What the resolution for this pdf file?

August 9, 2007 10:52 AM

How do I determine the resolution for images in a pdf? Or for a pdf that is one big raster image?

I work with a CAD program (Solidworks 2007) and often grab images of what I'm working on by saving them as jpg, tif or pdf files. Jpgs are saved at 72 dpi, and tifs at 92. For pdfs there's a "high quality" option and I'd like to know how high the quality is. I've got Acrobat 8.

posted by hydrophonic to computers & internet (5 comments total)

technically, PDF has no resolution. Images are made of samples and can be scaled to nearly any size.

Therefore the resolution of an image is the number of samples in each dimension divided by the size at which the image is being displayed.

If you extract the images from the PDF, unless the particular format contains resolution information, resolution needs to be synthesized.

PDF is designed such that the same image data can be displayed in multiple images across multiple pages in nearly any size and the resolution is therefore different.

From Adobe's documentation, they recommend that you think of an image as 1x1 in PDF units (1/72") and then when you draw the image, you scale it to the size you want.

If you open the file, you will discover that the contents don't make it easy for you to determine the number of samples in each dimension, and from a practical standpoint, there is no good way to do that without significant software tools for cracking the file.
posted by plinth at 11:05 AM on August 9, 2007

Actually, plinth, PDFs as a single file may not have a resolution, but the graphics within them do.

You can open up graphics embedded in a PDF by selecting the Touch Up Object tool, selecting the graphic in question, right-clicking or ctrl-clicking, and selecting Edit Image... from the pop-up menu. This will, by default, open the image up in Photoshop. If you don't have Photoshop, you can select another image editor on the Preferences, under TouchUp. Any image editor you use should be able to give you the dpi/ppi/resolution info.

Alternately, you can go to Tools -> Print Production -> Preflight

This will bring up your Preflight window. Scroll down until you see "List images below 250 ppi" and click the Execute button. This will run a script that will, surprise, list all images with resolutions below 250 points per inch. Any images below that resolution will be listed, along with their actual resolution. Click on the image names and Acrobat will highlight the image
posted by lekvar at 11:50 AM on August 9, 2007

If you like, you can post a link to or e-mail me a representative PDF file containing a raster image object and the corresponding TIFF version of the same image. If you do, I'll decompose the PDF file and let you know how the image object in the PDF compares to the TIFF image.
posted by RichardP at 11:51 AM on August 9, 2007

Thanks, lekvar, the Touch Up Object tool does just what I needed.
posted by hydrophonic at 1:06 PM on August 9, 2007

lekvar - hate to burst your bubble, but I was one of the primary engineers on Acrobat v1, v2, v3. I stand by my assertion: unless the embedded image contains resolution information, resolution is synthetic - it depends on how it is imaged on the page and since a given image can be imaged on a page in multiple sizes, each imprint has a different virtual resolution. The preflight is reporting virtual resolution, not real resolution.
posted by plinth at 6:34 AM on August 10, 2007