Fixed texture coordinate offset for both line vectors and volumes

fsl/fslview/gl/gl21/gllinevector_vert.glsl

@@ -16,8 +16,14 @@ varying vec3 fragTexCoord;
 void main(void) {
 
   // TODO check voxel +0.5 offset
+
+  // Round the vertex position to the nearest integer -
+  // this gives us the corresponding voxel coordinates
   fragVoxCoord = floor(vertex + 0.5);
-  fragTexCoord = fragVoxCoord / imageShape;
+
+  // Transform the voxel coordinates to texture
+  // coordinates, adding 0.5 to centre them
+  fragTexCoord = (fragVoxCoord + 0.5) / imageShape;
 
   gl_Position = gl_ModelViewProjectionMatrix * vec4(vertex, 1);
 }

fsl/fslview/gl/globject.py

@@ -601,7 +601,12 @@ def slice2D(dataShape, xax, yax, zpos, voxToDisplayMat, displayToVoxMat):
     vertices[:, zax] = zpos
 
     voxCoords = transform.transform(vertices, displayToVoxMat)
-    texCoords = voxCoords / dataShape
+
+    # offset by 0.5, because voxel coordinates are by
+    # default centered at 0 (i.e. the space of a voxel
+    # lies in the range [-0.5, 0.5]), but we want voxel
+    # coordinates to map to the effective range [0, 1]
+    texCoords = (voxCoords + 0.5) / dataShape
 
     return vertices, voxCoords, texCoords