char volume_tf_iso_frag[] = "uniform sampler3D baseTexture;\n"
                            "\n"
                            "uniform sampler1D tfTexture;\n"
                            "uniform float tfScale;\n"
                            "uniform float tfOffset;\n"
                            "\n"
                            "uniform float SampleDensityValue;\n"
                            "uniform float TransparencyValue;\n"
                            "uniform float IsoSurfaceValue;\n"
                            "\n"
                            "varying vec4 cameraPos;\n"
                            "varying vec4 vertexPos;\n"
                            "varying mat4 texgen;\n"
                            "\n"
                            "void main(void)\n"
                            "{ \n"
                            "    vec4 t0 = vertexPos;\n"
                            "    vec4 te = cameraPos;\n"
                            "    vec3 eyeDirection = normalize((te-t0).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"
                            "    t0 = t0 * texgen;\n"
                            "    te = te * texgen;\n"
                            "\n"
                            "    const float max_iteratrions = 2048.0;\n"
                            "    float num_iterations = ceil(length((te-t0).xyz)/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=(t0-te).xyz/float(num_iterations-1.0);\n"
                            "    vec3 texcoord = te.xyz;\n"
                            "    float previousV = texture3D( baseTexture, texcoord).a;\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"
                            "    while(num_iterations>0.0)\n"
                            "    {\n"
                            "\n"
                            "        float v = texture3D( baseTexture, texcoord).a;\n"
                            "\n"
                            "        float m = (previousV-IsoSurfaceValue) * (v-IsoSurfaceValue);\n"
                            "        if (m <= 0.0)\n"
                            "        {\n"
                            "            float r = (IsoSurfaceValue-v)/(previousV-v);\n"
                            "            texcoord = texcoord - r*deltaTexCoord;\n"
                            "\n"
                            "            v = texture3D( baseTexture, texcoord).a * tfScale + tfOffset;\n"
                            "            vec4 color = texture1D( tfTexture, v);\n"
                            "\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"
                            "            gl_FragColor = color;\n"
                            "\n"
                            "            return;\n"
                            "        }\n"
                            "\n"
                            "        previousV = v;\n"
                            "\n"
                            "        texcoord += deltaTexCoord;\n"
                            "\n"
                            "        --num_iterations;\n"
                            "    }\n"
                            "\n"
                            "    // we didn't find an intersection so just discard fragment\n"
                            "    discard;\n"
                            "\n"
                            "}\n"
                            "\n";
