Thanks all for your replies
You should bear in mind the constraints of my config :
under "pure" Windows Microsoft environment with its .tif specs
my Canon A2200 1.0d Camera without "jpg remote shoot" ( not implemented until now )
Note on cameras without remote shoot support, you can use shoot -dl -rm to shoot an image, download the file and delete it from the card. It's a little slower than remoteshoot, but probably still faster than DNG remote shoot if you only use the jpeg.
the results of the ocr is used to "measure" the quality of the scanning of the printed material.
I will post later the results of my experiments with dcraw and ImageMagick
I would suggest identifying what aspect of the tiff causes problems with your OCR. If it's the lens distortion, then using different programs isn't going to change anything (unless you find or create a lens profile to correct it). If it's something to do with the actual tiff structure or color values that confuses your OCR program, then maybe changing software would help.
While I know very little about OCR, my guess would be that the lens distortion is the problem. It's possible that a different zoom setting has less distortion.
Your DNG validates perfectly with adobe dng_validate, so I don't think there is any serious problem with CHDK DNG code on this camera.
Your dng has some unfixed bad pixels, which will put some spots on the image if your tiff conversion doesn't know to patch them. If you are using remoteshoot, you can can use -badpix to patch them in the download process. I doubt this causes any serious problem for OCR.
