fix low-bitdepth spng save

jcupitt · jcupitt · commit ca2796aa7ff7 · 2022-08-14T10:26:24.000+01:00
There was a missing shift, thanks jeffska See #2985
diff --git a/ChangeLog b/ChangeLog
@@ -4,6 +4,7 @@
 - fix libpng fallback when spng is disabled in meson
 - add "unlimited" to jpegload
 - better 0 detection in unpremultiply
+- fix low bitdepth spng save [jeffska]
 
 21/11/21 started 8.13
 - configure fails for requested but unmet dependencies [remicollet]
diff --git a/libvips/foreign/spngsave.c b/libvips/foreign/spngsave.c
@@ -270,14 +270,15 @@ vips_foreign_save_spng_pack( VipsForeignSaveSpng *spng,
 	VipsPel *q, VipsPel *p, size_t n )
 {
         int pixel_mask = 8 / spng->bitdepth - 1;
+	int shift = 8 - spng->bitdepth;
 
         VipsPel bits;
         size_t x;
 
         bits = 0;
         for( x = 0; x < n; x++ ) {
                 bits <<= spng->bitdepth;
-                bits |= p[x];
+		bits |= p[x] >> shift;
 
                 if( (x & pixel_mask) == pixel_mask )
                         *q++ = bits;
