ref: 0f644f38e939f3fa82a2a91e94d53c6e05750e44
dir: /src/sdf/ftbsdf.c/
#include <freetype/internal/ftobjs.h>
#include <freetype/internal/ftdebug.h>
#include <freetype/internal/ftmemory.h>
#include <freetype/fttrigon.h>
#include "ftsdf.h"
#include "ftsdferrs.h"
#include "ftsdfcommon.h"
/**************************************************************************
*
* useful macros
*
*/
#define ONE 65536 /* 1 in 16.16 */
/**************************************************************************
*
* structs
*
*/
/**************************************************************************
*
* @Struct:
* BSDF_TRaster
*
* @Description:
* This struct is used in place of @FT_Raster and is stored within the
* internal FreeType renderer struct. While rasterizing this is passed
* to the @FT_Raster_RenderFunc function, which then can be used however
* we want.
*
* @Fields:
* memory ::
* Used internally to allocate intermediate memory while raterizing.
*
*/
typedef struct BSDF_TRaster_
{
FT_Memory memory;
} BSDF_TRaster;
/**************************************************************************
*
* @Struct:
* ED
*
* @Description:
* Euclidean distance. It gets used for Euclidean distance transforms;
* it can also be interpreted as an edge distance.
*
* @Fields:
* dist ::
* Vector length of the `near` parameter. Can be squared or absolute
* depending on the `USE_SQUARED_DISTANCES` macro defined in file
* `ftsdfcommon.h`.
*
* near ::
* Vector to the nearest edge. Can also be interpreted as shortest
* distance of a point.
*
* alpha ::
* Alpha value of the original bitmap from which we generate SDF.
* Needed for computing the gradient and determining the proper sign
* of a pixel.
*
*/
typedef struct ED_
{
FT_16D16 dist;
FT_16D16_Vec near;
FT_Byte alpha;
} ED;
/**************************************************************************
*
* @Struct:
* BSDF_Worker
*
* @Description:
* A convenience struct that is passed to functions while generating
* SDF; most of those functions require the same parameters.
*
* @Fields:
* distance_map ::
* A one-dimensional array that gets interpreted as two-dimensional
* one. It contains the Euclidean distances of all points of the
* bitmap.
*
* width ::
* Width of the above `distance_map`.
*
* rows ::
* Number of rows in the above `distance_map`.
*
* params ::
* Internal parameters and properties required by the rasterizer. See
* file `ftsdf.h` for more.
*
*/
typedef struct BSDF_Worker_
{
ED* distance_map;
FT_Int width;
FT_Int rows;
SDF_Raster_Params params;
} BSDF_Worker;
/**************************************************************************
*
* initializer
*
*/
static const ED zero_ed = { 0, { 0, 0 }, 0 };
/**************************************************************************
*
* rasterizer functions
*
*/
/**************************************************************************
*
* @Function:
* bsdf_is_edge
*
* @Description:
* Check whether a pixel is an edge pixel, i.e., whether it is
* surrounded by a completely black pixel (zero alpha), and the current
* pixel is not a completely black pixel.
*
* @Input:
* dm ::
* Array of distances. The parameter must point to the current
* pixel, i.e., the pixel that is to be checked for being an edge.
*
* x ::
* The x position of the current pixel.
*
* y ::
* The y position of the current pixel.
*
* w ::
* Width of the bitmap.
*
* r ::
* Number of rows in the bitmap.
*
* @Return:
* 1~if the current pixel is an edge pixel, 0~otherwise.
*
*/
#ifdef CHECK_NEIGHBOR
#undef CHECK_NEIGHBOR
#endif
#define CHECK_NEIGHBOR( x_offset, y_offset ) \
if ( x + x_offset >= 0 && x + x_offset < w && \
y + y_offset >= 0 && y + y_offset < r ) \
{ \
num_neighbors++; \
\
to_check = dm + y_offset * w + x_offset; \
if ( to_check->alpha == 0 ) \
{ \
is_edge = 1; \
goto Done; \
} \
}
static FT_Bool
bsdf_is_edge( ED* dm, /* distance map */
FT_Int x, /* x index of point to check */
FT_Int y, /* y index of point to check */
FT_Int w, /* width */
FT_Int r ) /* rows */
{
FT_Bool is_edge = 0;
ED* to_check = NULL;
FT_Int num_neighbors = 0;
if ( dm->alpha == 0 )
goto Done;
if ( dm->alpha > 0 && dm->alpha < 255 )
{
is_edge = 1;
goto Done;
}
/* up */
CHECK_NEIGHBOR( 0, -1 );
/* down */
CHECK_NEIGHBOR( 0, 1 );
/* left */
CHECK_NEIGHBOR( -1, 0 );
/* right */
CHECK_NEIGHBOR( 1, 0 );
/* up left */
CHECK_NEIGHBOR( -1, -1 );
/* up right */
CHECK_NEIGHBOR( 1, -1 );
/* down left */
CHECK_NEIGHBOR( -1, 1 );
/* down right */
CHECK_NEIGHBOR( 1, 1 );
if ( num_neighbors != 8 )
is_edge = 1;
Done:
return is_edge;
}
#undef CHECK_NEIGHBOR
/**************************************************************************
*
* @Function:
* compute_edge_distance
*
* @Description:
* Approximate the outline and compute the distance from `current`
* to the approximated outline.
*
* @Input:
* current ::
* Array of Euclidean distances. `current` must point to the position
* for which the distance is to be caculated. We treat this array as
* a two-dimensional array mapped to a one-dimensional array.
*
* x ::
* The x coordinate of the `current` parameter in the array.
*
* y ::
* The y coordinate of the `current` parameter in the array.
*
* w ::
* The width of the distances array.
*
* r ::
* Number of rows in the distances array.
*
* @Return:
* A vector pointing to the approximate edge distance.
*
* @Note:
* This is a computationally expensive function. Try to reduce the
* number of calls to this function. Moreover, this must only be used
* for edge pixel positions.
*
*/
static FT_16D16_Vec
compute_edge_distance( ED* current,
FT_Int x,
FT_Int y,
FT_Int w,
FT_Int r )
{
/*
* This function, based on the paper presented by Stefan Gustavson and
* Robin Strand, gets used to approximate edge distances from
* anti-aliased bitmaps.
*
* The algorithm is as follows.
*
* (1) In anti-aliased images, the pixel's alpha value is the coverage
* of the pixel by the outline. For example, if the alpha value is
* 0.5f we can assume that the outline passes through the center of
* the pixel.
*
* (2) For this reason we can use that alpha value to approximate the real
* distance of the pixel to edge pretty accurately. A simple
* approximation is `(0.5f - alpha)`, assuming that the outline is
* parallel to the x or y~axis. However, in this algorithm we use a
* different approximation which is quite accurate even for
* non-axis-aligned edges.
*
* (3) The only remaining piece of information that we cannot
* approximate directly from the alpha is the direction of the edge.
* This is where we use Sobel's operator to compute the gradient of
* the pixel. The gradient give us a pretty good approximation of
* the edge direction. We use a 3x3 kernel filter to compute the
* gradient.
*
* (4) After the above two steps we have both the direction and the
* distance to the edge which is used to generate the Signed
* Distance Field.
*
* References:
*
* - Anti-Aliased Euclidean Distance Transform:
* http://weber.itn.liu.se/~stegu/aadist/edtaa_preprint.pdf
* - Sobel Operator:
* https://en.wikipedia.org/wiki/Sobel_operator
*/
FT_16D16_Vec g = { 0, 0 };
FT_16D16 dist, current_alpha;
FT_16D16 a1, temp;
FT_16D16 gx, gy;
FT_16D16 alphas[9];
/* Since our spread cannot be 0, this condition */
/* can never be true. */
if ( x <= 0 || x >= w - 1 ||
y <= 0 || y >= r - 1 )
return g;
/* initialize the alphas */
alphas[0] = 256 * (FT_16D16)current[-w - 1].alpha;
alphas[1] = 256 * (FT_16D16)current[-w ].alpha;
alphas[2] = 256 * (FT_16D16)current[-w + 1].alpha;
alphas[3] = 256 * (FT_16D16)current[ -1].alpha;
alphas[4] = 256 * (FT_16D16)current[ 0].alpha;
alphas[5] = 256 * (FT_16D16)current[ 1].alpha;
alphas[6] = 256 * (FT_16D16)current[ w - 1].alpha;
alphas[7] = 256 * (FT_16D16)current[ w ].alpha;
alphas[8] = 256 * (FT_16D16)current[ w + 1].alpha;
current_alpha = alphas[4];
/* Compute the gradient using the Sobel operator. */
/* In this case we use the following 3x3 filters: */
/* */
/* For x: | -1 0 -1 | */
/* | -root(2) 0 root(2) | */
/* | -1 0 1 | */
/* */
/* For y: | -1 -root(2) -1 | */
/* | 0 0 0 | */
/* | 1 root(2) 1 | */
/* */
/* [Note]: 92681 is root(2) in 16.16 format. */
g.x = -alphas[0] -
FT_MulFix( alphas[3], 92681 ) -
alphas[6] +
alphas[2] +
FT_MulFix( alphas[5], 92681 ) +
alphas[8];
g.y = -alphas[0] -
FT_MulFix( alphas[1], 92681 ) -
alphas[2] +
alphas[6] +
FT_MulFix( alphas[7], 92681 ) +
alphas[8];
FT_Vector_NormLen( &g );
/* The gradient gives us the direction of the */
/* edge for the current pixel. Once we have the */
/* approximate direction of the edge, we can */
/* approximate the edge distance much better. */
if ( g.x == 0 || g.y == 0 )
dist = ONE / 2 - alphas[4];
else
{
gx = g.x;
gy = g.y;
gx = FT_ABS( gx );
gy = FT_ABS( gy );
if ( gx < gy )
{
temp = gx;
gx = gy;
gy = temp;
}
a1 = FT_DivFix( gy, gx ) / 2;
if ( current_alpha < a1 )
dist = ( gx + gy ) / 2 -
square_root( 2 * FT_MulFix( gx,
FT_MulFix( gy,
current_alpha ) ) );
else if ( current_alpha < ( ONE - a1 ) )
dist = FT_MulFix( ONE / 2 - current_alpha, gx );
else
dist = -( gx + gy ) / 2 +
square_root( 2 * FT_MulFix( gx,
FT_MulFix( gy,
ONE - current_alpha ) ) );
}
g.x = FT_MulFix( g.x, dist );
g.y = FT_MulFix( g.y, dist );
return g;
}
/**************************************************************************
*
* @Function:
* bsdf_approximate_edge
*
* @Description:
* Loops over all the pixels and call `compute_edge_distance` only for
* edge pixels. This maked the process a lot faster since
* `compute_edge_distance` uses functions such as `FT_Vector_NormLen',
* which are quite slow.
*
* @InOut:
* worker ::
* Contains the distance map as well as all the relevant parameters
* required by the function.
*
* @Return:
* FreeType error, 0 means success.
*
* @Note:
* The function directly manipulates `worker->distance_map`.
*
*/
static FT_Error
bsdf_approximate_edge( BSDF_Worker* worker )
{
FT_Error error = FT_Err_Ok;
FT_Int i, j;
FT_Int index;
ED* ed;
if ( !worker || !worker->distance_map )
{
error = FT_THROW( Invalid_Argument );
goto Exit;
}
ed = worker->distance_map;
for ( j = 0; j < worker->rows; j++ )
{
for ( i = 0; i < worker->width; i++ )
{
index = j * worker->width + i;
if ( bsdf_is_edge( worker->distance_map + index,
i, j,
worker->width,
worker->rows ) )
{
/* approximate the edge distance for edge pixels */
ed[index].near = compute_edge_distance( ed + index,
i, j,
worker->width,
worker->rows );
ed[index].dist = VECTOR_LENGTH_16D16( ed[index].near );
}
else
{
/* for non-edge pixels assign far away distances */
ed[index].dist = 400 * ONE;
ed[index].near.x = 200 * ONE;
ed[index].near.y = 200 * ONE;
}
}
}
Exit:
return error;
}
/* END */