!!ARBfp1.0
# -----------------------------------------------------------------------------
# Copyright 2005 by University of Utah
#
# Hardware-Assisted Visibility Sorting
# 
# The program consists of the following steps:
#
# 1. Find the first and second entries in the fixed sized k-buffer list sorted 
#    by z (2+1 entries)
# 2. Perform a 3D pre-integrated transfer function lookup using front and back
#    scalar data values + the segment length computed from the depth values
#    of the first and second entries from the k-buffer.
# 3. Composite the color and opacity from the transfer funcion with the
#    color and opacity from the framebuffer. Discard winning k-buffer entry,
#    write the remaining k-buffer entries.
#
# The following textures are used:
# 
#   Tex 0: framebuffer (pbuffer, 2D RGBA 8/16 bpp int or 16/32 bpp float)
#   Tex 1: k-buffer entries 1 and 2(same)
#   Tex 2: transfer function (regular, 3D RGBA 8/16 bpp int)
#

# -----------------------------------------------------------------------------
# use the ATI_draw_buffers extension
OPTION ATI_draw_buffers;
# this may matter on future ATI hardware
OPTION ARB_precision_hint_nicest;

# -----------------------------------------------------------------------------
# input and temporaries
ATTRIB p = fragment.position; # fragment position in screen space
PARAM sz = program.local[0];  # texture scale and max gap length parameters
                              # {1/pw, 1/ph, max, not_used)}
PARAM half = { 0.5, 0.5, 0.0, 0.0 };
PARAM exp = { 0.0, 0.0, 0.0, 1.44269504 }; # 1/ln2 
	
TEMP a1, a2; # k-buffer entries
TEMP r0, r1; # sorted results
TEMP c, c0; # color and opacity
TEMP t; # temporary variable
TEMP colorBack,colorFront;
TEMP taud, zeta, gamma, Psi;

# -----------------------------------------------------------------------------
# compute texture coordinates from window position so that it is not 
# interpolated perspective correct.  Then look up the color and opacity from 
# the framebuffer
MUL t, p, sz; # t.xy = p.xy * sz.xy, only x and y are used for texture lookup
TEX c0, t, texture[0], 2D; # framebuffer color

# -----------------------------------------------------------------------------
# Check opacity and kill fragment if it is greater than tolerance
SUB t.w, 0.99, c0.w;
KIL t.w;

# -----------------------------------------------------------------------------
# set up the k-buffer entries a1, a2
# each k-buffer entry consists of the scalar data value in x or z and the 
# depth value in y or w
TEX a1, t, texture[1], 2D; # k-buffer entry 1
MOV a2, a1.zwzw; # k-buffer entry 2

# -----------------------------------------------------------------------------
# find fragment with minimum z (r0), save the other to r1

# r0 = min_d(a1, a2); r1 = max_d(a1, a2);
SUB t.w, a1.y, a2.y; # t.w < 0 iff a1.y < a2.y
CMP r1, t.w, a2, a1; # r1 = (a1.y < a2.y ? a2 : a1)
CMP r0, t.w, a1, a2; # r0 = (a1.y < a2.y ? a1 : a2)

# -----------------------------------------------------------------------------
# set up texture coordinates for transfer function lookup
MOV t.x, r0.x; # front scalar
MOV t.y, r1.x; # back scalar
SUB t.z, r1.y, r0.y; # z distance between front and back

# -----------------------------------------------------------------------------
# nullify fragment if distance is greater than unit scale (non-convexities)
SUB t.w, sz.z, t.z; 
CMP t.z, t.w, 0.0, t.z; 

# -----------------------------------------------------------------------------
# transfer function lookup
TEX colorFront, t.x, texture[3], 1D;
TEX colorBack,  t.y, texture[3], 1D;
MUL taud.x, t.z, colorBack.a;
MUL taud.y, t.z, colorFront.a;

# -----------------------------------------------------------------------------
# compute zeta = exp(-0.5*(taudf+taudb)
DP3 zeta.w, taud, half;
MUL zeta.w, exp.w, zeta.w; 
EX2 zeta.w, -zeta.w; 
	
# -----------------------------------------------------------------------------
# compute gamma = taud/(1+taud);
ADD t, taud, 1.0;
RCP t.x, t.x;
RCP t.y, t.y;
MUL gamma, taud, t;

# -----------------------------------------------------------------------------
# lookup Psi
TEX Psi.w, gamma, texture[2], 2D;

# -----------------------------------------------------------------------------
# compute color = cb(psi-zeta) + cf(1.0-psi) 
SUB t.w, Psi.w, zeta.w;
MUL colorBack, colorBack, t.w; 
SUB t.w, 1.0, Psi.w;
MUL colorFront, colorFront, t.w;
ADD c, colorBack, colorFront; 
SUB c.a, 1.0, zeta.w;

# -----------------------------------------------------------------------------
# nullify winning entry if the scalar value < 0
CMP c, r0.x, 0.0, c;
CMP c, r1.x, 0.0, c;

# -----------------------------------------------------------------------------
# composite color with the color from the framebuffer !!!front to back!!! 
SUB t.w, 1.0, c0.w;
MAD result.color[0], c, t.w, c0; 

# -----------------------------------------------------------------------------
# write remaining k-buffer entry and invalidate one entry
MOV r1.z, -1.0;
MOV r1.w, 10000.0;
MOV result.color[1], r1;

END