RF: Need to adjust voxValue by texZero so comparison works e.g. when neg cmap

is activate

fsleyes/assets/gl/gl14/glvolume_3d_frag.prog

@@ -281,9 +281,11 @@ MAD tempColour, tempVar.x, colour, finalColour;
 # 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
+SLT tempVar.x, skipTest.x, 0; # set x if should skip
+SLT tempVar.y, depth.w,    0; # set y if already have depth
+
+SUB tempVar.z, voxValue.x, {{ param_negCmap }}.y;   # set z if below threshold
+SLT tempVar.z, voxValue.x, 0.1;
 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;

fsleyes/assets/gl/gl21/glvolume_3d_frag.glsl

@@ -414,7 +414,7 @@ void main(void) {
          * current display range). The 0.1 threshold is
          * completely arbitrary, but seems to work well.
          */
-        if (depth == 0 && voxValue >= 0.1) {
+        if (depth == 0 && (voxValue - texZero) >= 0.1) {
           depth = (tex2ScreenXform * vec4(texCoord, 1.0)).z;
         }
       }