add comments

git-svn-id: http://svn.code.sf.net/p/netpbm/code/trunk@3600 9d0c8265-081b-0410-96cb-a4ca84ce46f8

1 files changed, 20 insertions, 0 deletions

diff --git a/converter/other/pngx.c b/converter/other/pngx.c
index 4bb09421..8295b979 100644
--- a/converter/other/pngx.c
+++ b/converter/other/pngx.c
@@ -416,6 +416,26 @@ void
 pngx_setFilter(struct pngx * const pngxP,
                int           const filterSet) {
 
+    /* This sets the allowed filters in the compressor.  The filters, and thus
+       the interpretation of 'filterSet', is specific to a filter method (aka
+       filter type), which you set with pngx_setIhdr.  There is only one
+       filter method defined today, though (PNG_FILTER_TYPE_BASE).
+
+       For filter method Base, 'filterSet' is the OR of the following masks,
+       each one allowing the compressor to use one filter.  Not that  the
+       compressor decides on a row-by-row basis what filter to use.
+
+         PNG_FILTER_NONE
+         PNG_FILTER_SUB
+         PNG_FILTER_UP
+         PNG_FILTER_AVG
+         PNG_FILTER_PAETH
+
+       There are also
+
+         PNG_NO_FILTERS
+         PNG_ALL_FILTERS
+    */
     png_set_filter(pngxP->png_ptr, 0, filterSet);
 }