[Desktop_printing] Update of the Printing Summit Agenda

Ralph Giles giles at ghostscript.com
Wed Jan 11 20:07:28 PST 2006

On Wed, Jan 11, 2006 at 08:46:24PM -0500, Michael Sweet wrote:

> The MaskDict entry in a Type 3 image defines the mask image, which
> for InterleaveType 1 requires you to provide an alpha mask with the
> same depth as the color data in the main image.  See pages 303 to
> 307 of the PostScript Language Reference Manual, Third Edition.

Yes, the mask component must be the same depth as the other components 
for an even data interleave, but it is converted to 1 bit. See the 
IterleaveType entry in Table 4.22, PRLM3 p.304:

"The mask sample must have the same number of bits as each color 
component of the image sample, with all bits set to the same value 
(that is, either all 0 or all 1); any other value will be treated
as if the bits were all 1."

And the mask component is only 1 bit in all other defined 

> (This *does* work, I use it in HTMLDOC's Level 3 output...)

With Ghostscript? Are you sure this isn't just a binary mask?

In any case, it's not correct to say that PDF and PostScript have the 
same graphics model. The addition of transparency groups and blend modes 
are still a huge difference even if PostScript did support PNG-style 
alpha masks.


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