Re: [Texmacs-dev] [TeXmacs] New experimental Pdf export facility

[Joris van der Hoeven]

On Tue, Nov 12, 2013 at 11:22:40PM +0100, Massimiliano Gubinelli wrote:
> > As to pure image formats (jpg, png, tiff, svg, etc.), I would like to avoid
> > depending on too many libraries.  Ideally, Qt should be able to do them
> > all for us.  If not, Qt in combination with a tool such as ImageMagick
> > which supports many different formats.  I agree that it is best to touch
> > images files as little as possible when including them in Pdf output.
> > Is there a way to include them in some kind of verbatim/escaped fashion?
> 
> > I still don't understand why libpng and the jpeg library would be necessary.
> 
> I was looking to how pdftex handle the problem. If I understand well their 
> code they can include directly jpg images without any external support (and 
> without uncompressing the jpg stream). For png they rely on libpng to read 
> the files. I think I can use Qt to read the actual bits and obtain enough 
> information (DPI, color depth, color map, to reconstruct a faithful 
> representation of the png data inside the pdf). 

I still don't understand something.
How exactly are png and jpg images stored in Pdf files?
If they are compressed in the standard png/jpg way,
then I would expect the images to be included in
a verbatim/escaped way.  If they are compressed in
an ad hoc Pdf way, then why would we need libpng,
except for obtaining the pixmaps (which Qt can give us)?

> The point is that ideally one should keep (lossy and lossless) compressed 
> formats compressed. Qt can read all the formats but give back only the 
> uncompressed bits, so this is almost useless if you want to put the image 
> into a compressed file format like PDF. The point is to try to maintain the 
> data compressed without wasting time in uncompressing it and then 
> recompressing it (in necessarily unoptimal ways). 

I agree that compressed images should not be touched.
But if we do not want to touch them, why do we need libpng?

> > There is also the question of producing image files directly from TeXmacs,
> > for instance when exporting mathematical formulas to Html.  What exactly
> > does Qt provide for the manipulation of image _files_?
> 
> Qt can produce images (especially raster) to any common format (png, maybe 
> jpg, pdf and eps) so I think there are no problem producing them. 

Does Qt also perform the corresponding compression?

Best wishes, --Joris