前言：
Freetype是一个跨平台、开源的字体渲染器，网上很多文章介绍，本人就不啰嗦了。本文重点在于实现文章标题所属的各种效果，不是Freetype的基本使用方法介绍文档，所以对于Freetype不熟悉的同学们请先学习下Freetype的基本用法，才可以使用本文中所提及的方法。
正文：
用FreeType实现矢量字体的粗体、斜体、描边、阴影效果不是一件容易的事，本人认为皆因Freetype的接口太过于底层化，Freetype没有对其进行上层包装，所以要实现这些对于软件/游戏来说的基本效果，都是件挺麻烦的事情。不过，问题总是会有解决方法的，这些效果的实现，请听本人一个个道来：
1. 加粗
加粗可以使用FreeType中的一个API来实现FT_Outline_Embolden，但是这个API不支持水平垂直方向加粗量的分别设置，所以，需要参照FT_Outline_Embolden的实现重新编写一个函数，GDI++已经做了这个事情，引用它的代码：
// 就是FT_Outline_Embolden
FT_Error Old_FT_Outline_Embolden( FT_Outline* outline, FT_Pos strength )
{
FT_Vector* points;
FT_Vector v_prev, v_first, v_next, v_cur;
FT_Angle rotate, angle_in, angle_out;
FT_Int c, n, first;
FT_Int orientation;
if ( !outline )
return FT_Err_Invalid_Argument;
strength /= 2;
if ( strength == 0 )
return FT_Err_Ok;
orientation = FT_Outline_Get_Orientation( outline );
if ( orientation == FT_ORIENTATION_NONE )
{
if ( outline->n_contours )
return FT_Err_Invalid_Argument;
else
return FT_Err_Ok;
}
if ( orientation == FT_ORIENTATION_TRUETYPE )
rotate = -FT_ANGLE_PI2;
else
rotate = FT_ANGLE_PI2;
points = outline->points;
first = 0;
for ( c = 0; c < outline->n_contours; c++ )
{
int last = outline->contours[c];
v_first = points[first];
v_prev = points[last];
v_cur = v_first;
for ( n = first; n <= last; n++ )
{
FT_Vector in, out;
FT_Angle angle_diff;
FT_Pos d;
FT_Fixed scale;
if ( n < last )
v_next = points[n + 1];
else
v_next = v_first;
/* compute the in and out vectors */
in.x = v_cur.x - v_prev.x;
in.y = v_cur.y - v_prev.y;
out.x = v_next.x - v_cur.x;
out.y = v_next.y - v_cur.y;
angle_in = FT_Atan2( in.x, in.y );
angle_out = FT_Atan2( out.x, out.y );
angle_diff = FT_Angle_Diff( angle_in, angle_out );
scale = FT_Cos( angle_diff / 2 );
if ( scale < 0x4000L && scale > -0x4000L )
in.x = in.y = 0;
else
{
d = FT_DivFix( strength, scale );
FT_Vector_From_Polar( &in, d, angle_in + angle_diff / 2 - rotate );
}
outline->points[n].x = v_cur.x + strength + in.x;
//伀偙傟傪僐儊儞僩傾僂僩偟偨偩偗
//outline->points[n].y = v_cur.y + strength + in.y;
v_prev = v_cur;
v_cur = v_next;
}
first = last + 1;
}
return FT_Err_Ok;
}
// 垂直加粗
FT_Error Vert_FT_Outline_Embolden( FT_Outline* outline, FT_Pos strength )
{
FT_Vector* points;
FT_Vector v_prev, v_first, v_next, v_cur;
FT_Angle rotate, angle_in, angle_out;
FT_Int c, n, first;
FT_Int orientation;
if ( !outline )
return FT_Err_Invalid_Argument;
strength /= 2;
if ( strength == 0 )
return FT_Err_Ok;
orientation = FT_Outline_Get_Orientation( outline );
if ( orientation == FT_ORIENTATION_NONE )
{
if ( outline->n_contours )
return FT_Err_Invalid_Argument;
else
return FT_Err_Ok;
}
if ( orientation == FT_ORIENTATION_TRUETYPE )
rotate = -FT_ANGLE_PI2;
else
rotate = FT_ANGLE_PI2;
points = outline->points;
first = 0;
for ( c = 0; c < outline->n_contours; c++ )
{
int last = outline->contours[c];
v_first = points[first];
v_prev = points[last];
v_cur = v_first;
for ( n = first; n <= last; n++ )
{
FT_Vector in, out;
FT_Angle angle_diff;
FT_Pos d;
FT_Fixed scale;
if ( n < last )
v_next = points[n + 1];
else
v_next = v_first;
/* compute the in and out vectors */
in.x = v_cur.x - v_prev.x;
in.y = v_cur.y - v_prev.y;
out.x = v_next.x - v_cur.x;
out.y = v_next.y - v_cur.y;
angle_in = FT_Atan2( in.x, in.y );
angle_out = FT_Atan2( out.x, out.y );
angle_diff = FT_Angle_Diff( angle_in, angle_out );
scale = FT_Cos( angle_diff / 2 );
if ( scale < 0x4000L && scale > -0x4000L )
in.x = in.y = 0;
else
{
d = FT_DivFix( strength, scale );
FT_Vector_From_Polar( &in, d, angle_in + angle_diff / 2 - rotate );
}
//outline->points[n].x = v_cur.x + strength + in.x;
//仾偙傟傪僐儊儞僩傾僂僩偟偨偩偗
outline->points[n].y = v_cur.y + strength + in.y;
v_prev = v_cur;
v_cur = v_next;
}
first = last + 1;
}
return FT_Err_Ok;
}
// 新的加粗函数
FT_Error New_FT_Outline_Embolden( FT_Outline* outline, FT_Pos str_h, FT_Pos str_v )
{
if ( !outline ) return FT_Err_Invalid_Argument;
int orientation = FT_Outline_Get_Orientation( outline );
if ( orientation == FT_ORIENTATION_NONE )
if ( outline->n_contours ) return FT_Err_Invalid_Argument;
Vert_FT_Outline_Embolden( outline, str_v );
Old_FT_Outline_Embolden( outline, str_h );
return FT_Err_Ok;
}
// 让一个字体槽加粗，并且填充其他的大小属性
void New_GlyphSlot_Embolden( FT_GlyphSlot slot , const Vector2Float &embolden)
{
if(embolden == Vector2Float::ZERO)
return;
FT_Library library = slot->library;
FT_Face face = slot->face;
FT_Error error;
FT_Pos xstr = embolden.x, ystr = embolden.y;
if ( slot->format != FT_GLYPH_FORMAT_OUTLINE &&
slot->format != FT_GLYPH_FORMAT_BITMAP )
return;
if ( slot->format == FT_GLYPH_FORMAT_OUTLINE )
{
FT_BBox oldBox;
FT_Outline_Get_CBox(&slot->outline , &oldBox);
error = New_FT_Outline_Embolden( &slot->outline, xstr , ystr);
if ( error )
return;
FT_BBox newBox;
FT_Outline_Get_CBox(&slot->outline , &newBox);
xstr = (newBox.xMax - newBox.xMin) - (oldBox.xMax - oldBox.xMin);
ystr = (newBox.yMax - newBox.yMin) - (oldBox.yMax - oldBox.yMin);
}
else if ( slot->format == FT_GLYPH_FORMAT_BITMAP )
{
xstr = FT_PIX_FLOOR( xstr );
if ( xstr == 0 )
xstr = 1 << 6;
ystr = FT_PIX_FLOOR( ystr );
error = FT_Bitmap_Embolden( library, &slot->bitmap, xstr, ystr );
if ( error )
return;
}
if ( slot->advance.x )
slot->advance.x += xstr;
if ( slot->advance.y )
slot->advance.y += ystr;
slot->metrics.width += xstr;
slot->metrics.height += ystr;
slot->metrics.horiBearingY += ystr;
slot->metrics.horiAdvance += xstr;
slot->metrics.vertBearingX -= xstr / 2;
slot->metrics.vertBearingY += ystr;
slot->metrics.vertAdvance += ystr;
if ( slot->format == FT_GLYPH_FORMAT_BITMAP )
slot->bitmap_top += ystr >> 6;
}
FT_Error Old_FT_Outline_Embolden( FT_Outline* outline, FT_Pos strength )
{
FT_Vector* points;
FT_Vector v_prev, v_first, v_next, v_cur;
FT_Angle rotate, angle_in, angle_out;
FT_Int c, n, first;
FT_Int orientation;
if ( !outline )
return FT_Err_Invalid_Argument;
strength /= 2;
if ( strength == 0 )
return FT_Err_Ok;
orientation = FT_Outline_Get_Orientation( outline );
if ( orientation == FT_ORIENTATION_NONE )
{
if ( outline->n_contours )
return FT_Err_Invalid_Argument;
else
return FT_Err_Ok;
}
if ( orientation == FT_ORIENTATION_TRUETYPE )
rotate = -FT_ANGLE_PI2;
else
rotate = FT_ANGLE_PI2;
points = outline->points;
first = 0;
for ( c = 0; c < outline->n_contours; c++ )
{
int last = outline->contours[c];
v_first = points[first];
v_prev = points[last];
v_cur = v_first;
for ( n = first; n <= last; n++ )
{
FT_Vector in, out;
FT_Angle angle_diff;
FT_Pos d;
FT_Fixed scale;
if ( n < last )
v_next = points[n + 1];
else
v_next = v_first;
/* compute the in and out vectors */
in.x = v_cur.x - v_prev.x;
in.y = v_cur.y - v_prev.y;
out.x = v_next.x - v_cur.x;
out.y = v_next.y - v_cur.y;
angle_in = FT_Atan2( in.x, in.y );
angle_out = FT_Atan2( out.x, out.y );
angle_diff = FT_Angle_Diff( angle_in, angle_out );
scale = FT_Cos( angle_diff / 2 );
if ( scale < 0x4000L && scale > -0x4000L )
in.x = in.y = 0;
else
{
d = FT_DivFix( strength, scale );
FT_Vector_From_Polar( &in, d, angle_in + angle_diff / 2 - rotate );
}
outline->points[n].x = v_cur.x + strength + in.x;
//伀偙傟傪僐儊儞僩傾僂僩偟偨偩偗
//outline->points[n].y = v_cur.y + strength + in.y;
v_prev = v_cur;
v_cur = v_next;
}
first = last + 1;
}
return FT_Err_Ok;
}
// 垂直加粗
FT_Error Vert_FT_Outline_Embolden( FT_Outline* outline, FT_Pos strength )
{
FT_Vector* points;
FT_Vector v_prev, v_first, v_next, v_cur;
FT_Angle rotate, angle_in, angle_out;
FT_Int c, n, first;
FT_Int orientation;
if ( !outline )
return FT_Err_Invalid_Argument;
strength /= 2;
if ( strength == 0 )
return FT_Err_Ok;
orientation = FT_Outline_Get_Orientation( outline );
if ( orientation == FT_ORIENTATION_NONE )
{
if ( outline->n_contours )
return FT_Err_Invalid_Argument;
else
return FT_Err_Ok;
}
if ( orientation == FT_ORIENTATION_TRUETYPE )
rotate = -FT_ANGLE_PI2;
else
rotate = FT_ANGLE_PI2;
points = outline->points;
first = 0;
for ( c = 0; c < outline->n_contours; c++ )
{
int last = outline->contours[c];
v_first = points[first];
v_prev = points[last];
v_cur = v_first;
for ( n = first; n <= last; n++ )
{
FT_Vector in, out;
FT_Angle angle_diff;
FT_Pos d;
FT_Fixed scale;
if ( n < last )
v_next = points[n + 1];
else
v_next = v_first;
/* compute the in and out vectors */
in.x = v_cur.x - v_prev.x;
in.y = v_cur.y - v_prev.y;
out.x = v_next.x - v_cur.x;
out.y = v_next.y - v_cur.y;
angle_in = FT_Atan2( in.x, in.y );
angle_out = FT_Atan2( out.x, out.y );
angle_diff = FT_Angle_Diff( angle_in, angle_out );
scale = FT_Cos( angle_diff / 2 );
if ( scale < 0x4000L && scale > -0x4000L )
in.x = in.y = 0;
else
{
d = FT_DivFix( strength, scale );
FT_Vector_From_Polar( &in, d, angle_in + angle_diff / 2 - rotate );
}
//outline->points[n].x = v_cur.x + strength + in.x;
//仾偙傟傪僐儊儞僩傾僂僩偟偨偩偗
outline->points[n].y = v_cur.y + strength + in.y;
v_prev = v_cur;
v_cur = v_next;
}
first = last + 1;
}
return FT_Err_Ok;
}
// 新的加粗函数
FT_Error New_FT_Outline_Embolden( FT_Outline* outline, FT_Pos str_h, FT_Pos str_v )
{
if ( !outline ) return FT_Err_Invalid_Argument;
int orientation = FT_Outline_Get_Orientation( outline );
if ( orientation == FT_ORIENTATION_NONE )
if ( outline->n_contours ) return FT_Err_Invalid_Argument;
Vert_FT_Outline_Embolden( outline, str_v );
Old_FT_Outline_Embolden( outline, str_h );
return FT_Err_Ok;
}
// 让一个字体槽加粗，并且填充其他的大小属性
void New_GlyphSlot_Embolden( FT_GlyphSlot slot , const Vector2Float &embolden)
{
if(embolden == Vector2Float::ZERO)
return;
FT_Library library = slot->library;
FT_Face face = slot->face;
FT_Error error;
FT_Pos xstr = embolden.x, ystr = embolden.y;
if ( slot->format != FT_GLYPH_FORMAT_OUTLINE &&
slot->format != FT_GLYPH_FORMAT_BITMAP )
return;
if ( slot->format == FT_GLYPH_FORMAT_OUTLINE )
{
FT_BBox oldBox;
FT_Outline_Get_CBox(&slot->outline , &oldBox);
error = New_FT_Outline_Embolden( &slot->outline, xstr , ystr);
if ( error )
return;
FT_BBox newBox;
FT_Outline_Get_CBox(&slot->outline , &newBox);
xstr = (newBox.xMax - newBox.xMin) - (oldBox.xMax - oldBox.xMin);
ystr = (newBox.yMax - newBox.yMin) - (oldBox.yMax - oldBox.yMin);
}
else if ( slot->format == FT_GLYPH_FORMAT_BITMAP )
{
xstr = FT_PIX_FLOOR( xstr );
if ( xstr == 0 )
xstr = 1 << 6;
ystr = FT_PIX_FLOOR( ystr );
error = FT_Bitmap_Embolden( library, &slot->bitmap, xstr, ystr );
if ( error )
return;
}
if ( slot->advance.x )
slot->advance.x += xstr;
if ( slot->advance.y )
slot->advance.y += ystr;
slot->metrics.width += xstr;
slot->metrics.height += ystr;
slot->metrics.horiBearingY += ystr;
slot->metrics.horiAdvance += xstr;
slot->metrics.vertBearingX -= xstr / 2;
slot->metrics.vertBearingY += ystr;
slot->metrics.vertAdvance += ystr;
if ( slot->format == FT_GLYPH_FORMAT_BITMAP )
slot->bitmap_top += ystr >> 6;
}
2. 斜体
斜体在FreeType中可以通过矩阵变换来实现，只要把矩阵设置成一个切边矩阵就可以了，方法如下：
// 倾斜度，越大就越斜
float lean = 0.5f;
FT_Matrix matrix;
matrix.xx = 0x10000L;
matrix.xy = lean * 0x10000L;
matrix.yx = 0;
matrix.yy = 0x10000L;
FT_Set_Transform( face, &matrix, 0 );
float lean = 0.5f;
FT_Matrix matrix;
matrix.xx = 0x10000L;
matrix.xy = lean * 0x10000L;
matrix.yx = 0;
matrix.yy = 0x10000L;
FT_Set_Transform( face, &matrix, 0 );
3. 描边
网上有不少文章说描边其实很简单，就是上下左右各移动一个像素渲染一次，最后在中间再渲染一次就可以了。但是，这种方法只对于位图字体有效，对于矢量字体，效果就不好了，特别是大字体，1个像素只是很细的边界而已，对于很小的字体，1像素又显得太大。
这里提供另一种实现方案，使用的是Freetype的API：
- 使用FT_Stroker_New创建一个笔触
- FT_Stroker_Set设置笔触为描边
- 把Load后的glyph通过FT_Glyph_Copy拷贝一份出来
- 对这个拷贝出来的glyph使用FT_Glyph_StrokeBorder设置成描边渲染
- 使用FT_Outline_Render渲染这个描边的glyph，渲染前要设置FT_Raster_Params参数成：
FT_Raster_Params params;
memset(¶ms, 0, sizeof(params));
params.flags = FT_RASTER_FLAG_AA | FT_RASTER_FLAG_DIRECT;
params.gray_spans = RasterCallback;
memset(¶ms, 0, sizeof(params));
params.flags = FT_RASTER_FLAG_AA | FT_RASTER_FLAG_DIRECT;
params.gray_spans = RasterCallback;
- 在回调函数RasterCallback中实现像素化到位图中
- 对原来的glyph执行8操作，在回调函数RasterCallback中实现像素化到位图中，像素化过程使用alphablend的方式绘制上去
- 把位图渲染到屏幕上或保存到文件中
这个方法是Freetype的一个example，只是很少有人注意而已，源码在这里http://www.freetype.org/freetype2/docs/tutorial/example2.cpp。
12. 阴影
阴影的实现就比较简单了，只要一个个像素偏移后多渲染几次就可以了，再次不多说。
