diff --git a/codecs/visdif/BUILD.md b/codecs/visdif/BUILD.md
new file mode 100644
index 00000000..2a77c28b
--- /dev/null
+++ b/codecs/visdif/BUILD.md
@@ -0,0 +1,14 @@
+This codec currently needs monkey-patching of Emscripten
+
+```
+$ docker run --rm -it -v $(PWD):/src squoosh-cpp "/bin/bash"
+# cat << EOF | patch /emsdk/upstream/emscripten/system/lib/dlmalloc.c
+659c659
+< #define MALLOC_ALIGNMENT ((size_t)(2 * sizeof(void *)))
+---
+> #define MALLOC_ALIGNMENT ((size_t)(16U))
+EOF
+# emcc --clear-cache
+# /emsdk/upstream/emscripten/embuilder build libdlmalloc --force
+# emmake make
+```
\ No newline at end of file
diff --git a/codecs/visdif/visdif.cpp b/codecs/visdif/visdif.cpp
index 0825cdea..0fcb1d16 100644
--- a/codecs/visdif/visdif.cpp
+++ b/codecs/visdif/visdif.cpp
@@ -5,12 +5,19 @@
 using namespace emscripten;
 using namespace butteraugli;
 
+#define GAMMA 2.2
+
+static float SrgbToLinear[256];
+
+inline void gammaLookupTable() {
+  SrgbToLinear[0] = 0;
+  for (int i = 1; i < 256; ++i) {
+    SrgbToLinear[i] = static_cast<float>(255.0 * pow(i / 255.0, GAMMA));
+  }
+}
+
 // Turns an interleaved RGBA buffer into 4 planes for each color channel
-void planarize(std::vector<ImageF>& img,
-               const uint8_t* rgba,
-               int width,
-               int height,
-               float gamma = 2.2) {
+void planarize(std::vector<ImageF>& img, const uint8_t* rgba, int width, int height) {
   assert(img.size() == 0);
   img.push_back(ImageF(width, height));
   img.push_back(ImageF(width, height));
@@ -22,10 +29,10 @@ void planarize(std::vector<ImageF>& img,
     float* const row_b = img[2].Row(y);
     float* const row_a = img[3].Row(y);
     for (int x = 0; x < width; x++) {
-      row_r[x] = 255.0 * pow(rgba[(y * width + x) * 4 + 0] / 255.0, gamma);
-      row_g[x] = 255.0 * pow(rgba[(y * width + x) * 4 + 1] / 255.0, gamma);
-      row_b[x] = 255.0 * pow(rgba[(y * width + x) * 4 + 2] / 255.0, gamma);
-      row_a[x] = 255.0 * pow(rgba[(y * width + x) * 4 + 3] / 255.0, gamma);
+      row_r[x] = SrgbToLinear[rgba[(y * width + x) * 4 + 0]];
+      row_g[x] = SrgbToLinear[rgba[(y * width + x) * 4 + 1]];
+      row_b[x] = SrgbToLinear[rgba[(y * width + x) * 4 + 2]];
+      row_a[x] = SrgbToLinear[rgba[(y * width + x) * 4 + 3]];
     }
   }
 }
@@ -37,6 +44,7 @@ class VisDiff {
 
  public:
   VisDiff(std::string ref_img, int width, int height) {
+    gammaLookupTable();
     planarize(this->ref_img, (uint8_t*)ref_img.c_str(), width, height);
     this->width = width;
     this->height = height;
diff --git a/codecs/visdif/visdif.wasm b/codecs/visdif/visdif.wasm
index ebb11eab..2ac3ca8b 100755
Binary files a/codecs/visdif/visdif.wasm and b/codecs/visdif/visdif.wasm differ