Travails with my camera

Flickr causing aliasing

A zone plate following scaing in Flickr

I’ve noticed a few problems with Flickr causing what I suspect to be aliasing in images it has resized. I decided to create a zoneplate to test this theory. I created a PNG file using Gstreamer using the following command:

gst-launch -e videotestsrc num-buffers=1 pattern=zone-plate kx2=128 ky2=128 kt=2 ! video/x-raw-rgb, color-matrix=hdtv, framerate=25/1, width=4272, height=2848 ! pngenc compression-level=9 snapshot=FALSE ! multifilesink location="%05d.png"

This creates a zoneplate in accordance with Drewery, J.O., “The zone plate as a television test pattern”, BBC Research Department, 1978

The original image consists of concentric circles which get closer and closer together as they move towards the edge. At the edge the circles are 1 pixel wide. When you scale the image, some of the information is necessarily lost above a certain frequency. So for a perfect scaler, you’d expect the image I linked to to be a rectangle of detail surrounded by grey. You can’t get a perfect scaler, so we get what is shown. The detail sort of bleeds off into grey with extra circles added. The aliasing is the extra detail the scaler has added. On real pictures, this makes the picture artificially sharp and adds information where there wasn’t any in the original.

May need to experiment with slight blurring of the original to reduce the high frequencies in the image prior to scaling.

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One response

  1. However, it doesn’t look as bad as the scaler in Photoshop, see: http://www.talkphotography.co.uk/forums/showthread.php?t=440245

    October 12, 2012 at 4:37 pm

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