Preview and screen resolution

I’ve been messing around with Gnuplot and Ghostscript lately, figuring out ways to generate PDF and PNG plots more efficiently. I hope to have a post about that soon, but in the meantime I thought I’d mention something I discovered about Preview.

After Safari, Preview is my favorite Apple-written program. It just works, serving as an all-purpose interactive graphics toolkit. It’s a better PDF viewer than Adobe’s Reader and handles most bitmapped images, too. Using it is so easy I never paid much attention to its preferences. But as I was generating and reviewing PDF and PNG versions of the same plots, I noticed that the PDFs appeared bigger on my screen than the PNGs, even though the PNGs were set to a 72 dpi resolution and both versions were being viewed at 100% scale.

I opened Preview’s preferences to see what was going on. Here’s what I found:

Preview PDF preferences

Preview bitmap preferences

The key is the “Define 100% scale as” option. With the options set as shown, bitmapped images, like PNGs, are shown pixel-perfect when the scale is at 100%, regardless of any dpi setting in the file. PDFs, on the other hand, are adjusted to avoid the pixel=point problem that I talked about last fall when the new MacBook Airs were announced. With this setting, a 5-inch wide PDF plot will not show up as just 360 pixels wide on my screen, but will account for the higher resolution of the screen and actually be 5 inches wide.

Measuring a PDF plot

(I shifted the ruler as I was taking the photo, but it really is almost exactly 5 inches wide.)

Which raises a question: If Preview is smart to account for the screen resolution, why aren’t other applications? For example, Pages ’08, Numbers ’08,1 and OmniGraffle all think a point is a pixel, so I have to zoom in to see things at approximately the size they’ll be when printed.2 Obviously, there’s a way for programs to access the screen resolution. Why don’t they use it and give us on-screen displays that match the printed size?

  1. I haven’t updated to the most recent versions because I don’t feel like buying an ’09 product in ’11. If you have the ’09 versions of Pages and Numbers, I’d like to hear if they still think points are pixels. 

  2. OmniOutliner is worse: it not only thinks points are pixels, it doesn’t have a zoom feature that allows me to see things at their “real” size.