Apple’s Subpixel Font Rendering is Bugged in Mac OS X

On September 20, 2006 I submitted the following Mac OS X bug report to Apple via the ADC Bug Reporter. The bug has to do with OS X’s subpixel font rendering, which is activated if the font smoothing style (in the Appearance prefs pane) is set to “Medium – best for Flat Panel.”

    The ATI X1900 XT video card supports screen rotation in the OS X Displays control panel. A Mac Pro was setup with an X1900, a 30″ Apple Cinema Display, and a rotated Samsung 1600×1200 DVI display.

    See a similar setup:

    ‘Font smoothing style’ in the Appearance prefs pane is set to “Medium – best for Flat Panel.” This setting uses subpixeling, providing up to 3x the horizontal resolution of the screen’s native pixel width, etc. (More about sub-pixeling here: )

    Sub-pixeling depends upon the orientation of the RGB elements of each sub-pixel cluster that makes up a pixel. When a screen is rotated 90-degrees, the RGB “stripes” run horizontal, not vertical. As can be seen from the below images, where the first image (“broken”) shows the rotated Samsung and the second image (“proper”) shows the standard orientation Apple Cinema 30″, OS X’s sub-pixeling does NOT take into account the new orientation of the sub-pixel clusters, resulting in a rather bold and crude rendering of text on the rotated first (“broken”) image:

    Apple needs to correct this so that people don’t have to abandon sub-pixeling and use ‘Standard’ (greyscale) anti-aliasing. Microsoft takes sub-pixel cluster orientation into account with Windows Mobile (CE) on the Pocket PC’s. Surely Apple can get this working with rotated screens under OS X.

    Thanks. I’ve encountered, so far, one other user complaining about this issue.

I just checked the status of the big report (Problem ID 4740732, for any Mac OS X core developers at Apple reading this) and I find it’s still “Open,” which means it’s unlikely to be addressed in Leopard – the initial release, anyway. Even just the option of selecting a different font smoothing method for each screen would be a win, here.

Apple, please address this.

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