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Unified Diff: src/utils/SkTextureCompressor_Utils.h

Issue 669243003: Add utils to better quantize grayscale values to three bit indices while (Closed) Base URL: https://skia.googlesource.com/skia.git@master
Patch Set: Update tests to reflect new qunatization method Created 6 years, 2 months ago
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Index: src/utils/SkTextureCompressor_Utils.h
diff --git a/src/utils/SkTextureCompressor_Utils.h b/src/utils/SkTextureCompressor_Utils.h
new file mode 100755
index 0000000000000000000000000000000000000000..9b115a29647b95525cf445db686a65d92f4849fa
--- /dev/null
+++ b/src/utils/SkTextureCompressor_Utils.h
@@ -0,0 +1,68 @@
+/*
+* Copyright 2014 Google Inc.
+*
+* Use of this source code is governed by a BSD-style license that can be
+* found in the LICENSE file.
+*/
+
+#ifndef SkTextureCompressorUtils_DEFINED
+#define SkTextureCompressorUtils_DEFINED
+
+namespace SkTextureCompressor {
+
+ // In some compression formats used for grayscale alpha, i.e. coverage masks, three
+ // bit indices are used to represent each pixel. A compression scheme must therefore
+ // quantize the full eight bits of grayscale to three bits. The simplest way to do
+ // this is to take the top three bits of the grayscale value. However, this does not
+ // provide an accurate quantization: 192 will be quantized to 219 instead of 185. In
+ // our compression schemes, we let these three-bit indices represent the full range
+ // of grayscale values, and so when we go from three bits to eight bits, we replicate
+ // the three bits into the lower bits of the eight bit value. Below are two different
+ // techniques that offer a quality versus speed tradeoff in terms of quantization.
+#if 1
+ // Divides each byte in the 32-bit argument by three.
+ static inline uint32_t MultibyteDiv3(uint32_t x) {
+ const uint32_t a = (x >> 2) & 0x3F3F3F3F;
+ const uint32_t ar = (x & 0x03030303) << 4;
+
+ const uint32_t b = (x >> 4) & 0x0F0F0F0F;
+ const uint32_t br = (x & 0x0F0F0F0F) << 2;
+
+ const uint32_t c = (x >> 6) & 0x03030303;
+ const uint32_t cr = x & 0x3F3F3F3F;
+
+ return a + b + c + (((ar + br + cr) >> 6) & 0x03030303);
+ }
+
+ // Takes a loaded 32-bit integer of four 8-bit greyscale values and returns their
+ // quantization into 3-bit values, used by LATC and R11 EAC. Instead of taking the
+ // top three bits, the function computes the best three-bit value such that its
+ // reconstruction into an eight bit value via bit replication will yield the best
+ // results. In a 32-bit integer taking the range of values from 0-255 we would add
+ // 18 and divide by 36 (255 / 36 ~= 7). However, since we are working in constrained
+ // 8-bit space, our algorithm is the following:
+ // 1. Shift right by one to give room for overflow
+ // 2. Add 9 (18/2)
+ // 3. Divide by 18 (divide by two, then by three twice)
+ static inline uint32_t ConvertToThreeBitIndex(uint32_t x) {
+ x = (x >> 1) & 0x7F7F7F7F; // 1
+ x = x + 0x09090909; // 2
+
+ // Need to divide by 18... so first divide by two
+ x = (x >> 1) & 0x7F7F7F7F;
+
+ // Now divide by three twice
+ x = MultibyteDiv3(x);
+ x = MultibyteDiv3(x);
+ return x;
+ }
+#else
+ // Moves the top three bits of each byte in the 32-bit argument to the least
+ // significant bits of their respective byte.
+ static inline uint32_t ConvertToThreeBitIndex(uint32_t x) {
+ return (x >> 5) & 0x07070707;
+ }
+#endif
+}
+
+#endif // SkTextureCompressorUtils_DEFINED
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