Planar mapping OpenGL shader

[Auradrummer]

Hello guys,

I made my first "from scratch" shader, and worked very well. But, as I'm new in shader programming, I would be pleased if you, masters, could evaluate it for me.

This shader can project a image over another, blending them, where the first image is a mapped one and the second is a decal, with shader generated planar mapping.

I considered that how many calculus are made out of shaders, better. And I don't know how to pass and retrieve 'user data' in the callback. So I made two global variables.

This is the code:

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//callback:
virtual void OnSetConstants(video::IMaterialRendererServices* services, s32 userData)
	{

        //objDim and extents are globals position2df

        int texture = 0;
        services->setPixelShaderConstant("tex0",(float*)(&texture),1);

        int texture2 = 1;
        services->setPixelShaderConstant("tex1",(float*)(&texture2),1);

        services->setPixelShaderConstant("extents", reinterpret_cast<irr::f32*>(&extents),2);
        services->setPixelShaderConstant("objDim", reinterpret_cast<irr::f32*>(&objDim),2);
	}

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//globals initialization (the material creation is the same as the tutorial, so, I omitted here

//the region that will receive the decal is the meshBuffer(2) of the object.

    aabbox3df bbox = object->getMesh()->getMeshBuffer(2)->getBoundingBox();
    objDim.Width =  bbox.MaxEdge.X - bbox.MinEdge.X;
    objDim.Height = bbox.MaxEdge.Z - bbox.MinEdge.Z;
    extents.Width = bbox.MaxEdge.X;
    extents.Height = bbox.MaxEdge.Z;

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//vertex shader
uniform vec2 objDim;
uniform vec2 extents;

void main()
{
	gl_TexCoord[0] = gl_MultiTexCoord0;
	gl_TexCoord[1].s = (extents.x - gl_Vertex.x) / objDim.x;
	gl_TexCoord[1].t = (gl_Vertex.z - extents.y) / objDim.y;
	gl_Position = ftransform();
}

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//fragment  shader
sampler2D tex0 : register(s0);
sampler2D tex1 : register(s1);

void main()
{
	vec4 ColorHull = texture2D(tex0,gl_TexCoord[0].st);
	vec4 ColorDecal = texture2D(tex1,gl_TexCoord[1].st);
	
	vec4 Color = ColorDecal * ColorDecal.a/2 + ColorHull * (1-ColorDecal.a/2);
	Color.a = 1.0;
	
	gl_FragColor = Color;
} 

That is. Simple no?

[serengeor]

Put the screenshot, so someone who's not familiar with it could see what it does.

[nespa]

don't you pass "extents" and "objDim" to vertex program?

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services->setVertexShaderConstant("extents", reinterpret_cast<irr::f32*>(&extents),2);
        services->setVertexShaderConstant("objDim", reinterpret_cast<irr::f32*>(&objDim),2); 

In your code seems to be passed to pixel program!