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[base] Improve the matrix degeneracy check. 

Also fixes #1251.

* src/base/ftcalc.c (FT_Matrix_Check): To avoid overflow, scale by shifting.
* include/freetype/internal/ftcalc.h (FT_Matrix_Check): Update description.
This commit is contained in:
Alexei Podtelezhnikov 2023-08-21 23:23:22 -04:00
parent a3f44aadbc
commit 97251fd5aa
2 changed files with 27 additions and 49 deletions
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4
include/freetype/internal/ftcalc.h
View File

@ -332,9 +332,9 @@ FT_BEGIN_HEADER
* Based on geometric considerations we use the following inequality to
* identify a degenerate matrix.
*
* 50 * abs(xx*yy - xy*yx) < xx^2 + xy^2 + yx^2 + yy^2
* 32 * abs(xx*yy - xy*yx) < xx^2 + xy^2 + yx^2 + yy^2
*
* Value 50 is heuristic.
* Value 32 is heuristic.
*/
FT_BASE( FT_Bool )
FT_Matrix_Check( const FT_Matrix* matrix );

72
src/base/ftcalc.c
View File

@ -749,65 +749,43 @@
FT_BASE_DEF( FT_Bool )
FT_Matrix_Check( const FT_Matrix* matrix )
{
FT_Matrix m;
FT_Fixed val[4];
FT_Fixed nonzero_minval, maxval;
FT_Fixed temp1, temp2;
FT_UInt i;
FT_Fixed xx, xy, yx, yy;
FT_Fixed val;
FT_Int shift;
FT_ULong temp1, temp2;
if ( !matrix )
return 0;
val[0] = FT_ABS( matrix->xx );
val[1] = FT_ABS( matrix->xy );
val[2] = FT_ABS( matrix->yx );
val[3] = FT_ABS( matrix->yy );
xx = matrix->xx;
xy = matrix->xy;
yx = matrix->yx;
yy = matrix->yy;
val = FT_ABS( xx ) | FT_ABS( xy ) | FT_ABS( yx ) | FT_ABS( yy );
/*
* To avoid overflow, we ensure that each value is not larger than
*
* int(sqrt(2^31 / 4)) = 23170 ;
*
* we also check that no value becomes zero if we have to scale.
*/
maxval = 0;
nonzero_minval = FT_LONG_MAX;
for ( i = 0; i < 4; i++ )
{
if ( val[i] > maxval )
maxval = val[i];
if ( val[i] && val[i] < nonzero_minval )
nonzero_minval = val[i];
}
/* we only handle 32bit values */
if ( maxval > 0x7FFFFFFFL )
/* we only handle non-zero 32-bit values */
if ( !val || val > 0x7FFFFFFFL )
return 0;
if ( maxval > 23170 )
/* Scale matrix to avoid the temp1 overflow, which is */
/* more stringent than avoiding the temp2 overflow. */
shift = FT_MSB( val ) - 12;
if ( shift > 0 )
{
FT_Fixed scale = FT_DivFix( maxval, 23170 );
if ( !FT_DivFix( nonzero_minval, scale ) )
return 0; /* value range too large */
m.xx = FT_DivFix( matrix->xx, scale );
m.xy = FT_DivFix( matrix->xy, scale );
m.yx = FT_DivFix( matrix->yx, scale );
m.yy = FT_DivFix( matrix->yy, scale );
xx >>= shift;
xy >>= shift;
yx >>= shift;
yy >>= shift;
}
else
m = *matrix;
temp1 = FT_ABS( m.xx * m.yy - m.xy * m.yx );
temp2 = m.xx * m.xx + m.xy * m.xy + m.yx * m.yx + m.yy * m.yy;
temp1 = 32U * (FT_ULong)FT_ABS( xx * yy - xy * yx );
temp2 = (FT_ULong)( xx * xx ) + (FT_ULong)( xy * xy ) +
(FT_ULong)( yx * yx ) + (FT_ULong)( yy * yy );
if ( temp1 == 0 ||
temp2 / temp1 > 50 )
if ( temp1 <= temp2 )
return 0;
return 1;