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Date Posted: 00:12:32 07/04/00 Tue
Author: Jay
Subject: Re: Jpeg Quality on Read Request
In reply to: Michel 's message, "Re: Jpeg Quality on Read Request" on 17:29:55 07/03/00 Mon

Hi Michel,

I will try restating a little clear. The current form of the components will read in what was saved at that quality. What I am suggesting is a way to display the saved image at a "progress level/value". Why you may ask? We are discovering that the displaying of the images slows down as the number of images are loaded and displayed on our paint canvas. This is on machines with limited memory. So if I could display the image at a level display quality it would speed up the total paint process.

Here are a few assumptions I am making: (some may not be important)
1. The jpeg is saved at 100 quality
2. An original image of 10 Meg could be saved as 100kb.
3. Loading the image from the 100kb stream to a TJpegGraphic of ~10Meg
4. The displayed image is stretched to a width and height saved in our data file. So this image may be displayed as a 800x600 image or a 10x10 pixel display area.


So if assumption 3 is true, this is the where the addition of this “display progress” feature would be used. From this example the 10meg expanded image may now be expanded to 1Meg with a “diplay progress” of 10 percent. As I write this, I see that this like subsampling the image.

In my mind I guess I am thinking of the way the early web jpegs would fill the screen by displaying a rough (blocks) of the image. As the browser received move jpeg bits it is slowly filled in. If the data transfer is stopped the image is displayed as the rough image. I guess I am hoping that this same method could be used with our application.

I don’t know if this helps clarify what I am thinking or not. Or if is something that can be done or should be done.

Thanks,

Jay.

>
> Hi Jay,
>
> I am still a bit confused on this. Firstly, Envision
> does not support progressive display of jpeg files and
> that in itself will not affect the quality of the
> image, it only affects how the image is drawn when it
> is displayed. For dithering, as far as I know all JPEG
> images (except for gray scale) are saved in 24 bit
> true color. I know Delphi's TJpegImage has a smoothing
> property, but it is only used during the progressive
> display. After the image is displayed, the quality is
> the same.
>
> Best regards,
>
> Michel
>
> > 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;
> >
> > Description
> >
> > 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?
> >
> > Thanks,
> > Jay.
> >
> > >
> > > 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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