Merge pull request #211 from ImagingDataCommons/feat/gamma-correction

feat: Add gamma correction / PS

Author: fedorov

src/color.js

@@ -364,26 +364,43 @@ class PaletteColorLookupTable {
       ]
       const maxInput = Math.max(...maxValues)
       const maxOutput = 255
+
+      // Apply gamma correction to compensate for display gamma (brightens mid-tones)
+      // When a linear intensity value i (0 to 1) is looked up, we actually want to look up
+      // the color at position i^gamma where gamma = 1/2.2 ≈ 0.4545
+      // This pre-brightens the colors for mid-tones
+      const gammaInverse = 1.0 / 2.2 // ≈ 0.4545
+
       if (this[_attrs].bitsPerEntry === 16 && maxInput > 255) {
         /*
          * Only palettes with 256 entries and 8 bit per entry are supported for
          * display.  Therefore, data need to rescaled and resampled.
          */
         const n = 256
-        const step = this[_attrs].numberOfEntries / n
         this[_attrs].data = new Array(n)
         for (let i = 0; i < n; i++) {
-          const j = i * step
+          // Apply gamma correction: for palette position i, look up the color
+          // that would be at the gamma-corrected position
+          const normalizedPos = i / (n - 1)
+          const gammaCorrectedPos = Math.pow(normalizedPos, gammaInverse)
+          const lutIndex = Math.round(gammaCorrectedPos * (this[_attrs].numberOfEntries - 1))
+
           this[_attrs].data[i] = [
-            Math.round(rescale(redLUT[j], 0, maxInput, 0, maxOutput)),
-            Math.round(rescale(greenLUT[j], 0, maxInput, 0, maxOutput)),
-            Math.round(rescale(blueLUT[j], 0, maxInput, 0, maxOutput))
+            Math.round(rescale(redLUT[lutIndex], 0, maxInput, 0, maxOutput)),
+            Math.round(rescale(greenLUT[lutIndex], 0, maxInput, 0, maxOutput)),
+            Math.round(rescale(blueLUT[lutIndex], 0, maxInput, 0, maxOutput))
           ]
         }
       } else {
         this[_attrs].data = new Array(this[_attrs].numberOfEntries)
         for (let i = 0; i < this[_attrs].numberOfEntries; i++) {
-          this[_attrs].data[i] = [redLUT[i], greenLUT[i], blueLUT[i]]
+          // Apply gamma correction: for palette position i, look up the color
+          // that would be at the gamma-corrected position
+          const normalizedPos = i / (this[_attrs].numberOfEntries - 1)
+          const gammaCorrectedPos = Math.pow(normalizedPos, gammaInverse)
+          const lutIndex = Math.round(gammaCorrectedPos * (this[_attrs].numberOfEntries - 1))
+
+          this[_attrs].data[i] = [redLUT[lutIndex], greenLUT[lutIndex], blueLUT[lutIndex]]
         }
       }
     }