[Git][freetype/freetype][master-patch-ea68] [base] Improve the matrix degeneracy check.

[Alexei Podtelezhnikov (@apodtele)]

Alexei Podtelezhnikov pushed to branch master-patch-ea68 at FreeType / FreeType

Commits:

• 1cdd9df2
  by Alexei Podtelezhnikov at 2023-08-21T23:23:22-04:00

  [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.
  

2 changed files:

• include/freetype/internal/ftcalc.h
• src/base/ftcalc.c

Changes:

include/freetype/internal/ftcalc.h

---

...	...	@@ -332,9 +332,9 @@ FT_BEGIN_HEADER
332	332	* Based on geometric considerations we use the following inequality to
333	333	* identify a degenerate matrix.
334	334	*
335		- * 50 * abs(xx*yy - xy*yx) < xx^2 + xy^2 + yx^2 + yy^2
	335	+ * 32 * abs(xx*yy - xy*yx) < xx^2 + xy^2 + yx^2 + yy^2
336	336	*
337		- * Value 50 is heuristic.
	337	+ * Value 32 is heuristic.
338	338	*/
339	339	FT_BASE( FT_Bool )
340	340	FT_Matrix_Check( const FT_Matrix* matrix );

src/base/ftcalc.c

---

...	...	@@ -749,65 +749,43 @@
749	749	FT_BASE_DEF( FT_Bool )
750	750	FT_Matrix_Check( const FT_Matrix* matrix )
751	751	{
752		- FT_Matrix m;
753		- FT_Fixed val[4];
754		- FT_Fixed nonzero_minval, maxval;
755		- FT_Fixed temp1, temp2;
756		- FT_UInt i;
	752	+ FT_Fixed xx, xy, yx, yy;
	753	+ FT_Fixed val;
	754	+ FT_Int shift;
	755	+ FT_ULong temp1, temp2;
757	756
758	757
759	758	if ( !matrix )
760	759	return 0;
761	760
762		- val[0] = FT_ABS( matrix->xx );
763		- val[1] = FT_ABS( matrix->xy );
764		- val[2] = FT_ABS( matrix->yx );
765		- val[3] = FT_ABS( matrix->yy );
766		-
767		- /*
768		- * To avoid overflow, we ensure that each value is not larger than
769		- *
770		- * int(sqrt(2^31 / 4)) = 23170 ;
771		- *
772		- * we also check that no value becomes zero if we have to scale.
773		- */
774		-
775		- maxval = 0;
776		- nonzero_minval = FT_LONG_MAX;
	761	+ xx = matrix->xx;
	762	+ xy = matrix->xy;
	763	+ yx = matrix->yx;
	764	+ yy = matrix->yy;
	765	+ val = FT_ABS( xx ) | FT_ABS( xy ) | FT_ABS( yx ) | FT_ABS( yy );
777	766
778		- for ( i = 0; i < 4; i++ )
779		- {
780		- if ( val[i] > maxval )
781		- maxval = val[i];
782		- if ( val[i] && val[i] < nonzero_minval )
783		- nonzero_minval = val[i];
784		- }
785		-
786		- /* we only handle 32bit values */
787		- if ( maxval > 0x7FFFFFFFL )
	767	+ /* we only handle non-zero 32-bit values */
	768	+ if ( !val || val > 0x7FFFFFFFL )
788	769	return 0;
789	770
790		- if ( maxval > 23170 )
791		- {
792		- FT_Fixed scale = FT_DivFix( maxval, 23170 );
793		-
	771	+ /* Scale matrix to avoid the temp1 overflow, which is */
	772	+ /* more stringent than avoiding the temp2 overflow. */
794	773
795		- if ( !FT_DivFix( nonzero_minval, scale ) )
796		- return 0; /* value range too large */
	774	+ shift = FT_MSB( val ) - 12;
797	775
798		- m.xx = FT_DivFix( matrix->xx, scale );
799		- m.xy = FT_DivFix( matrix->xy, scale );
800		- m.yx = FT_DivFix( matrix->yx, scale );
801		- m.yy = FT_DivFix( matrix->yy, scale );
	776	+ if ( shift > 0 )
	777	+ {
	778	+ xx >>= shift;
	779	+ xy >>= shift;
	780	+ yx >>= shift;
	781	+ yy >>= shift;
802	782	}
803		- else
804		- m = *matrix;
805	783
806		- temp1 = FT_ABS( m.xx * m.yy - m.xy * m.yx );
807		- temp2 = m.xx * m.xx + m.xy * m.xy + m.yx * m.yx + m.yy * m.yy;
	784	+ temp1 = 32U * (FT_ULong)FT_ABS( xx * yy - xy * yx );
	785	+ temp2 = (FT_ULong)( xx * xx ) + (FT_ULong)( xy * xy ) +
	786	+ (FT_ULong)( yx * yx ) + (FT_ULong)( yy * yy );
808	787
809		- if ( temp1 == 0 ||
810		- temp2 / temp1 > 50 )
	788	+ if ( temp1 <= temp2 )
811	789	return 0;
812	790
813	791	return 1;

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