[bsdf] Added function to approximate edge distances.
* src/sdf/ftbsdf.c (compute_edge_distance): Added function to approximate edges given only the array of alpha values representing pixel coverage. This function uses the Gustavson's algorithm for approximating. * src/sdf/ftbsdf.c (bsdf_approximate_edge): This function loops through the entire bitmap and for edge pixel (found using `bsdf_is_edge') compute approximate edge distances (using `compute_edge_distance').
This commit is contained in:
Anuj Verma
parent
44e9d12f0f
commit
e883c6dc50
245
src/sdf/ftbsdf.c
245
src/sdf/ftbsdf.c
@ -234,5 +234,250 @@
|
||||
|
||||
#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 distances. This parameter is an array of Euclidean
|
||||
* distances. The `current' must point to the position for which
|
||||
* the distance is to be caculated. We treat this array as a 2D
|
||||
* array mapped to a 1D 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:
|
||||
* FT_16D16_Vec ::
|
||||
* 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 is the function which is based on the paper presented */
|
||||
/* by Stefan Gustavson and Robin Strand which is used to app- */
|
||||
/* roximate edge distance from anti-aliased bitmaps. */
|
||||
/* */
|
||||
/* The algorithm is as follows: */
|
||||
/* */
|
||||
/* * 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 then we can assume the the outline */
|
||||
/* passes through the center of the pixel. */
|
||||
/* */
|
||||
/* * So, we can use that alpha value to approximate the real */
|
||||
/* distance of the pixel to edge pretty accurately. A real */
|
||||
/* simple approximation is ( 0.5f - alpha ), assuming that */
|
||||
/* the outline is parallel to the x or y axis. But in this */
|
||||
/* algorithm we use a different approximation which is qui- */
|
||||
/* te accurate even for non axis aligned edges. */
|
||||
/* */
|
||||
/* * 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 the 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. */
|
||||
/* */
|
||||
/* * 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 nothing but root(2) in 16.16 */
|
||||
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:
|
||||
* This is a handy function which loops through all the pixels, and
|
||||
* calls `compute_edge_distance' function only for edge pixels. This
|
||||
* maked the process a lot faster since `compute_edge_distance' uses
|
||||
* some functions such as `FT_Vector_NormLen' which are quite slow.
|
||||
*
|
||||
* @Input:
|
||||
* worker ::
|
||||
* Contains the distance map as well as all the relevant parameters
|
||||
* required by the function.
|
||||
*
|
||||
* @Return:
|
||||
* FT_Error ::
|
||||
* FreeType error, 0 means success.
|
||||
*
|
||||
* @Note:
|
||||
* The function dosen't have any actual output, it do computation on
|
||||
* the `distance_map' parameter of the `worker' and put the data in
|
||||
* that distance map itself.
|
||||
*
|
||||
*/
|
||||
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 ) )
|
||||
{
|
||||
/* for edge pixels approximate the edge distance */
|
||||
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 */
|
||||
|
||||
Loading…
Reference in New Issue
Block a user