char volume_lit_tf_frag[] = "uniform sampler3D baseTexture;\n"
                            "uniform sampler1D tfTexture;\n"
                            "uniform float SampleDensityValue;\n"
                            "uniform float TransparencyValue;\n"
                            "uniform float AlphaFuncValue;\n"
                            "\n"
                            "varying vec4 cameraPos;\n"
                            "varying vec4 vertexPos;\n"
                            "varying mat4 texgen;\n"
                            "\n"
                            "void main(void)\n"
                            "{ \n"
                            "    vec3 t0 = (texgen * vertexPos).xyz;\n"
                            "    vec3 te = (texgen * cameraPos).xyz;\n"
                            "    \n"
                            "    vec3 eyeDirection = normalize(te-t0);\n"
                            "\n"
                            "    if (te.x>=0.0 && te.x<=1.0 &&\n"
                            "        te.y>=0.0 && te.y<=1.0 &&\n"
                            "        te.z>=0.0 && te.z<=1.0)\n"
                            "    {\n"
                            "        // do nothing... te inside volume\n"
                            "    }\n"
                            "    else\n"
                            "    {\n"
                            "        if (te.x<0.0)\n"
                            "        {\n"
                            "            float r = -te.x / (t0.x-te.x);\n"
                            "            te = te + (t0-te)*r;\n"
                            "        }\n"
                            "\n"
                            "        if (te.x>1.0)\n"
                            "        {\n"
                            "            float r = (1.0-te.x) / (t0.x-te.x);\n"
                            "            te = te + (t0-te)*r;\n"
                            "        }\n"
                            "\n"
                            "        if (te.y<0.0)\n"
                            "        {\n"
                            "            float r = -te.y / (t0.y-te.y);\n"
                            "            te = te + (t0-te)*r;\n"
                            "        }\n"
                            "\n"
                            "        if (te.y>1.0)\n"
                            "        {\n"
                            "            float r = (1.0-te.y) / (t0.y-te.y);\n"
                            "            te = te + (t0-te)*r;\n"
                            "        }\n"
                            "\n"
                            "        if (te.z<0.0)\n"
                            "        {\n"
                            "            float r = -te.z / (t0.z-te.z);\n"
                            "            te = te + (t0-te)*r;\n"
                            "        }\n"
                            "\n"
                            "        if (te.z>1.0)\n"
                            "        {\n"
                            "            float r = (1.0-te.z) / (t0.z-te.z);\n"
                            "            te = te + (t0-te)*r;\n"
                            "        }\n"
                            "    }\n"
                            "\n"
                            "    const float max_iteratrions = 2048.0;\n"
                            "    float num_iterations = ceil(length(te-t0)/SampleDensityValue);\n"
                            "    if (num_iterations<2.0) num_iterations = 2.0;\n"
                            "\n"
                            "    if (num_iterations>max_iteratrions) \n"
                            "    {\n"
                            "        num_iterations = max_iteratrions;\n"
                            "    }\n"
                            "\n"
                            "    vec3 deltaTexCoord=(te-t0)/float(num_iterations-1.0);\n"
                            "    vec3 texcoord = t0;\n"
                            "\n"
                            "    float normalSampleDistance = 1.0/512.0;\n"
                            "    vec3 deltaX = vec3(normalSampleDistance, 0.0, 0.0);\n"
                            "    vec3 deltaY = vec3(0.0, normalSampleDistance, 0.0);\n"
                            "    vec3 deltaZ = vec3(0.0, 0.0, normalSampleDistance);\n"
                            "    \n"
                            "    vec4 fragColor = vec4(0.0, 0.0, 0.0, 0.0); \n"
                            "    while(num_iterations>0.0)\n"
                            "    {\n"
                            "        float v = texture3D( baseTexture, texcoord).a;\n"
                            "        vec4 color = texture1D( tfTexture, v);\n"
                            "\n"
                            "        float a = v;\n"
                            "        float px = texture3D( baseTexture, texcoord + deltaX).a;\n"
                            "        float py = texture3D( baseTexture, texcoord + deltaY).a;\n"
                            "        float pz = texture3D( baseTexture, texcoord + deltaZ).a;\n"
                            "\n"
                            "        float nx = texture3D( baseTexture, texcoord - deltaX).a;\n"
                            "        float ny = texture3D( baseTexture, texcoord - deltaY).a;\n"
                            "        float nz = texture3D( baseTexture, texcoord - deltaZ).a;\n"
                            "\n"
                            "        vec3 grad = vec3(px-nx, py-ny, pz-nz);\n"
                            "        if (grad.x!=0.0 || grad.y!=0.0 || grad.z!=0.0)\n"
                            "        {\n"
                            "            vec3 normal = normalize(grad);\n"
                            "            float lightScale = 0.1 +  abs(dot(normal.xyz, eyeDirection))*0.9;\n"
                            "\n"
                            "            color.x *= lightScale;\n"
                            "            color.y *= lightScale;\n"
                            "            color.z *= lightScale;\n"
                            "        }\n"
                            "\n"
                            "\n"
                            "        float r = color[3]*TransparencyValue;\n"
                            "        if (r>AlphaFuncValue)\n"
                            "        {\n"
                            "            fragColor.xyz = fragColor.xyz*(1.0-r)+color.xyz*r;\n"
                            "            fragColor.w += r;\n"
                            "        }\n"
                            "\n"
                            "        if (fragColor.w<color.w)\n"
                            "        {\n"
                            "            fragColor = color;\n"
                            "        }\n"
                            "        texcoord += deltaTexCoord; \n"
                            "\n"
                            "        --num_iterations;\n"
                            "    }\n"
                            "\n"
                            "    fragColor.w *= TransparencyValue;\n"
                            "\n"
                            "    if (fragColor.w>1.0) fragColor.w = 1.0; \n"
                            "    if (fragColor.w<AlphaFuncValue) discard;\n"
                            "    \n"
                            "    gl_FragColor = fragColor;\n"
                            "}\n";
