I'm using the listview control to display thumbnails for a number of
relatively large images (~4MB each).

I was thinking, it would speed things up if I could cache a thumbnail for
each image, and display that, rather than the actual image. I'm not really
totally sure how to do that...

I've been looking in to the Image.GetThumbNailImage function, but it doesn't
seem to be working as expected.

If I use this, does the main image still get loaded if a cached thumbnail
exists?

Is there a better way of doing this?

It seems to be running quite slowly, and I'm not sure why.

Dale

Hi Dale,

    Basically what you would want to do is create a Dictionary...

    Dim myimagecache as new Dictionary(Of String, Bitmap)

    myimagecache .Add(filename, bitmapobject)  << Adding

    myimagecache .Item(filename) << Retreiving

    To be honest I wouldn't use this feature, not only is it slow but it can
also return embedded thumbnails in images which don't always match the
actual image.  Also the quality returned isn't as great as it can be.

    Yup, you need to perform any caching yourself.  One thing to remember
though is when you want to remove images from your cache, dispose of them
first....

    Dim myimage as Bitmap = myimagecache .Item(filename)
    myimagecache.Remove(filename)
    Call myimage.Dispose()

    Of course if you are clearing the entire cache you will need to do this
for each individual bitmap.  It won't take long but will make sure your app
runs smoothly and you don't run out of resources.

    One thing I have done in the past to get lightning fast thumbnails was
to use this,

    http://www.vbaccelerator.com/home/net/code/libraries/shell_projects/Thumbnail_Extraction/article.asp

    It's a hell of allot faster than using GetThumbnailImage or even
creating the thumbnail yourself.


Nick.

This looks very interesting, and should suit my needs. I'll have to take a
closer look at it.

Do you happen know off the top of your head if it is relying on the file
extension to determine the file type? Or does it look at the actual file?
One thing I've done, to try to discourage novice users from monkeying with
the underlying data outside my program, is to strip off the file extensions.
Looking at how this is working would tend to make me believe that the file
extensions might be important for this to work properly.

Dale

Hi Dale,

    Umm,  well as it's using Explorer, my understanding is that the file
extension will play a big part.  The best thing to do would be to have a try
tbh, but I think if Explorer treats the image differently after changing the
extension then so will this method.

    Out of interest are you thumbnailing images that are packed with the
application, or images that are created by the application at runtime?  If
the latter, one thing you could do, is create an index file that holds the
file type of each file and then just associate it by a key of some kind.

    <CacheIndex>
        <CacheItem>
            <Key>key goes in here</Key>
            <OriginalType>original file extension goes in
here</OriginalType>
            <MD5Hash>hash of file data goes in here</MD5Hash>
        </CacheItem>
        <CacheItem>
            <Key>key goes in here</Key>
            <OriginalType>original file extension goes in
here</OriginalType>
            <MD5Hash>hash of file data goes in here</MD5Hash>
        </CacheItem>
         ...
    </CacheIndex>

    I wrote "MD5Hash" there too as if you don't want the  user to change the
files in any way, you could also hash them and compare the hash with the
actual file data upon retrieval, if it differs then you know it's been
tampered with.

Nick.

Show quoteHide quote

That was my thought too. I'm in the midst of some restructuring right now
though (see my other question on looking for an XML tutorial for details),
so I may put the extension back in. It isn't really *crucial* that the user
doesn't change the files. The program just will yield some odd results if
they do (which I figure they can deal with if they are monkeying with the
database ;).

Basically they are images that the user imports in to an internal database.
The underlying goal here is to create an interactive anatomical atlas
program using high resolution images. Basically, it is a tool to help
students learn/study anatomy. The user (or the prof) brings in an image,
tags it with certain attributes (i.e. this is x,y,z). The images then become
'clickable' and will bring up a description of what ever you clicked on. The
description is then also clickable, and contains links to related topics,
and images within the database (this is where the thumbnails come in... in
the search box, the user is presented with thumbnails of the 'hits').

Dale

Hi Dale

    Here's one I wrote a while back, a little dated now as it was written
for .net 1.0, but it should still work,

    http://www.npsoftware.co.uk/Tutorials/tut_data_persistence_XML.php

    Personally I don't use databases unless I really need to, I think they
are overused and not everyone wants SQL Server installed, even if it is an
Express edition.  Access databases fill the middle ground I guess but still,
you can edit XML by hand.

    Aaah okay, then there is allot to think about in that respect, as I'm
not entirely how your app will be setup.  Like how is the application going
to be accessed? Client based instancing or a Web Browser?  What data exactly
is going to be shared?

    If it's per user instancing then you have no need to use SQL, you can
even encrypt and base 64 XML nodes so the user won't be able to read them
without a bit of "hacking" anyway.  If you want to share the thumbnails and
indexing file through a web browser then you could get away with creating a
web service that incorporates a singleton instance of your indexing
reader/writer, but that is a little more work than just using a database in
that respect.

    Decisions eh? lol!  Good luck anyway!

Nick.

Show quote Hide quote I'm throwing around the term 'database' a bit too liberally here. Basically
I'm building up a collection of XML files, and image files (JPGs), that will
be stored in a data directory.  I tend to stay away from more 'formal'
databases as well (unless I have a really strong call for it). I've been
working with Chris's example (dunawayc), and having a lot of success with
it. Only problem, is I have to do a lot of backend restructuring, but in the
end I think its worth it.

Show quoteHide quoteI'm going 'old school' on it. I despise webbased apps for this kind of
thing. The images are too big to deliver efficiently (the ones I'm working
at the moment with total more than 10GB...), so most people end of scaling
the images down. Trying to learn, this can be *really* frustrating, as
you'll be looking at the file, and you just want to zoom in 'a little bit'
but you're stuck, and you know the original image was higher resolution, but
they cut it back to make the CD/DVD etc.

You also then have to worry a lot more about security/access rights if the
database is shared between users.

So, yeah, the whole thing is going to be released on a DVD, and the user can
do what the heck they like with it. I've pretty well decided now that if the
user messes with the back end stuff, then it serves them right if the
program crashes on them. (they'll just have to delete the files they changed
to get it to work again.).

Lots of fun. Luckily I have the next 3 months to get it all working as I
want it to. Then I go back to school (and next year's class gets to play
with my program and find all the bugs in it ;) ).

Dale

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