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Date Posted: 10:47:26 06/30/00 Fri
Author: Jay
Subject: Re: Jpeg Quality on Read Request
In reply to: Michel 's message, "Re: Jpeg Quality on Read Request" on 18:29:38 06/29/00 Thu

Hi Michel,

Here is a little bit more info on what I was talking about.

This is the property I was thinking of.

property Performance: TJPEGPerformance;

type TJPEGPerformance = (jpBestQuality, jpBestSpeed);

Use Performance to set the performance of the JPEG image when reading, that is, for decompressing files. This property is not used for writing out files.

Performance affects the display of the internal bitmap image. This image is a 256 color bitmap. Setting Performance to jpBestSpeed may cause some dithering in the bitmap image.

But what I am thinking is closer to what I described is the property ProgressiveDisplay: Boolean;


Use ProgressiveDisplay to turn on or off the incremental display of an image when reading in files. For example, if the JPEG data source file is on the hard drive, progressively displaying it slows down the display time: in this case setting ProgressiveDisplay to False speeds up the displaying of the image. If the file must be downloaded, set ProgressiveDisplay to True if the downloading process will take as long as the progressive displaying of the image.

ProgressiveDisplay is used only for decompression, but is affected by the manner in which the image was compressed. Images that were compressed with ProgressiveEncoding set to False cannot take advantage of ProgressiveDisplay. Setting ProgressiveDisplay to True for these images has no effect.

My thought was having a percent of ProgressiveDisplayed.
So the user has a feel for the image.

Does this help clarify my earlier comments?


> Hi Jay,
> I am not familiar with this, but I don't think if it
> is possible. As far as I know, the JPEG quality factor
> is used only when saving. It controls parameters used
> in the JPEG lossy compression algorithm. When reading
> a JPEG, the quality does not apply, the image is read
> with the quality that it was saved.
> Best regards,
> Michel
> > Hi Michel,
> > I am working on a project were we are using the
> > Envision Components. Love Them. The App is a simple
> > desktop publishing application. We save all raster
> > images out as jpegs to our file. It would nice if
> the
> > Jpeg class had an option on reading of the jpeg from
> > the stream at a set quality. This would provide a
> > simple way to speed up the displaying of images on
> > machine with less resource. It would be great if it
> > was a percentage. I know TJpegImage has a similar
> > property but it is only a few possible states.
> >
> > Thanks
> > Jay.

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