// Raycasting fragment program implementation
fragment_out fragment_main( vertex_fragment IN,
uniform sampler2D tex,
uniform sampler3D volume_tex,
uniform float stepsize
)
{
fragment_out OUT;
float2 texc = ((IN.Pos.xy / IN.Pos.w) + 1) / 2; // find the right place to lookup in the backside buffer
float4 start = IN.TexCoord; // the start position of the ray is stored in the texturecoordinate
float4 back_position = tex2D(tex, texc);
float3 dir = float3(0,0,0);
dir.x = back_position.x - start.x;
dir.y = back_position.y - start.y;
dir.z = back_position.z - start.z;
float len = length(dir.xyz); // the length from front to back is calculated and used to terminate the ray
float3 norm_dir = normalize(dir);
float delta = stepsize;
float3 delta_dir = norm_dir * delta;
float delta_dir_len = length(delta_dir);
float3 vec = start;
float4 col_acc = float4(0,0,0,0);
float alpha_acc = 0;
float length_acc = 0;
float4 color_sample;
float alpha_sample;
///////////////////////////////////////////////////////////
/////i do not understand the fellowing code//////////////
///////////////////////////////////////////////////////////
for(int i = 0; i < 450; i++)
{
color_sample = tex3D(volume_tex,vec);
alpha_sample = color_sample.a * stepsize;
col_acc += (1.0 - alpha_acc) * color_sample * alpha_sample * 3;
alpha_acc += alpha_sample;
vec += delta_dir;
length_acc += delta_dir_len;
if(length_acc >= len || alpha_acc > 1.0) break; // terminate if opacity > 1 or the ray is outside the volume
}
OUT.Color = col_acc;
return OUT;
}
