source: downloads/Samples/Media/materials/programs/skinningTwoWeightsVp.glsl @ 1

// Example GLSL program for skinning with two bone weights per vertex

attribute vec4 blendIndices;
attribute vec4 blendWeights;

// 3x4 matrix, passed as vec4's for compatibility with GL 2.0
// GL 2.0 supports 3x4 matrices
// Support 24 bones ie 24*3, but use 72 since our parser can pick that out for sizing
uniform vec4 worldMatrix3x4Array[72];
uniform mat4 viewProjectionMatrix;
uniform vec4 lightPos[2];
uniform vec4 lightDiffuseColour[2];

void main()
{
        vec3 blendPos = vec3(0,0,0);
        vec3 blendNorm = vec3(0,0,0);
        
        for (int bone = 0; bone < 2; ++bone)
        {
                // perform matrix multiplication manually since no 3x4 matrices
        // ATI GLSL compiler can't handle indexing an array within an array so calculate the inner index first
            int idx = int(blendIndices[bone]) * 3;
        // ATI GLSL compiler can't handle unrolling the loop so do it manually
        // ATI GLSL has better performance when mat4 is used rather than using individual dot product
        // There is a bug in ATI mat4 constructor (Cat 7.2) when indexed uniform array elements are used as vec4 parameter so manually assign
                mat4 worldMatrix;
                worldMatrix[0] = worldMatrix3x4Array[idx];
                worldMatrix[1] = worldMatrix3x4Array[idx + 1];
                worldMatrix[2] = worldMatrix3x4Array[idx + 2];
                worldMatrix[3] = vec4(0);
                // now weight this into final 
            float weight = blendWeights[bone];
                blendPos += (gl_Vertex * worldMatrix).xyz * weight;
                
                mat3 worldRotMatrix = mat3(worldMatrix[0].xyz, worldMatrix[1].xyz, worldMatrix[2].xyz);
                blendNorm += (gl_Normal * worldRotMatrix) * weight;

        }

        // apply view / projection to position
        gl_Position = viewProjectionMatrix * vec4(blendPos, 1);

        // simple vertex lighting model
        vec3 lightDir0 = normalize(
                lightPos[0].xyz -  (blendPos.xyz * lightPos[0].w));
        vec3 lightDir1 = normalize(
                lightPos[1].xyz -  (blendPos.xyz * lightPos[1].w));
                
        gl_FrontSecondaryColor = vec4(0);
        gl_FrontColor = vec4(0.5, 0.5, 0.5, 1.0) 
                + clamp(dot(lightDir0, blendNorm), 0.0, 1.0) * lightDiffuseColour[0]
                + clamp(dot(lightDir1, blendNorm), 0.0, 1.0) * lightDiffuseColour[1];

        gl_TexCoord[0] = gl_MultiTexCoord0;
        
}