ref: 36ee71714f5f84876cd3e9b479dbbd72a5124412
dir: /src/sdf/ftsdfcommon.h/
/****************************************************************************
*
* ftsdfcommon.h
*
* Auxiliary data for Signed Distance Field support (specification only).
*
* Copyright (C) 2020-2021 by
* David Turner, Robert Wilhelm, and Werner Lemberg.
*
* Written by Anuj Verma.
*
* 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.
*
*/
/****************************************************
*
* This file contains common functions and properties
* for both the 'sdf' and 'bsdf' renderers.
*
*/
#ifndef FTSDFCOMMON_H_
#define FTSDFCOMMON_H_
#include <ft2build.h>
#include FT_CONFIG_CONFIG_H
#include <freetype/freetype.h>
FT_BEGIN_HEADER
/**************************************************************************
*
* default values (cannot be set individually for each renderer)
*
*/
/* default spread value */
#define DEFAULT_SPREAD 8
/* minimum spread supported by the renderer */
#define MIN_SPREAD 2
/* maximum spread supported by the renderer */
#define MAX_SPREAD 32
/**************************************************************************
*
* common definitions (cannot be set individually for each renderer)
*
*/
/* If this macro is set to 1 the rasterizer uses squared distances for */
/* computation. It can greatly improve the performance but there is a */
/* chance of overflow and artifacts. You can safely use it up to a */
/* pixel size of 128. */
#ifndef USE_SQUARED_DISTANCES
#define USE_SQUARED_DISTANCES 0
#endif
/**************************************************************************
*
* common macros
*
*/
/* convert int to 26.6 fixed-point */
#define FT_INT_26D6( x ) ( x * 64 )
/* convert int to 16.16 fixed-point */
#define FT_INT_16D16( x ) ( x * 65536 )
/* convert 26.6 to 16.16 fixed-point */
#define FT_26D6_16D16( x ) ( x * 1024 )
/* Convenience macro to call a function; it */
/* jumps to label `Exit` if an error occurs. */
#define FT_CALL( x ) do \
{ \
error = ( x ); \
if ( error != FT_Err_Ok ) \
goto Exit; \
} while ( 0 )
/*
* The macro `VECTOR_LENGTH_16D16` computes either squared distances or
* actual distances, depending on the value of `USE_SQUARED_DISTANCES`.
*
* By using squared distances the performance can be greatly improved but
* there is a risk of overflow.
*/
#if USE_SQUARED_DISTANCES
#define VECTOR_LENGTH_16D16( v ) ( FT_MulFix( v.x, v.x ) + \
FT_MulFix( v.y, v.y ) )
#else
#define VECTOR_LENGTH_16D16( v ) FT_Vector_Length( &v )
#endif
/**************************************************************************
*
* common typedefs
*
*/
typedef FT_Vector FT_26D6_Vec; /* with 26.6 fixed-point components */
typedef FT_Vector FT_16D16_Vec; /* with 16.16 fixed-point components */
typedef FT_Fixed FT_16D16; /* 16.16 fixed-point representation */
typedef FT_Fixed FT_26D6; /* 26.6 fixed-point representation */
typedef FT_Byte FT_SDFFormat; /* format to represent SDF data */
typedef FT_BBox FT_CBox; /* control box of a curve */
/**************************************************************************
*
* common functions
*
*/
/*
* Original algorithm:
*
* https://github.com/chmike/fpsqrt
*
* Use this to compute the square root of a 16.16 fixed point number.
*/
static FT_16D16
square_root( FT_16D16 val )
{
FT_ULong t, q, b, r;
r = (FT_ULong)val;
b = 0x40000000L;
q = 0;
while ( b > 0x40L )
{
t = q + b;
if ( r >= t )
{
r -= t;
q = t + b;
}
r <<= 1;
b >>= 1;
}
q >>= 8;
return (FT_16D16)q;
}
/**************************************************************************
*
* format and sign manipulating functions
*
*/
/*
* Convert 16.16 fixed point value to the desired output format.
* In this case we reduce 16.16 fixed point value to normalized
* 8-bit values.
* The `max_value` in the parameter is the maximum value in the
* distance field map and is equal to the spread. We normalize
* the distances using this value instead of computing the maximum
* value for the entire bitmap.
* You can use this function to map the 16.16 signed values to any
* format required. Do note that the output buffer is 8-bit, so only
* use 8-bit format for `FT_SDFFormat`, or increase buffer size in
* `ftsdfrend.c`.
*/
static FT_SDFFormat
map_fixed_to_sdf( FT_16D16 dist, FT_16D16 max_value )
{
FT_SDFFormat out;
FT_16D16 udist;
/* normalize the distance values */
dist = FT_DivFix( dist, max_value );
udist = dist < 0 ? -dist : dist;
/* Reduce the distance values to 8 bits, +1/-1 in */
/* 16.16 takes the 16th bit. So we right shift the */
/* number by 9 to make it fit in the 7 bit range. */
/* 1 bit is reserved for the sign. */
udist >>= 9;
/* Since char can only store max positive value */
/* of 127 we need to make sure it does not wrap */
/* around and give a negative value. */
if ( dist > 0 && udist > 127 )
udist = 127;
if ( dist < 0 && udist > 128 )
udist = 128;
/* Output the data; negative values are from [0, 127] and positive */
/* from [128, 255]. One important thing is that negative values */
/* are inverted here, that means [0, 128] maps to [-128, 0] linearly. */
/* More on that in `freetype.h` near `FT_RENDER_MODE_SDF` */
out = dist < 0 ? 128 - (FT_SDFFormat)udist :
(FT_SDFFormat)udist + 128;
return out;
}
/*
* Invert the signed distance packed into the corresponding format.
* So if the values are negative they will become positive in the
* chosen format.
*
* [Note]: This function should only be used after converting the
* 16.16 signed distance values to `FT_SDFFormat`, if that
* conversion has not been done, then simply invert the sign
* and use the above function to pack the values.
*/
static FT_SDFFormat
invert_sign( FT_SDFFormat dist ) {
return 255 - dist;
}
FT_END_HEADER
#endif /* FTSDFCOMMON_H_ */
/* END */