cHiddenImageData vs imageData

[prothero at earthednet.org]

I'm drawing symbols on an image. The live code tutorial give examples that use 
the cHiddenImageData of image "image"
The cHiddenImageData seems to place my changes all along the left side of the image. If I use "the image data", I get reasonable placements for my symbols.

There is no entry in the documents (6.2.2) for cHiddenImageData. What is it? Why would it be used instead of the plain image data?

Thanks,
Bill

William Prothero
http://es.earthednet.org