Jaggies

Vincent Bockaert, 123di.com

Hardly a technical term, jaggies refer to the visible "steps" of diagonal lines or edges in a digital image. Also referred to as "aliasing", these steps are simply a consequence of the regular, square layout of a pixel.

Increasing Resolution Reduces the Visibility of Jaggies

Jaggies become less visible as the sensor or image resolution increases. The crops below are from pictures of a flower against a blue sky taken with digital cameras with different resolutions[1]. The low resolution cameras show very visible jaggies. As we increase the camera resolution from A to D, the steps become almost invisible in crop D. But they are still present when the image is enlarged, as shown in crop E.

A.
76,800 pixels
B.
307,200 pixels
C.
1.2 megapixel
D.
5 megapixel
E. Red zone in D
8X enlarged

Anti-aliasing Reduces the Visibility of Jaggies

Digital camera images undergo natural anti-aliasing because the pixels that measure the edges receive information from both sides of the edge. In this example the pixels that measure the yellow edge of the flower will also measure some blue sky resulting in values that are somewhere between yellow and blue. This makes the edges softer than in theoretical example F which has no anti-aliasing.

E. Red zone in D
8X enlarged
F.
No anti-aliasing

If the sensor has a color filter array, the interpolation of the missing information (demosaicing) uses information of surrounding pixels and will therefore cause additional anti-aliasing.

Sharpening Increases the Visibility of Jaggies

Sharpening will increases edge-contrast (reduce anti-aliasing) and make jaggies more visible, as shown in the sharpening topic. For the same reason, the jaggies in this rooftop against a bright sky are visible because the contrast of the image made the edge sharper.

Footnote

  1. (1) Simulated results, only crop D is from a real camera.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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