explain the magic width and height values in resize command

1 files changed, 13 insertions, 1 deletions

diff --git a/USE.md b/USE.md
index f4facae..bdd3993 100644
--- a/USE.md
+++ b/USE.md
@@ -16,5 +16,17 @@ Alternative:
 
 ## Resize PDF with crop marks
 
-A3 oversize to A4 oversize:
+Ghostscript resizing is done by first defining target size
+and then tell to resize content to fit that target with `FitPage`.
+
+For a PDF document containing bleed and/or crop marks,
+setting a simple target `PAPERSIZE` would either scale too much
+or (e.g. with `-dUseArtBox`) would loose bleed and crop marks.
+Instead we look up original width and height with the command `pdfinfo`
+and explicitly set those values scaled down by 21/29.7
+(the ratio between A3 and A4 page formats).
+
+Example command,
+for a PDF document with width 910.24 and height 1258.9:
+
 	gs -o output.pdf -sDEVICE=pdfwrite -dDEVICEWIDTHPOINTS=643.60333 -dDEVICEHEIGHTPOINTS=890.131 -dFIXEDMEDIA -dFitPage -dCompatibilityLevel=1.4 input.pdf