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Date Posted: 07:49:38 01/29/01 Mon
Author: Michel
Subject: Re: JPEG scan bombing using Gray 256
In reply to: Mark Bailey 's message, "JPEG scan bombing using Gray 256" on 17:17:39 01/28/01 Sun


Hi Mark,

Many scanners return an image format of color256 when scanning gray scale. There are very similar indeed, a gray 256 is really a color 256 where the palette contains only gray values.

The Jpeg format does not support color 256 images, but
does support gray 256. So when saving with the TJpegGraphic class, the ImageFormat must be ifTrueColor or ifGray256, otherwise an exception will be raised.

To prevent this error, what you can do is to use the TConvertToGrayTransform to ensure the image format is ifGray256 before saving it.

Another option, which is more low level is to change the internal FImageFormat data member of the TJpegGraphic class, using a new class which has access to the protected data member. I would only do this if you are absolutely sure that the image is a gray scale image. For example:

type

TProtectedJpegGraphic = class(TJpegGraphic);

...

TProtectedJpegGraphic(JpegGraphic).FImageFormat := ifGray256;

Best regards,

Michel
>Hi,
>I am trying to scan images as Gray256 into a JPEG file.
>When I use TrueColor no problems but if I change to
>ifGray256 I get following exception raised in
>EnJpgGr.pas
>at procedure TJpegGraphic.SaveToStream(Stream:
>TStream);
>
>raise
>EJpegError.Create(msgJpegMustBeTrueColorOrGrayscale);
>
>Anyone got any suggestions please.
>
>Regards,
>Mark

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