fix(ImageResliceMapper): normalize slab sampling direction

Ensure the slab sampling direction is normalized before computing
step sizes and traversal distances.
Previously, normalWCVSOutput was used directly, which could be a
non-unit vector. This caused incorrect scaling of sampling steps,
leading to inconsistent sampling density and potential visual artifacts
when traversing the slab.

Author: daker

Sources/Rendering/OpenGL/ImageResliceMapper/index.js

@@ -1431,7 +1431,8 @@ function vtkOpenGLImageResliceMapper(publicAPI, model) {
         '// Get the first and last samples',
         'int numSlices = 1;',
         'float scaling = min(min(spacing.x, spacing.y), spacing.z) * 0.5;',
-        'vec3 normalxspacing = scaling * normalWCVSOutput;',
+        'vec3 slabNormal = normalize(normalWCVSOutput);',
+        'vec3 normalxspacing = scaling * slabNormal;',
         'float distTraveled = length(normalxspacing);',
         'int trapezoid = 0;',
         'while (distTraveled < slabThickness * 0.5)',
@@ -1441,7 +1442,7 @@ function vtkOpenGLImageResliceMapper(publicAPI, model) {
         '  if (distTraveled > slabThickness * 0.5)',
         '  {',
         '    // Before stepping outside the slab, sample at the boundaries',
-        '    normalxspacing = normalWCVSOutput * slabThickness * 0.5 / fnumSlices;',
+        '    normalxspacing = slabNormal * slabThickness * 0.5 / fnumSlices;',
         '    trapezoid = slabTrapezoid;',
         '  }',
         '  vec3 worldPosNeg = vertexWCVSOutput.xyz - fnumSlices * normalxspacing * vboScaling;',