forked from minhngoc25a/freetype2
148 lines
4.0 KiB
C
148 lines
4.0 KiB
C
/****************************************************************************
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*
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* ftsdfcommon.c
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*
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* Auxiliary data for Signed Distance Field support (body).
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*
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* Copyright (C) 2020-2021 by
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* David Turner, Robert Wilhelm, and Werner Lemberg.
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*
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* Written by Anuj Verma.
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*
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* This file is part of the FreeType project, and may only be used,
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* modified, and distributed under the terms of the FreeType project
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* license, LICENSE.TXT. By continuing to use, modify, or distribute
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* this file you indicate that you have read the license and
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* understand and accept it fully.
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*
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*/
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#include "ftsdf.h"
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#include "ftsdfcommon.h"
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/**************************************************************************
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*
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* common functions
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*
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*/
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/*
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* Original algorithm:
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*
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* https://github.com/chmike/fpsqrt
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*
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* Use this to compute the square root of a 16.16 fixed point number.
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*/
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FT_LOCAL_DEF( FT_16D16 )
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square_root( FT_16D16 val )
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{
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FT_ULong t, q, b, r;
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r = (FT_ULong)val;
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b = 0x40000000L;
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q = 0;
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while ( b > 0x40L )
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{
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t = q + b;
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if ( r >= t )
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{
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r -= t;
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q = t + b;
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}
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r <<= 1;
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b >>= 1;
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}
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q >>= 8;
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return (FT_16D16)q;
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}
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/**************************************************************************
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*
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* format and sign manipulating functions
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*
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*/
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/*
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* Convert 16.16 fixed point values to the desired output format.
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* In this case we reduce 16.16 fixed point values to normalized
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* 8-bit values.
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*
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* The `max_value` in the parameter is the maximum value in the
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* distance field map and is equal to the spread. We normalize
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* the distances using this value instead of computing the maximum
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* value for the entire bitmap.
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*
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* You can use this function to map the 16.16 signed values to any
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* format required. Do note that the output buffer is 8-bit, so only
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* use an 8-bit format for `FT_SDFFormat`, or increase the buffer size in
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* `ftsdfrend.c`.
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*/
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FT_LOCAL_DEF( FT_SDFFormat )
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map_fixed_to_sdf( FT_16D16 dist,
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FT_16D16 max_value )
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{
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FT_SDFFormat out;
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FT_16D16 udist;
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/* normalize the distance values */
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dist = FT_DivFix( dist, max_value );
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udist = dist < 0 ? -dist : dist;
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/* Reduce the distance values to 8 bits. */
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/* */
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/* Since +1/-1 in 16.16 takes the 16th bit, we right-shift */
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/* the number by 9 to make it fit into the 7-bit range. */
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/* */
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/* One bit is reserved for the sign. */
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udist >>= 9;
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/* Since `char` can only store a maximum positive value */
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/* of 127 we need to make sure it does not wrap around and */
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/* give a negative value. */
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if ( dist > 0 && udist > 127 )
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udist = 127;
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if ( dist < 0 && udist > 128 )
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udist = 128;
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/* Output the data; negative values are from [0, 127] and positive */
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/* from [128, 255]. One important thing is that negative values */
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/* are inverted here, that means [0, 128] maps to [-128, 0] linearly. */
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/* More on that in `freetype.h` near the documentation of */
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/* `FT_RENDER_MODE_SDF`. */
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out = dist < 0 ? 128 - (FT_SDFFormat)udist
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: (FT_SDFFormat)udist + 128;
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return out;
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}
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/*
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* Invert the signed distance packed into the corresponding format.
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* So if the values are negative they will become positive in the
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* chosen format.
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*
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* [Note]: This function should only be used after converting the
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* 16.16 signed distance values to `FT_SDFFormat`. If that
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* conversion has not been done, then simply invert the sign
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* and use the above function to pack the values.
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*/
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FT_LOCAL_DEF( FT_SDFFormat )
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invert_sign( FT_SDFFormat dist )
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{
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return 255 - dist;
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}
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/* END */
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