Dpi 'n' PNG? How?

I need to upload some images to AnimalsTaiwan’s upcoming online store, and I’m being told that it’s better for the images to be at least 200 dpi and in PNG format.

My questions are thus:

(1) How do I find out the dpi of the images I am about to upload?

(2) How do I convert a RAW file or JPEG file to PNG format?

Thank you for any help for this tech idiot!

Sean

OK, I worked it out (saved to Windows photo viewer and then saved from there to PNg format and then checked the properties from that image).

But, if my image is only 92 dpi but 11 MB in size, and the site suggests 4 MB max, how can I reduce the size of the file but increase the dpi? Or is this impossible? Do I need to photograph the images in digital at a higher dpi?

Oh, it’s all so confusing! :s

What are they pictures of ? Photos or handdrawn ? DPI is only for printed matter, not websites, you just need to make them the right size in pixels.
I think you need a new tech adviser…

EDIT:

[quote]Do I need to photograph the images in digital at a higher dpi? [/quote]See ? You don’t use .png for photos. give em here and I’ll do it. You only need to ask me to turn up, I have some experience of an online store you know…

Here’s the site, Fluffy:

http://www.cafepress.com

And here’s the specs for images:

http://www.cafepress.com/cp/info/help/images.aspx

http://www.cafepress.com/cp/info/help/help_image_sizes.aspx

Any help or advice would be greatly appreciated. One free mug for you if you can! :wink:

So they’re not for a website, they are for printing on a mug ? Make your mind up :s
If you right click on the file you should be able to see its DPI somewhere. To change it and convert it you’ll probably need some art proggy. GIMP would do it. I could do it for you, but not sure how to get those big files to me though…
When you right click on the file, do you get a “convert to file format” option ?
DPI = dots per inch. So if divide the pixels by the DPI, you will get the printed size in inches. Do you have an image 1663 x 600 (for a mug) ?

If you might to send/receive large files, try this site out. I’ve used the free account a few times and it works quite well.

http://www.sendthisfile.com/

Here’s a simplified explanation: Let’s assume that 200 DPI (dots per inch, or d/in) is what the machines used for printing at Cafepress are physically able to do. They can print 200 tiny dots (in any desired colour) on one inch of paper, t-shirt, whatever.

On the other hand, your images are measured in pixels. If you want to get the best possible result, you’ll want to resize your picture so that one pixel corresponds to exactly one dot being printed (thereby printing at the printer’s “ideal DPI setting”).

To do that, you’ll have to know the exact size of the printing area. For the “Ash Grey T-Shirt” (no. 5 on your specs list) this area is 10 inches by 10 inches (on the front). To calculate the best image size in pixels, just multiply the width of 10 in by 200 d/in and you get 2000 dots. Since we said we want one pixel to correspond to one dot, that means our image has to be 2000 pixels wide. Do the same for the height and you get a required image size of 2000 pixels by 2000 pixels. Of course, that’s exactly what’s written in the specs (image size at ideal dpi).

So now we know you’ll need a 2000 x 2000 pixels image to print on a 10 x 10 inch area with 200 dpi. At this point, you can completely forget about the dpi setting and simply resize/crop your image accordingly so that it ends up being 2000 pixels wide and 2000 pixels high. While many programs allow you to save a DPI value with the image (which is “for information purposes only” and doesn’t change the picture’s actual dimensions in pixels
unless you’ve set your software to measure in inches instead of pixels
), Cafepress would completely ignore this DPI value since they allow you to select the physical size of the printed image during upload, thereby determining the DPI (same math as above).

In one sentence: Get the dimensions in pixels right, forget about the DPI. If your image has the ideal image size in pixels as given in the specs, it’ll always pint at the “ideal DPI”, resulting in the best possible quality.

However, if you’re trying to use a photo which simply doesn’t have 2000 x 2000 pixels but you still want to have it printed on the entire 10 x 10 in area (instead of leaving white borders or including it in a more complex design), you’ll have to look at the DPI specs again. In this case, crop the image in order to get the required square form (let’s say your photo is 2200 x 1700: you’d crop it to 1700 x 1700). Then check if it is still within the allowed DPI range (100-300 DPI for the t-shirt): 1700 “dots” printed on 10 inches would give you 1700/10=170 DPI, which would be OK (though not as good as 200 DPI), so you could simply upload it (no need to resample it to 2000 x 2000). If it were less than 1000 x 1000 pixels (or larger than 3000 x 3000), Cafepress wouldn’t accept it (or not allow you to print it over the entire printable area, I’m not sure how exactly Cafepress works).

BTW, IfranView is a nice litte free software for resizing, cropping etc. (you can even set the DPI value which would later be ignored by Cafepress).

hypermegaglobal, thanks very much for the detailed explanation - much appreciated.

cfimages, that’s a very useful link - thank you.

So, Fluffy, did you get all that? When can I send you the files? :smiley:

You can be our marketing assistant (plenty of room to work your way up :wink: ).

[quote=“hypermegaglobal”]…Then check if it is still within the allowed DPI range (100-300 DPI for the t-shirt): 1700 “dots” printed on 10 inches would give you 1700/10=170 DPI, which would be OK (though not as good as 200 DPI)…[/quote]That’s a very useful explanation, HMG. Thanks. One idle, curious question: from what you wrote, it sounds as if printing machines can vary the dpi up to the maximum limit, is this correct? So with the example of the 1700*1700 image, would it actually be printed at 170 dpi and not at 200 dpi with some kind of reprocessing applied?

hypermegaglobal’s explanation is spot-on. But I have to wonder if you really need to down-rez the pic. Stray Dog said that it needs to be “at least 200dpi.” I’m assuming they’re just suggesting a minimum to ensure decent print quality and they can handle anything above 200dpi.

Yep, got all that. Makes sense to me. Just get a picture the size in pixels they want, and I’ll see which format works best. Or get that Irfanview and try converting them yourself. Try .png, and .jpg with quality set to high.

Joesax, remember I wrote it’s a simplified example :wink:. The printer will have to raster the image (halftoning) in any case to print different colours and shades of grey (with the exception of dye-sublimation printers which are apparently used for some of the products at Cafepress). You’re right, though, printers can’t usually vary their DPI like I wrote, but this is the way Cafepress uses the term DPI in the specs.

Alidarbac, Cafepress writes that 200 DPI will give “ideal” results (smaller images will look worse, larger ones won’t really look better). If an image is above the ideal size but within the maximum permitted size, I also don’t see the need to downsize it (I suspect larger images might lead to better results in some cases, but I don’t know enough about the kind of printers they use to be sure).