char volume_lit_frag[] = "uniform sampler3D baseTexture;\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"
                         "    vec4 t0 = vertexPos;\n"
                         "    vec4 te = cameraPos;\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"
                         "    vec3 eyeDirection = normalize((te-t0).xyz);\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=(te-t0).xyz/float(num_iterations-1.0);\n"
                         "    vec3 texcoord = t0.xyz;\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"
                         "        vec4 color = texture3D( baseTexture, texcoord);\n"
                         "\n"
                         "        float a = color.a;\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"
                         "        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"
                         "\n";
