ENH: Adjust depth values in GL14

fsleyes/assets/gl/gl14/glvolume_3d_frag.prog

@@ -122,12 +122,16 @@ MOV skipTest,    1;
 MOV finalColour, startingColour;
 
 
-# Set depth.w > 0 if depth has
-# not been set, < 0 otherwise.
-# If depth has already been set,
-# we want to pass through its
-# initial value.
-SUB depth.w, 0, finalColour.a;
+# Set depth.w < 0 if depth has already been
+# set in a previous pass, > 0 otherwise. If
+# depth has already been set, we want to pass
+# through that value, rather than calculating
+# it again. Initial value for a depth texture
+# is 1 (and depth textures are clamped to 0
+# [closer to screen], 1 [farther from screen]),
+# so if it is < 1, we assume it has already
+# been set.
+SUB depth.w, depth.x, 1;
 CMP depth.w, depth.w, -1, 1;
 
 
@@ -209,7 +213,10 @@ MIN skipTest.x, skipTest.x, tempVar.x;
 
 
 # Check whether the accumulated
-# colour alpha is already high enough
+# colour alpha is already high enough.
+# If it is, we don't consider the
+# values from any more samples on the
+# ray.
 SLT tempVar.x,  finalColour.a, 0.95;
 MAD tempVar.x,  tempVar.x,     2.0, -1;
 MIN skipTest.x, skipTest.x,    tempVar.x;
@@ -250,32 +257,53 @@ MUL colour.rgb, colour, colour.a;
 SUB tempVar.x, 1, finalColour.a;
 MAD tempColour, tempVar.x, colour, finalColour;
 
+# Figure out if we should consider this
+# sample for depth calculation:
+#  - skipTest.x tells us whether this sample
+#    should be discarded
+#  - depth.w tells us whether we already have
+#    a depth value
+#  - {{ param_screenSize }}.z tells us whether
+#    we are modulating alpha by voxel intensity
+#    (depth is calculated differently depending
+#    on whether we are or not)
+#
+# If modulating alpha by voxel intensity, we
+# take the depth of the first sample on the ray
+# with intensity >= 0.1 (voxel values are
+# normalised to [0, 1] w.r.t. current display
+# range).
+#
+# If not blending by voxel intensity, we
+# take the depth of the first sample.
+#
+# We accumulate all of these conditionals into
+# tempVar.x, such that it is == 0 if we should
+# take the depth value from this sample, < 0
+# otherwise.
+SLT tempVar.x, skipTest.x, 0;                       # set if should skip
+SLT tempVar.y, depth.w,    0;                       # set if already have depth
+SLT tempVar.z, voxValue.x, 0.1;                     # set if below threshold
+ADD tempVar.z, tempVar.z, {{ param_screenSize }}.z; # and blending by intensity
+SGE tempVar.z, tempVar.z, 2;
+ADD tempVar.x, tempVar.x, tempVar.y;
+ADD tempVar.x, tempVar.x, tempVar.z;
+MUL tempVar.x, tempVar.x, -1;
 
-# Calculate the screen depth of
-# the current ray position. If we
-# have just taken our first sample,
-# save the depth, as we will use
-# it for the final fragment depth.
-
-# Figure out if this is the first
-# sample taken by testing whether
-# skipTest.x > 0, and depth.w > 0
-# (the latter is true if the
-# startingColour.a == 0).
-ADD depth.y, depth.w,  skipTest.x;
-MUL depth.y, depth.y, -1;
-
-DP4 depth.z, {{ param_tex2ScreenXform }}, texCoord;
-CMP depth.x, depth.y,  depth.z, depth.x;
+# Calculate the screen depth of the
+# current ray position, storing it
+# in tempvar.z
+DP4 tempVar.z, {{ param_tex2ScreenXform }}, texCoord;
 
+# Save the depth value to depth.x, based on
+# conditional tests above.
+CMP depth.x, tempVar.x, depth.x, tempVar.z;
 
 # If we have just set the depth,
 # set depth.w to < 0 so that we
 # don't overwrite the depth on a
 # subsequent sample.
-SUB depth.y, depth.w,  skipTest.x;
-MAD depth.y, depth.y, -1,   0.5;
-CMP depth.w, depth.y,  1,  -1;
+CMP depth.w, tempVar.x, depth.w, -1;
 
 
 # Only update the accumulated colour
@@ -294,9 +322,25 @@ CMP finalColour, skipTest.x, finalColour, tempColour;
 ADD texCoord.xyz,     texCoord,     {{ param_rayStep }};
 ADD clipTexCoord.xyz, clipTexCoord, {{ param_rayStep }};
 
-
 {% endfor %}
 
+# set tempVar.w to -1 if this is the final
+# pass, and we still don't have a depth
+# value, 0 otherwise.
+SGE tempVar.x, {{ param_settings }}.z, 1;
+SGE tempVar.y, depth.w,                1;
+ADD tempVar.x, tempVar.x,              tempVar.y;
+SGE tempVar.x, tempVar.x,              2;
+MUL tempVar.w, tempVar.x,             -1;
+
+# Calculate the depth value corresponding
+# to the very first sample in this ray.
+ADD tempVar.xyz, {{ param_rayStep }},         {{ varying_texCoord }};
+DP4 tempVar.z,   {{ param_tex2ScreenXform }}, tempVar;
+
+# Use it if we don't already have a depth
+CMP depth.x, tempVar.w, tempVar.z, depth.x;
+
 
 # If startingColour.a == 0 and
 #    finalColour.a    == 0,

fsleyes/gl/gl14/glvolume_funcs.py

@@ -256,6 +256,14 @@ def draw3D(self, xform=None, bbox=None):
     with glroutines.enabled((gl.GL_VERTEX_ARRAY)), \
          glroutines.disabled((gl.GL_BLEND)):
 
+        # The depth value for a fragment will
+        # not necessary be set at the same
+        # time that the fragment colour is
+        # set, so we need to use <= for depth
+        # testing so that unset depth values
+        # do not cause depth clipping.
+        gl.glDepthFunc(gl.GL_LEQUAL)
+
         for i in range(outerLoop):
 
             settings    = list(settings)
@@ -280,6 +288,8 @@ def draw3D(self, xform=None, bbox=None):
 
             dest, src = src, dest
 
+    gl.glDepthFunc(gl.GL_LESS)
+
     shader.unloadAtts()
     shader.unload()