Ghosting and compression of stereoscopic images

The JPEG is currently the most widely used format for presenting picture in the Internet. It's a lossy compression, meaning that part of the information present in the original picture is lost.

An inapropriate compression can damage the quality of the pictures. A too compressed image will loose details.

For stereo images, a feature of the JPEG compression JPEG must also be taken into acount: chrominance sub-sampling. To simplify, one can say that the compression of the "color" information is not carried out pixel per pixel but per block of 2 or 4 pixels which are then regarded as having the same color.

That becomes inconvenient for the stereoscopic images where one must code the two views (left and right) in a single image:

• Anaglyphs : color spectrum is devided in two, one part for the left view and the other part for the right view.
• Interlaced images: half of the pixel are used to represent the left image and the other half the right image.

Chrominance sub-sampling mix left and right views making them bleed on each other. That will produce annoying ghosts looking like shiny lines.

Fortunately many software have the option to disable chroma sub-sampling when saving a JPEG file, quality of the stereo picture is then greatly improved (at the cost of a slightly larger file).

AnaBuilder web site has a more detailed description of this subject and also lists the compatible software: Click here .

This page demonstrates the effect of chroma subsampling on some types of stereo pictures:

Red-cyan anaglyph

Lossless PNG compression (152 KB)	Standard JPEG with chroma subsampling (23 KB)	JPEG without chroma subsampling (37 KB)
	Differences with lossless image	Differences with lossless image

Horizontal interlacing

Lossless PNG compression (158 KB)	Standard JPEG with chroma subsampling (26 KB)	JPEG without chroma subsampling (36 KB)
	Differences with lossless image	Differences with lossless image

Sharp 3D format (subpixel vertical interlacing)

Lossless PNG compression (167 KB)	Standard JPEG with chroma subsampling (24 KB)	JPEG without chroma subsampling (45 KB)
	Differences with lossless image	Differences with lossless image

3D DLP® HDTV format (diagonal, checkerboard style interlacing)

Lossless PNG compression (188 KB)	Standard JPEG with chroma subsampling (28 KB)	JPEG without chroma subsampling (39 KB)
	Differences with lossless image	Differences with lossless image