Image Size and Formats

[GeorgeB]

No help in the forums, and couldn't find any help googling elsewhere.

I've been trying to learn about various filetypes (image formats) and how they print and how they view on a computer. So I've been saving in various image formats.

Starting with a new image, canvas set to 346x130 pixels, print size 3.6 x 1.35 inches, resolution at 96 px/in. Three layers, background, image, text. Flattened before saving. Following are my results:

Filetypes comparison:

FORMAT.......WIDTH.....HEIGHT.....DPI.....PX............CM

PDN.............3.6"........1.35".........96.....346 x 130.....n/a

PNG & GIF.....4.81"......1.81".........72.....346 x 130.....12.21 x 4.59

JPG & TIFF....3.60"......1.35".........96.....346 x 130.....9.15 x 3.44

(+BMP)

The above are the sizes as reflected in Irfanview's Image Resize dialogue; same results in Xnview. Same results when printed out (unless I specify the image print size for the PNG/GIF formats, rather than the original image size).

But, when I open the PNG into a new tab, it shows the dimensions I want.

This might be a different issue: when I print that PNG through PDN, I don't see any options to print it at actual size. (Full page fax, 8x10, 5x7, etc. -- nothing that comes close to 3.6 x 1.35 in.) Which is why I rely on Irfanview for printing chores.

Visually, in a browser, they all appear the same size.

So am I doing something wrong? Just plain old misunderstanding image formats?

And why am I getting two different sizes from the same PDN file?

And as long as I'm here, are there any guidelines on how best to save for both print (desktop printer) and computer (web) uses?

Obviously, I am so-o-o-o confused!

--GeorgeB

[MiguelPereira]

physical dimensions and dpi are only used on printing, when you are viewing the images on the computer you see them at their resolution.

PNG & GIF.....4.81"......1.81".........72.....346 x 130.....12.21 x 4.59

JPG & TIFF....3.60"......1.35".........96.....346 x 130.....9.15 x 3.44

check the values on the smaller pic with the higher value of dpi the printing size will be smaller because you will have more pixels(dots) per inch on a smaller image. so if you have a larger image you want to print with the size you print another one that is smaller, you have to increase the dpi value of the first one so that it prints the size you want.

[GeorgeB]

I'm still not getting it.

I set the image and canvas size in PDN to 346x130, DPI 96; that automatically sets the print size to 3.6 in x 1.35 in. When I save that image in PNG format, I still get 346x130, but the DPI is 72, and the file size is 4.81" x 1.81". I see no means or method of adjusting the DPI when I save as PNG. And saving to .tif keeps the parameters as I originally set them.

So maybe my question really is: why does PDN change the DPI from 96 to 72, and alter the print size? To my way of thinking, it would seem that if I set the image/canvas size before starting, it should save to those parameters regardless of format, PNG or JPG or TIF.

I just tried an experiment: I opened a TIF in Irfanview, then saved it as a PNG file (.png). That PNG file has the same size and DPI as the TIF, and is different from the PNG saved via the PDN "save as" dialogue.

I noticed something else: the PNG file saved directly from PDN at 4.81" x1.81" and 72 DPI has a file size of 74K; the PNG save from the TIF (which was saved directly from PDN) and is 3.6" x 1.35" and 96 DPI, has a file size of 135K. Which may or may not be relevant to this discussion, just thought I'd point it out.

So either something is happening during the save process in PDN which I don't understand, or something's screwed up in my computer (and/or my brain! )

If nothing else, I can live with saving to TIF and then converting to other formats via Iview.

I'm really floundering here, so any further elucidation would be most appreciated.

--GeorgeB

[MiguelPereira]

the dpi value is not part of the image itself some programs probably might save the last dpi used in the file within it's metadata, but that value can be changed for variable printing sizes