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Diffstat (limited to 'src/sdf/ftsdfcommon.c')
-rw-r--r-- | src/sdf/ftsdfcommon.c | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/src/sdf/ftsdfcommon.c b/src/sdf/ftsdfcommon.c new file mode 100644 index 000000000..031f2bab8 --- /dev/null +++ b/src/sdf/ftsdfcommon.c @@ -0,0 +1,147 @@ +/**************************************************************************** + * + * ftsdfcommon.c + * + * Auxiliary data for Signed Distance Field support (body). + * + * 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. + * + */ + + +#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 */ |