The Incredible Shrinking Image

May 9, 2008 12:40 PM

How do I resize a very large image to a smaller size yet keep text detail legible?

Long version:

I have a document (hi-res Google mape export with lots of small text detail) at the following resolution: 5000 x 7000 @ 72ppi. This is ~70 x 97 inches were I to print it.

I don't want to print it that big. I want to print it at ~25 x 36 inches.

Problem is, when I resize it thus, small text details seem to be illegible on screen (per Photoshop's "print preview", anyway). Of course, this is at just 72 ppi, so I can increase the ppi and shrink the image. But I can't get even, say, 300ppi, as my document would have to be shrunk to a mere 8 inches in height (without resampling).

It appears that I can get it to my desired size of ~25 x 36 at 138ppi without resampling. Is that the best I can do, or is there some other tomfoolery I can try?

Remember, I'm trying to get small text legible after shrinking it. I'm assuming the higher the ppi the better for print legibility, as long as it's not being resampled. Yeah?

Put another way, even if my "shrunk" image @ 138ppi looks hard to read in Photoshop, if I run it off on a high-dpi printer, will it be legible? What should I be aiming for here?

posted by sprocket87 to computers & internet (13 comments total)

Your math seems a little suspect. If your image is 5000 pixels wide and you print it out so it's 25 inches wide, that's a resolution of 200 pixels per inch. 5000 divided by 25 equals 200.

Higher pixels per inch will not render your print illegible; laser printers commonly print at 600 or 1200 pixels per inch and we're all perfectly happy using them. Now if your writing is only, say, 12 PIXELS high in your source image, then it's going to be 1/16" tall in your finished printout. Whether you consider that legible is up to you and the people who will be viewing/using the print.

Resampling doesn't add data, by the way, and therefore can't improve the quality of your finished image. Sharpening or adjusting the colors in your image may improve legibility but this is hard to generalize about.
posted by bcwinters at 12:55 PM on May 9, 2008

Text can usually still be legible at around 150PPI without too much concern. However, I wonder if it might just be easier to downsize the image, kick the PPI up to maybe 300 (or 250) and then just use Photoshop to rewrite the text on top the old text.

Use a thick white stroke on the text (maybe 3 - 5px), and it'll completely cover the old text and still look professional and readable.
posted by revmitcz at 12:55 PM on May 9, 2008

Shrink it to 138ppi, crop it so it'll print on a bog-standard inkjet, and print it out. Does it look legible?
posted by RustyBrooks at 1:15 PM on May 9, 2008

When I say "shrink" I really mean just adjust the PPI in photoshop, you aren't actually going to rescale it. And when I say crop, I mean just crop out a section big enough to get a representative sample and small enough to print in 8x10
posted by RustyBrooks at 1:19 PM on May 9, 2008

Thanks for the feedback so far.

bcwinters, I was simply trusting Photoshop with the numbers I used - I posted exactly what it said about the image properties.

Although I just realized I had "resample image" checked in the Image Size palette in Photoshop. I unchecked that, plugged in my desired dimension of Wx36", and it automatically selected a resolution of 194 PPI. Okay, is that progress? The resized image at "actual pixels" looks quite legible - though the "print size" preview is very small.

revmitcz, rewriting the text isn't really an option. Like I said this is a Google Map (which is at a high level of zoom), and there are literally hundreds of street names. It'd be practically impossible to do all that work, let alone try to fit it all in a legible manner without words running over each other.
posted by sprocket87 at 1:24 PM on May 9, 2008

Ah, yes, you don't want to "resample image" if your goal is preserving the image quality. With "resample image" turned off, Photoshop keeps all your pixels intact and just tells the output device or other image-viewing program what the DPI should be--the data in the image actually remains the same, which is what you want.

How are you getting this output, by the way?
posted by bcwinters at 1:28 PM on May 9, 2008

Note: I should say that when I resized the image with "resample image" unchecked, the "actual pixels" view in Photoshop looks identical to the source image - which makes sense, since it's compensated for the increase in pixel count by reduction in dimensions (or however you explain that).

However, the "print size" view in Photoshop causes it to look mighty tiny. Is that just Photoshop being ignorant and going to a default zoom level, regardless of what it will actually look like printed?

I'd run a test print but the only printer I have at my disposal at the moment is a crummy Dell laser that yields illegible prints anyway :)
posted by sprocket87 at 1:30 PM on May 9, 2008

bcwinters, I'm planning to take this to Kinko's (or the like). I called and a tech claimed that the target resolution for their large-format printers would be 300 DPI max.
posted by sprocket87 at 1:31 PM on May 9, 2008

Be sure whenever you resize an image in Photoshop that you are in RGB or CMYK mode. (You can tell via the menu Image > Mode.) If it's Indexed (which it usually is when you start with a GIF) then you should change it to RGB before resizing. Resizing in a restricted palette with do awful things to the details of your image.
posted by aught at 1:37 PM on May 9, 2008

Thanks for the tip aught. I'm in RGB, this is a raster graphic output from Google Maps (originally saved as PNG).
posted by sprocket87 at 1:39 PM on May 9, 2008

Ahh, this article is very helpful in making this clearer. It also confirms that Photoshop's "Print size" view mode is pretty much useless :)

I'm going to run off a test sample when I get the opportunity and we'll see how it looks.
posted by sprocket87 at 1:49 PM on May 9, 2008

When you make an image smaller, Photoshop has fomulae to determine what to throw away. Inevitably, when you go from large to very small, a lot gets thrown away and the image tends to look a bit fuzzy. The best method is to reduce the size of the image in steps, and after each step (usually by half in any given direction) sharpen.

So you might go from 5000 to 2500, then sharpen. Then down to 1250 and sharpen again. The only downside to this is the inevitable sharpening detritus that photoshop will introduce around contrast points. You simply have to be judicious in your sharpening - just enough but not too much.

That said, I don't know why you want to reduce anything for printing. If you are printing high quality you often want to be printing at 300dpi. 150 is a line-screen number for very high quality offset printing, not dpi for desktop printing. Over 300 tends to be overkill, but there's no harm in it. 72dpi will always look pretty crappy in comparison to 300.

5000 X 7000 should yield a very nice 16 X 20 at 300, and will easily yield good quality at the size you want. Resize the image such that the number of pixels never changes, but the dpi changes and you'll be fine. You never want to throw away information if you can help it.
posted by johngumbo at 3:56 PM on May 9, 2008

Things to consider- the DPI/PPI of your monitor when viewing the print preview. And the technique your printer will use to layer colors, and at what DPI it *actually* prints. Printers love to say they are 5000x8000 DPI, but their engine only really prints 300x600 and the printer's logic attempts to resample it.

If it was me, I'd convert the image to CMYK, then convert the image's DPI/PPI to a value that your printer will natively accept. You are looking at a loss of quality if you convert from 72 to 138, and then the printer changes it to something else and then expands *that* to fit the page. See if the printer will natively print @ 144, and then change it to that. I'd suspect you'd have the least loss in that scenario.
posted by gjc at 10:43 AM on May 10, 2008