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 /**************************************************************************** * * ftsdfcommon.c * * Auxiliary data for Signed Distance Field support (body). * * Copyright (C) 2020-2022 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. * */ #include "ftsdf.h" #include "ftsdfcommon.h" /************************************************************************** * * common functions * */ /* * Original algorithm: * * https://github.com/chmike/fpsqrt * * Use this to compute the square root of a 16.16 fixed point number. */ FT_LOCAL_DEF( 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 values to the desired output format. * In this case we reduce 16.16 fixed point values 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 an 8-bit format for `FT_SDFFormat`, or increase the buffer size in * `ftsdfrend.c`. */ FT_LOCAL_DEF( 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. */ /* */ /* Since +1/-1 in 16.16 takes the 16th bit, we right-shift */ /* the number by 9 to make it fit into the 7-bit range. */ /* */ /* One bit is reserved for the sign. */ udist >>= 9; /* Since `char` can only store a maximum 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 the documentation of */ /* `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. */ FT_LOCAL_DEF( FT_SDFFormat ) invert_sign( FT_SDFFormat dist ) { return 255 - dist; } /* END */