//=============================================================================
//
// Project:   SharpImage
// Module:    shader-vm.frag
// Language:  OpenGL Shading Language (GLSL)
// Author:    Dan Mueller
// Date:      $Date: 2007-07-06 10:57:00 +1000 (Fri, 06 Jul 2007) $
// Revision:  $Revision: 2 $
//
// Copyright (c) Queensland University of Technology (QUT) 2007.
// All rights reserved.
//
// This software is distributed WITHOUT ANY WARRANTY; without even
// the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
// PURPOSE.  See the copyright notices for more information.
//
//=============================================================================
uniform sampler3D sam3Tex0;  // Sampler for transfer function
uniform sampler3D sam3Tex1;  // Sampler for value image
uniform sampler3D sam3Tex2;  // Sampler for gradient image
varying vec3 pos3Tex1;       // Current coord of value image
varying vec3 pos3Tex2;       // Current coord of gradient image
uniform float fSamplingRate; // Current sampling rate

// Adjust the alpha value for the current sampling rate
float adjustAlphaForSamplingRate( float a )
{
    return 1.0 - pow( 1.0 - a,  1.0 / fSamplingRate );
}

void main()
{
    // Interpolate images
    float value = texture3D( sam3Tex1, pos3Tex1 ).a;
    float gradmag = texture3D( sam3Tex2, pos3Tex2 ).a;

    // Interpolate transfer function
    vec3 pos3Tf = vec3( value, 1.0-gradmag, 0.0 );
    vec4 val4Tf = texture3D( sam3Tex0, pos3Tf );

    // Discard if no contribution
    if (val4Tf.a <= 0.0) discard;

    // Adjust alpha for sampling rate
    val4Tf.a = adjustAlphaForSamplingRate( val4Tf.a );

    // Set color
    gl_FragColor = val4Tf;
}