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diff --git a/third_party/jpeg-xl/lib/jxl/alpha.h b/third_party/jpeg-xl/lib/jxl/alpha.h
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+++ b/third_party/jpeg-xl/lib/jxl/alpha.h
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+// Copyright (c) the JPEG XL Project Authors. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#ifndef LIB_JXL_ALPHA_H_
+#define LIB_JXL_ALPHA_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <limits>
+
+#include "lib/jxl/base/compiler_specific.h"
+
+namespace jxl {
+
+// A very small value to avoid divisions by zero when converting to
+// unpremultiplied alpha. Page 21 of the technical introduction to OpenEXR
+// (https://www.openexr.com/documentation/TechnicalIntroduction.pdf) recommends
+// "a power of two" that is "less than half of the smallest positive 16-bit
+// floating-point value". That smallest value happens to be the denormal number
+// 2^-24, so 2^-26 should be a good choice.
+static constexpr float kSmallAlpha = 1.f / (1u << 26u);
+
+struct AlphaBlendingInputLayer {
+  const float* r;
+  const float* g;
+  const float* b;
+  const float* a;
+};
+
+struct AlphaBlendingOutput {
+  float* r;
+  float* g;
+  float* b;
+  float* a;
+};
+
+// Note: The pointers in `out` are allowed to alias those in `bg` or `fg`.
+// No pointer shall be null.
+void PerformAlphaBlending(const AlphaBlendingInputLayer& bg,
+                          const AlphaBlendingInputLayer& fg,
+                          const AlphaBlendingOutput& out, size_t num_pixels,
+                          bool alpha_is_premultiplied, bool clamp);
+// Single plane alpha blending
+void PerformAlphaBlending(const float* bg, const float* bga, const float* fg,
+                          const float* fga, float* out, size_t num_pixels,
+                          bool alpha_is_premultiplied, bool clamp);
+
+void PerformAlphaWeightedAdd(const float* bg, const float* fg, const float* fga,
+                             float* out, size_t num_pixels, bool clamp);
+
+void PerformMulBlending(const float* bg, const float* fg, float* out,
+                        size_t num_pixels, bool clamp);
+
+void PremultiplyAlpha(float* JXL_RESTRICT r, float* JXL_RESTRICT g,
+                      float* JXL_RESTRICT b, const float* JXL_RESTRICT a,
+                      size_t num_pixels);
+void UnpremultiplyAlpha(float* JXL_RESTRICT r, float* JXL_RESTRICT g,
+                        float* JXL_RESTRICT b, const float* JXL_RESTRICT a,
+                        size_t num_pixels);
+
+}  // namespace jxl
+
+#endif  // LIB_JXL_ALPHA_H_