Commit 97251fd5aa2a90041cf4f397a5e887b8d60ab0c2
2023-08-21T23:23:22
[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.
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diff --git a/include/freetype/internal/ftcalc.h b/include/freetype/internal/ftcalc.h
index d1baa39..d9aea23 100644
--- a/include/freetype/internal/ftcalc.h
+++ b/include/freetype/internal/ftcalc.h
@@ -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 );
diff --git a/src/base/ftcalc.c b/src/base/ftcalc.c
index 442b08d..9e87abd 100644
--- a/src/base/ftcalc.c
+++ b/src/base/ftcalc.c
@@ -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 );
-
- /*
- * 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;
+ 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 );
- 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 )
- {
- FT_Fixed scale = FT_DivFix( maxval, 23170 );
-
+ /* Scale matrix to avoid the temp1 overflow, which is */
+ /* more stringent than avoiding the temp2 overflow. */
- if ( !FT_DivFix( nonzero_minval, scale ) )
- return 0; /* value range too large */
+ shift = FT_MSB( val ) - 12;
- 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 );
+ if ( shift > 0 )
+ {
+ 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;