char volume_tf_frag[] = "uniform sampler3D baseTexture;\n"
                        "uniform sampler1D tfTexture;\n"
                        "uniform float sampleDensity;\n"
                        "uniform float transparency;\n"
                        "uniform float alphaCutOff;\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"
                        "    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)/sampleDensity);\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"
                        "    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 r = color[3]*transparency;\n"
                        "        if (r>alphaCutOff)\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 *= transparency;\n"
                        "\n"
                        "    if (fragColor.w>1.0) fragColor.w = 1.0; \n"
                        "    if (fragColor.w<alphaCutOff) discard;\n"
                        "    \n"
                        "    gl_FragColor = fragColor;\n"
                        "}\n"
                        "\n";