Point to pixel conversion

Question

On Mac OS X, I need to convert a point measurement to pixel measurement.

The formula which I know is

pixel   = point * resolution (in terms of dpi) / 72

I have few measurements which I want to convert to pixels. Although reverse cases would also be possible.

How to do this in Cocoa or Quartz? Does it depend on axis? Means would 5 pixels in Y-axis would be same as 5 pixels in X-axis in terms of points? Or is it safe to assume that resolution is same for both X and Y axis?

Please note that I do not want to make any assumption about resolution.

Answer 1

You probably don't want to convert anything to pixels. OS X now works in points; so for example when you draw a rectangle you are giving its dimensions in points, not pixels.

A OS X Quartz point is related to, but not the same as, a (computer) printing point - the two used to be the same, 72 points = 1". However WYSIWYG has become "some scale of" and 72 points (note not pixels) on screen is not a physical inch as screen pixel densities have increased. However 72 points is still an "abstract" inch.

In OS X you draw in points, the OS takes care of mapping those points to the physical pixels on the screen; which roughly translates to screens up to a certain density being treated as 72ppi (pixels/inch) or 1 pixel/point, and higher density screens being treated as 144ppi or 2 pixel/point - for example these are the ppi assigned to standard and Retina screenshots.

If you really, really need to know what a point translates to in pixels you can find out, but this changes depending on what screen a window is on.

For details of all of this you can start with Points Don’t Correspond to Pixels and then read the rest of the High Resolution Guidelines for OS X that reference is part of. How to find point to backing store mapping, if you really, really need to know, is covered.

HTH