Re: Transparent png

On 5/23/2017 9:37 AM, Tim Roberts wrote:
> Arkadi Colson wrote:
>> Does anybody know if it's possible to use tranparent png images with the
>> code below:
>>
>> self.data[x][y] = Paragraph(
>>                               '<img src="' + filename + '"
>> valign="middle"/>', style=self.style_def)
> Wouldn't it have been quicker just to try it?
>
> Although I don't know the answer, I will point out that transparency has
> always been a tricky issue with PostScript and, by extension, PDF.  Both
> standards were designed with paper and printing in mind, where
> transparency does not exist.  Most professional print shops will not
> accept PDF files with embedded transparency, because it's not clear what
> that means.
>
I know that transparency isn't supported in some graphics formats, and 
that there have been issues with that in various programs/browsers/etc. 
that didn't understand the concept, because they were designed prior to 
the existence of the concept.

However, the concept seems extremely straightforward for printing: 
transparent means "don't print there".  This seems to be exactly the 
same concept as stripping images for the 4-color printing process: color 
images are reduced to "print density of each of CYMK" for each color for 
each pixel, and the 0 density is (effectively) transparent.

Could you elaborate on the issues that professional print shops have 
with transparency? It doesn't seem to be that they can't produce a 
density of 0 for swath of certain (even all) colors in areas of the 
printed page: except for prints consisting totally of color pictures, 
nearly all pages in all publications have areas where no ink of any 
color is laid on the paper.  Is it because the concept of transparency 
in image files (particularly, those embedded in Postscript or PDF) 
hasn't been sufficiently standardized? Or what?