Add utils to better quantize grayscale values to three bit indices while
compressing coverage masks.
Signed-off-by: Pavel Krajcevski <pavel@cs.unc.edu>
BUG=skia:
Review URL: https://codereview.chromium.org/669243003
diff --git a/tests/TextureCompressionTest.cpp b/tests/TextureCompressionTest.cpp
index 7dd285d..568d4d1 100644
--- a/tests/TextureCompressionTest.cpp
+++ b/tests/TextureCompressionTest.cpp
@@ -255,7 +255,39 @@
// and that the three bits saved per pixel are computed from the top three
// bits of the luminance value.
const uint64_t kIndexEncodingMap[8] = { 1, 7, 6, 5, 4, 3, 2, 0 };
- const uint64_t kIndex = kIndexEncodingMap[lum >> 5];
+
+ // Quantize to three bits in the same way that we do our LATC compression:
+ // 1. Divide by two
+ // 2. Add 9
+ // 3. Divide by two
+ // 4. Approximate division by three twice
+ uint32_t quant = static_cast<uint32_t>(lum);
+ quant >>= 1; // 1
+ quant += 9; // 2
+ quant >>= 1; // 3
+
+ uint32_t a, b, c, ar, br, cr;
+
+ // First division by three
+ a = quant >> 2;
+ ar = (quant & 0x3) << 4;
+ b = quant >> 4;
+ br = (quant & 0xF) << 2;
+ c = quant >> 6;
+ cr = (quant & 0x3F);
+ quant = (a + b + c) + ((ar + br + cr) >> 6);
+
+ // Second division by three
+ a = quant >> 2;
+ ar = (quant & 0x3) << 4;
+ b = quant >> 4;
+ br = (quant & 0xF) << 2;
+ c = quant >> 6;
+ cr = (quant & 0x3F);
+ quant = (a + b + c) + ((ar + br + cr) >> 6);
+
+ const uint64_t kIndex = kIndexEncodingMap[quant];
+
const uint64_t kConstColorEncoding =
SkEndian_SwapLE64(
255 |