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Date Posted: 18:03:30 02/28/01 Wed
Author: Michel
Subject: Re: Grayscale JPEG and Palette
In reply to: Bob Villiers 's message, "Grayscale JPEG and Palette" on 15:09:28 02/28/01 Wed

Hi Bob,

I did come across your thread on the Borland forums and started to think about this.

Indeed, gray JPEG images don't have a palette, but when displaying on windows, it is very common to create a gray palette where every entry has the same value for the RGB components. This saves on memory rather than using true color.

The JPEG support in Envision (and many JPEG software including Delphi) is based on the "independent jpeg group library" which handles the conversion automatically from a palette to the JPEG "format". Other than that, there is not much else to worry about.

Best regards,


>I am working at the moment with grayscale JPeg images
>and had believed that a grayscale (8bit) Jpeg
>contained a colour table. I had been lead to this
>belief because both PaintShop Pro and Envision
>automatically produce a palette when a grayscale Jpeg
>is recovered.
>Now I am told that all JPegs are 24 bit and that
>production of a palette is only a function of
>decompression. So how does TJpegGraphic distinguish
>between a greyscale and colour image?

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