This ;) (Comments on my comments welcome -- still not sure what to do
about e.g. the color-balancing formula)
2015-11-26 20:54 GMT+01:00 Werner LEMBERG <w...@gnu.org>:
>>> This sounds like a good compromise, thanks.
>>>
>>> Can we all agree on this filter as the default for `light_filter'?
>>>
>>> Alexei, what value should `default_filter' have?
>>
>> Please leave the light one alone. Dave's filter should be default.
>
> So *what* shall I change?
>
>
> Werner
>
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From e9b3098dbbcfe7a313982cb47ab8689eb331e2df Mon Sep 17 00:00:00 2001
From: Nikolaus Waxweiler <madig...@gmail.com>
Date: Thu, 26 Nov 2015 22:52:19 +0100
Subject: [PATCH] Change default LCD filter to normalized, color-balanced
---
include/freetype/ftlcdfil.h | 39 +++++++++++++++++++++++++--------------
src/base/ftlcdfil.c | 6 ++----
2 files changed, 27 insertions(+), 18 deletions(-)
diff --git a/include/freetype/ftlcdfil.h b/include/freetype/ftlcdfil.h
index 3a62528..eb48513 100644
--- a/include/freetype/ftlcdfil.h
+++ b/include/freetype/ftlcdfil.h
@@ -69,28 +69,28 @@ FT_BEGIN_HEADER
* linear color space, then alpha blended in that space, and transformed
* back to the output color space.
*
- * When linear light blending is used, the default FIR5 filtering
- * weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as
- * they have been designed for black on white rendering while lacking
- * gamma correction. To preserve color neutrality, weights for a FIR5
- * filter should be chosen according to two free parameters `a' and `c',
- * and the FIR weights should be
+ * To preserve color neutrality and reduce color fringing to a
+ * minimum, weights for a FIR5 filter should be chosen according to
+ * two free parameters `a' and `c', and the FIR weights should be
*
* {
* [a - c, a + c, 2 * a, a + c, a - c] .
* }
*
* This formula generates equal weights for all the color primaries
- * across the filter kernel, which makes it colorless. One suggested
- * set of weights is
+ * across the filter kernel, which makes it colorless or
+ * color-balanced. One possible set of weights is
*
* {
* [0x10, 0x50, 0x60, 0x50, 0x10] ,
* }
*
- * where `a' has value 0x30 and `c' value 0x20. The weights in filter
- * may have a sum larger than 0x100, which increases coloration slightly
- * but also improves contrast.
+ * where `a' has value 0x30 and `c' value 0x20, however the sum goes
+ * above 0x100, leading to non-linearities and increased
+ * color-fringing. This can be used to increase contrast at the cost
+ * of colorfulness, but stem darkening achieves contrast much better if
+ * the rendering system can do linear alpha blending and gamma
+ * correction.
*/
@@ -111,10 +111,21 @@ FT_BEGIN_HEADER
* The default filter reduces color fringes considerably, at the cost
* of a slight blurriness in the output.
*
+ * It is a normalized and color-balanced five-tap filter that is more
+ * forgiving to screens with non-ideal gamma curves and viewing
+ * angles. Note that while color-fringing is reduced, it can only be
+ * minimized by using linear alpha blending and gamma correction to
+ * render glyphs onto surfaces.
+ *
* FT_LCD_FILTER_LIGHT ::
- * The light filter is a variant that produces less blurriness at the
- * cost of slightly more color fringes than the default one. It might
- * be better, depending on taste, your monitor, or your personal vision.
+ * The light filter is a variant that is sharper at the cost of
+ * slightly more color fringes than the default one.
+ *
+ * It is a normalized and color-balanced three-tap filter that is less
+ * forgiving to screens with non-ideal gamma curves and viewing
+ * angles. This filter works best when the rendering system uses
+ * linear alpha blending and gamma correction to render glyphs onto
+ * surfaces.
*
* FT_LCD_FILTER_LEGACY ::
* This filter corresponds to the original libXft color filter. It
diff --git a/src/base/ftlcdfil.c b/src/base/ftlcdfil.c
index b50383c..5ee7e0a 100644
--- a/src/base/ftlcdfil.c
+++ b/src/base/ftlcdfil.c
@@ -305,12 +305,10 @@
FT_Library_SetLcdFilter( FT_Library library,
FT_LcdFilter filter )
{
+ static const FT_Byte default_filter[5] =
+ { 0x08, 0x4d, 0x56, 0x4d, 0x08 };
static const FT_Byte light_filter[5] =
{ 0x00, 0x55, 0x56, 0x55, 0x00 };
- /* the values here sum up to a value larger than 256, */
- /* providing a cheap gamma correction */
- static const FT_Byte default_filter[5] =
- { 0x10, 0x40, 0x70, 0x40, 0x10 };
if ( !library )
--
2.5.0
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