How to create a thumbnail without reading the whole jpeg file

Is there a way to read a jpeg partially to construct a smaller sized
image?
For example if I have a 1000x1000pixel jpeg, i want to read only every
tenth pixel to create a 100x100 preview thumbnail. Is this possible?
I dont want to use the previously created thumbnail in the exif data,
or the thumbnails created by the explorer shell, i want the thumbnail
size to be flexible(could be almost as large as the original the
image) and high quality.

Thanks.

Date:Tue, 26 Jun 2007 19:43:00 -0700 Author:

Re: How to create a thumbnail without reading the whole jpeg file

Tedy Pranolo wrote:

> Is there a way to read a jpeg partially to construct a smaller sized
> image?
> For example if I have a 1000x1000pixel jpeg, i want to read only every
> tenth pixel to create a 100x100 preview thumbnail. Is this possible?
> I dont want to use the previously created thumbnail in the exif data,
> or the thumbnails created by the explorer shell, i want the thumbnail
> size to be flexible(could be almost as large as the original the
> image) and high quality.
>
> Thanks.

No, because a jpeg is not stored as a series of pixels, it's stored as an 
encoded byte stream, so there is no "every tenth pixel" to read - to get 
every 10th pixel you have to decode the whole image.

Date:Wed, 27 Jun 2007 14:52:32 +1200 Author:

Re: How to create a thumbnail without reading the whole jpeg file

In addition to Nik's comment you may be able to extract a thumbnail from 
the file if it has one. Some cameras put one in for quick display of 
their directories.

See the GetThumbnailImage method.

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Bob Powell [MVP]
Visual C#, System.Drawing

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Tedy Pranolo wrote:

> Is there a way to read a jpeg partially to construct a smaller sized
> image?
> For example if I have a 1000x1000pixel jpeg, i want to read only every
> tenth pixel to create a 100x100 preview thumbnail. Is this possible?
> I dont want to use the previously created thumbnail in the exif data,
> or the thumbnails created by the explorer shell, i want the thumbnail
> size to be flexible(could be almost as large as the original the
> image) and high quality.
> 
> Thanks.
>

Date:Wed, 27 Jun 2007 20:48:34 +0200 Author: