PNG does not preserve scanned image dpi resolution?

[adon]

I've just started scanning pictures using PhotoShop and TWAIN. I've been saving to PNG format. I noticed that Photoshop6 does not preserve the dpi resolution of the scan. However, JPG does preserve the dpi resolution. Is this PNG's fault or Photoshop's?

thanks!

 

[zefir]

Adon: the resolution (as defineded by scanner dpi) is a completely unnecessary
information in an image file. The file size is in all graphics file formats defined
by the number of x-pixels and y-pixels. More is not necessary.
E.g. computer graphics don´t have any dpi.
File formats for the Web (GIF, JPEG, PNG) don´t use dpi. A picture on a web page
is shown by the source pixels, unless it is scaled in HTML, also in pixel-width or
pixel height.
So it´s not a bug, when a file doesn´t preserve the "history of scanning".
Please be aware that PhS tells you "Your image has 72dpi", if no information
is embedded. In fact, the SIZE in inch was selected by PSh´s default in a way that it
results in 72 dpi (you may have scanned by 600dpi, but the information is not
embedded). A terrible nonsense, this information, by the way. ----Gernot

 

[adon]

Thanks for your reply.

I subsequently scanned Usenet archives and found out that PS ignores this annotation for PNG, unfortunately. I do think that the resolution data makes a difference when transferring the image to print media. I'd like to store how large the original image is, for when I print.

For now, I'm using TIFF to store images instead of PNG. But that may chance once I find a different program to accept TWAIN scans other than PS that can retain resolution information in PNG format.

thanks!

 

[pixl8r]

I think the problem you are having with PNG's is inherrent to the file type and not necessarily to PS.

Industry standard is definitely TIFF if you are trying to keep resolution without sacrificing it to compression. If you need to keep a bit of (disk)space you can use the LZW compression, but I wouldn't necessarily recommend it.

I think that Gernot's post above is a bit too literal. DPI and PPI(pixels per inch) are directly related and have a lot to do with computer graphics and final output whether for web or print.

Good luck

 

[adon]

pixl8r,

I do know that PNG can store resolution information. It is Adobe's fault for not making use of it. At least that is what I gather from Usenet postings.

As far as file formats go, PNG is a loss-less format that compresses better than TIFF w/ LZW, in general. That's why I was using it.

Why do you not recommend LZW?

thanks.

 

[pixl8r]

Only because it is not "lossless" either and you don't save enough space using it to justify the loss. Might as well just keep them big (in file size).

I think PNG's are a great file format but I haven't seen a place to "adjust" the resolution in any graphics application. One thing I can think of that might work is that you increase the overall pixel width/height and drop ppi to 72.

Maybe that is what Gernot was trying to say. LOL

 

[zefir]

Adon: It´s not a fault, to omit the resolution. For printing you need
the NEW resolution in source pixels per inch on the paper, and this is
different to the SCAN resolution, if the image is scaled (as usual).
For the Web, any dpi information is useless, and PNG is mainly
a Web format. In DTP applications "nobody" uses PNG
(PageMaker cannot read PNG). ---Gernot

 

[pixl8r]

Yeah I would never send a PNG to DTP. I would send a TIFF or EPS. Wow JT that almost looked like you knew what you were doing!