Programming iOS 4

Points and Pixels

A point is a dimensionless location described by an x-coordinate and a y-coordinate. When you draw in a graphics context, you specify the points at which to draw, and this works regardless of the device’s resolution, because Core Graphics maps your drawing nicely onto the physical output (using the base CTM, along with any anti-aliasing and smoothing). Therefore, throughout this chapter I’ve concerned myself with graphics context points, disregarding their relationship to screen pixels.

However, pixels do exist. A pixel is a physical, integral, dimensioned unit of display in the real world. Whole-numbered points effectively lie between pixels, and this can matter if you’re fussy, especially on a single-resolution device. For example, if a vertical path with whole-number coordinates is stroked with a line width of 1, half the line falls on each side of the path, and the drawn line on the screen of a single-resolution device will seem to be 2 pixels wide (because the device can’t illuminate half a pixel).

You will sometimes encounter advice suggesting that if this effect is objectionable, you should try shifting the line’s position by 0.5, to center it in its pixels. This advice may appear to work, but it makes some simple-minded assumptions. A more sophisticated approach is to obtain the UIView’s contentScaleFactor property (on iOS 4.0 and later). This value will be either 1.0 or 2.0, so you can divide by it to convert from pixels to points. Consider also that the ...