aboutsummaryrefslogtreecommitdiff
path: root/freetype/include/ftlcdfil.h
blob: 439ddfd511be434d6d79fb5a637045dd736dfd5d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
/***************************************************************************/
/*                                                                         */
/*  ftlcdfil.h                                                             */
/*                                                                         */
/*    FreeType API for color filtering of subpixel bitmap glyphs           */
/*    (specification).                                                     */
/*                                                                         */
/*  Copyright 2006-2008, 2010, 2013, 2014 by                               */
/*  David Turner, Robert Wilhelm, and Werner Lemberg.                      */
/*                                                                         */
/*  This file is part of the FreeType project, and may only be used,       */
/*  modified, and distributed under the terms of the FreeType project      */
/*  license, LICENSE.TXT.  By continuing to use, modify, or distribute     */
/*  this file you indicate that you have read the license and              */
/*  understand and accept it fully.                                        */
/*                                                                         */
/***************************************************************************/


#ifndef __FT_LCD_FILTER_H__
#define __FT_LCD_FILTER_H__

#include <ft2build.h>
#include <freetype.h>

#ifdef FREETYPE_H
#error "freetype.h of FreeType 1 has been loaded!"
#error "Please fix the directory search order for header files"
#error "so that freetype.h of FreeType 2 is found first."
#endif


FT_BEGIN_HEADER

  /***************************************************************************
   *
   * @section:
   *   lcd_filtering
   *
   * @title:
   *   LCD Filtering
   *
   * @abstract:
   *   Reduce color fringes of LCD-optimized bitmaps.
   *
   * @description:
   *   The @FT_Library_SetLcdFilter API can be used to specify a low-pass
   *   filter, which is then applied to LCD-optimized bitmaps generated
   *   through @FT_Render_Glyph.  This is useful to reduce color fringes
   *   that would occur with unfiltered rendering.
   *
   *   Note that no filter is active by default, and that this function is
   *   *not* implemented in default builds of the library.  You need to
   *   #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file
   *   in order to activate it.
   *
   *   FreeType generates alpha coverage maps, which are linear by nature.
   *   For instance, the value 0x80 in bitmap representation means that
   *   (within numerical precision) 0x80/0xFF fraction of that pixel is
   *   covered by the glyph's outline.  The blending function for placing
   *   text over a background is
   *
   *   {
   *     dst = alpha * src + (1 - alpha) * dst    ,
   *   }
   *
   *   which is known as OVER.  However, when calculating the output of the
   *   OVER operator, the source colors should first be transformed to a
   *   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
   *
   *   {
   *     [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
   *
   *   {
   *     [0x10, 0x50, 0x60, 0x50, 0x10]    ,
   *   }
   *
   *   where `a' has value 0x30 and `b' value 0x20.  The weights in filter
   *   may have a sum larger than 0x100, which increases coloration slightly
   *   but also improves contrast.
   */


  /****************************************************************************
   *
   * @enum:
   *   FT_LcdFilter
   *
   * @description:
   *   A list of values to identify various types of LCD filters.
   *
   * @values:
   *   FT_LCD_FILTER_NONE ::
   *     Do not perform filtering.  When used with subpixel rendering, this
   *     results in sometimes severe color fringes.
   *
   *   FT_LCD_FILTER_DEFAULT ::
   *     The default filter reduces color fringes considerably, at the cost
   *     of a slight blurriness in the output.
   *
   *   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.
   *
   *   FT_LCD_FILTER_LEGACY ::
   *     This filter corresponds to the original libXft color filter.  It
   *     provides high contrast output but can exhibit really bad color
   *     fringes if glyphs are not extremely well hinted to the pixel grid.
   *     In other words, it only works well if the TrueType bytecode
   *     interpreter is enabled *and* high-quality hinted fonts are used.
   *
   *     This filter is only provided for comparison purposes, and might be
   *     disabled or stay unsupported in the future.
   *
   * @since:
   *   2.3.0
   */
  typedef enum  FT_LcdFilter_
  {
    FT_LCD_FILTER_NONE    = 0,
    FT_LCD_FILTER_DEFAULT = 1,
    FT_LCD_FILTER_LIGHT   = 2,
    FT_LCD_FILTER_LEGACY  = 16,

    FT_LCD_FILTER_MAX   /* do not remove */

  } FT_LcdFilter;


  /**************************************************************************
   *
   * @func:
   *   FT_Library_SetLcdFilter
   *
   * @description:
   *   This function is used to apply color filtering to LCD decimated
   *   bitmaps, like the ones used when calling @FT_Render_Glyph with
   *   @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V.
   *
   * @input:
   *   library ::
   *     A handle to the target library instance.
   *
   *   filter ::
   *     The filter type.
   *
   *     You can use @FT_LCD_FILTER_NONE here to disable this feature, or
   *     @FT_LCD_FILTER_DEFAULT to use a default filter that should work
   *     well on most LCD screens.
   *
   * @return:
   *   FreeType error code.  0~means success.
   *
   * @note:
   *   This feature is always disabled by default.  Clients must make an
   *   explicit call to this function with a `filter' value other than
   *   @FT_LCD_FILTER_NONE in order to enable it.
   *
   *   Due to *PATENTS* covering subpixel rendering, this function doesn't
   *   do anything except returning `FT_Err_Unimplemented_Feature' if the
   *   configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
   *   defined in your build of the library, which should correspond to all
   *   default builds of FreeType.
   *
   *   The filter affects glyph bitmaps rendered through @FT_Render_Glyph,
   *   @FT_Outline_Get_Bitmap, @FT_Load_Glyph, and @FT_Load_Char.
   *
   *   It does _not_ affect the output of @FT_Outline_Render and
   *   @FT_Outline_Get_Bitmap.
   *
   *   If this feature is activated, the dimensions of LCD glyph bitmaps are
   *   either larger or taller than the dimensions of the corresponding
   *   outline with regards to the pixel grid.  For example, for
   *   @FT_RENDER_MODE_LCD, the filter adds up to 3~pixels to the left, and
   *   up to 3~pixels to the right.
   *
   *   The bitmap offset values are adjusted correctly, so clients shouldn't
   *   need to modify their layout and glyph positioning code when enabling
   *   the filter.
   *
   * @since:
   *   2.3.0
   */
  FT_EXPORT( FT_Error )
  FT_Library_SetLcdFilter( FT_Library    library,
                           FT_LcdFilter  filter );


  /**************************************************************************
   *
   * @func:
   *   FT_Library_SetLcdFilterWeights
   *
   * @description:
   *   Use this function to override the filter weights selected by
   *   @FT_Library_SetLcdFilter.  By default, FreeType uses the quintuple
   *   (0x00, 0x55, 0x56, 0x55, 0x00) for FT_LCD_FILTER_LIGHT, and (0x10,
   *   0x40, 0x70, 0x40, 0x10) for FT_LCD_FILTER_DEFAULT and
   *   FT_LCD_FILTER_LEGACY.
   *
   * @input:
   *   library ::
   *     A handle to the target library instance.
   *
   *   weights ::
   *     A pointer to an array; the function copies the first five bytes and
   *     uses them to specify the filter weights.
   *
   * @return:
   *   FreeType error code.  0~means success.
   *
   * @note:
   *   Due to *PATENTS* covering subpixel rendering, this function doesn't
   *   do anything except returning `FT_Err_Unimplemented_Feature' if the
   *   configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
   *   defined in your build of the library, which should correspond to all
   *   default builds of FreeType.
   *
   *   This function must be called after @FT_Library_SetLcdFilter to have
   *   any effect.
   *
   * @since:
   *   2.4.0
   */
  FT_EXPORT( FT_Error )
  FT_Library_SetLcdFilterWeights( FT_Library      library,
                                  unsigned char  *weights );

  /* */


FT_END_HEADER

#endif /* __FT_LCD_FILTER_H__ */


/* END */