From 858a25f2cd059ce2abf55e16708f53988e2ff380 Mon Sep 17 00:00:00 2001
From: Brianna Major <brianna.major@kitware.com>
Date: Tue, 7 Nov 2023 13:42:41 -0500
Subject: [PATCH] fix(shiftedOpacity): Do not shift opacity values of 0

---
 src/components/VtkTwoView.vue    | 2 +-
 src/utils/vtk-helpers.ts         | 8 +++++++-
 src/vtk/PiecewiseWidget/index.js | 6 +++++-
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/src/components/VtkTwoView.vue b/src/components/VtkTwoView.vue
index 7963ed303..85986d3aa 100644
--- a/src/components/VtkTwoView.vue
+++ b/src/components/VtkTwoView.vue
@@ -824,7 +824,7 @@ export default defineComponent({
                     opFunc.preset,
                     opFunc.mappingRange,
                     opFunc.shift,
-                    opFunc.shiftAlpha,
+                    opFunc.shiftAlpha
                   );
                   if (opacityPoints) {
                     pwf.setPoints(opacityPoints);
diff --git a/src/utils/vtk-helpers.ts b/src/utils/vtk-helpers.ts
index a760d778d..bb4d86b9c 100644
--- a/src/utils/vtk-helpers.ts
+++ b/src/utils/vtk-helpers.ts
@@ -129,7 +129,13 @@ export function getShiftedOpacityFromPreset(
 
     const [xmin, xmax] = effectiveRange;
     const width = xmax - xmin;
-    return points.map(([x, y]) => [(x - xmin) / width + shift, y - shiftAlpha]);
+    return points.map(([x, y]) => {
+      // Non-zero values should be affected by shift
+      // but preset values of zero should not
+      const shifted = y && y - shiftAlpha;
+      const yVal = Math.max(Math.min(shifted, 1), 0);
+      return [(x - xmin) / width + shift, yVal];
+    });
   }
   return null;
 }
diff --git a/src/vtk/PiecewiseWidget/index.js b/src/vtk/PiecewiseWidget/index.js
index e1725340f..d6d9ab62c 100644
--- a/src/vtk/PiecewiseWidget/index.js
+++ b/src/vtk/PiecewiseWidget/index.js
@@ -52,7 +52,11 @@ export function samplePiecewiseLinear(
       } else if (slope === -Infinity) {
         sampledPoints.push(0);
       } else {
-        sampledPoints.push(slope * (sx - p1[0]) + (p1[1] - shiftAlpha));
+        // Non-zero values should be affected by shift
+        // but original values of zero should not
+        let value = slope * (sx - p1[0]) + p1[1];
+        value = value && value - shiftAlpha;
+        sampledPoints.push(value);
       }
     }
   }