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I'm working with an InDesign document and turning it into HTML/CSS. It is using mm for sizes, but I need the measurements in pixels.

Height measures as 66px in InDesign using the measuring tool.

I set the zoom to "Actual Size" then take a screenshot. Measuring the screenshot in photoshop gives a height of 88px.

My question is what measurement is correct, and why is the other one incorrect? Why do these two methods of measurement give different results?

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    Can't you just change the InDesign document to reflect pixels? (in the preferences) – Scott Jul 7 '15 at 21:04
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The measurement tool calculates the pixel size based upon the page size and reports that regardless of zoom level (8.5 inches is 612px).

"Actual size" is calculated based upon a assumed screen density and size (both of which are basically not ever correct), and is an attempt to simulate physical inches. This false assumption is why an image of a ruler at actual size rarely coincides with a real-world ruler unless you are a) lucky; or b) have an OS and monitor which conspire to make this happen

Imagine the letter O placed at 12 pt. The measurement tool will give you the pixel dimensions of the O somewhere in the neighborhood of 10px (depending on font). If you zoom to 500% and take a screencap, then the actual number of pixels used to render that O at zoom will be far greater. The measurement tool would still show 10px.

If you want/need the screen capture to be pixel perfect, you need to figure out the correct zoom level for "actual size." If what inDesign calls actual size is 100%, then you'd want to set your zoom to something like 75% (66/88*100%). ({Measured PX} divided by {screen capture result PX} times {zoom level at time of screen capture}).

  • Thanks. I don't quite get this If actual size is 100%, then you'd want to set your zoom to something like 75% (66/88*100%) - could you explain that a bit more? – developerbmw Jul 7 '15 at 21:15
  • Updated: your measured px divided by the resultant px times the zoom level at time of capture. – Yorik Jul 7 '15 at 21:19
  • And, sometimes I do math "not good." So I apologize in advance if I got the ratios inverted or something. – Yorik Jul 7 '15 at 21:19
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Unlike Photoshop, InDesign generally doesn't have a native resolution because it is based on Vector design.

It used to always be important to understand DPI (dots per inch), although this discussion at Adobe mentions some things that may be related to your issue and points out that it should be PPI (pixels per inch)

Generally if you are using per pixel measurements, you'll want to set PPI/DPI to 72 pixels per inch as that's been the standard for screen resolution. (Although this is often incorrect also)

As for an accurate answer, there may need to be more information, but as first link mentions, it may be best to either flatten the InDesign image, or use Photoshop for raster pixel measuring. There's not enough information in your question to say if there's a fundamental flaw, but most likely it's a different in the way the images are being interpreted as pixels based on PPI resolution of the file.

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