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improve gray approximation for LCD LERP

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git-svn-id: http://skia.googlecode.com/svn/trunk@1359 2bbb7eff-a529-9590-31e7-b0007b416f81
This commit is contained in:
reed@google.com 2011-05-18 11:58:10 +00:00
parent c518134fa2
commit bde3c8ed97
1 changed files with 12 additions and 10 deletions
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22
src/ports/SkFontHost_FreeType.cpp
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@ -72,11 +72,12 @@
using namespace skia_advanced_typeface_metrics_utils;
// SK_FREETYPE_LCD_LERP should be 0...256, where 0 means no color reduction
// and 256 means 100% color reduction (e.g. gray)
// SK_FREETYPE_LCD_LERP should be 0...256
// 0 means no color reduction (e.g. just as returned from FreeType)
// 256 means 100% color reduction (e.g. gray)
//
#ifndef SK_FREETYPE_LCD_LERP
#define SK_FREETYPE_LCD_LERP 128
#define SK_FREETYPE_LCD_LERP 96
#endif
//////////////////////////////////////////////////////////////////////////
@ -986,17 +987,18 @@ extern void CopyFreetypeBitmapToVerticalLCDMask(const SkGlyph& dest, const FT_Bi
using namespace skia_freetype_support;
#endif
static int lerp(int start, int end, int percent) {
return start + ((end - start) * percent >> 8);
static int lerp(int start, int end) {
SkASSERT((unsigned)SK_FREETYPE_LCD_LERP <= 256);
return start + ((end - start) * (SK_FREETYPE_LCD_LERP) >> 8);
}
static uint16_t packTriple(unsigned r, unsigned g, unsigned b) {
if (SK_FREETYPE_LCD_LERP) {
SkASSERT((unsigned)SK_FREETYPE_LCD_LERP <= 256);
unsigned sum = r + 2 * g + b >> 2;
r = lerp(r, sum, SK_FREETYPE_LCD_LERP);
g = lerp(g, sum, SK_FREETYPE_LCD_LERP);
b = lerp(b, sum, SK_FREETYPE_LCD_LERP);
// want (a+b+c)/3, but we approx to avoid the divide
unsigned ave = (5 * (r + g + b) + b) >> 4;
r = lerp(r, ave);
g = lerp(g, ave);
b = lerp(b, ave);
}
return SkPackRGB16(r >> 3, g >> 2, b >> 3);
}