError(

                                 SceneJS.errors.NODE_CONFIG_EXPECTED,

                                 "texture layer " + i + " blendMode value is unsupported - " +

[Benjamin Drucker]

Upon further research, I think this concept is called "decal" in OpenGL terminology.  So this is not a new problem.