GetThumbnailImage produces a larger file

jacob · November 7th, 2005, 07:58 AM

I am generating thumbnails on-the-fly for a web application; actually profile pictures. The problem is that the original picture has a much smaller size byte-wise than the thumbnail, which is not what I expected.

Scenario... One picture is 434x614 pixels in size and 59kb and fairly good quality. After creating the thumbnail, the image is 200x307 pixels and the size is 144kb!

How can this be? And how do I reduce the size (bytes) according to the size (pixels)?

I would assume that I would be able to get a thumbnail with a size (bytes) 1/4 of the original image. Why do I get a smaller image with a bigger size (bytes)?

My method for generating the thumbnail is as follows...

Code:

private string GenerateThumb(string sourcepath, string folder, string prefix, int width, 
    bool overwrite, bool deletecurrupt)
{
    double factor;
    System.Drawing.Image thumbnail, fullimage;        

    string file = Path.GetFileName(sourcepath);
    string filethumb = folder + (folder.EndsWith(@"\")? "" : @"\") 
        + prefix + "." + file;
    if(overwrite || (!overwrite && !File.Exists(filethumb)))
    {
        try
        {
            fullimage = System.Drawing.Image.FromFile(sourcepath);
            factor = fullimage.Width/(double)width;
            thumbnail = fullimage.GetThumbnailImage(width, 
            (int)(fullimage.Height/factor), null, IntPtr.Zero);
            fullimage.Dispose();

            string directory = Path.GetDirectoryName(filethumb);
            if(!Directory.Exists(directory))
                Directory.CreateDirectory(directory);

            thumbnail.Save(filethumb);
            thumbnail.Dispose();
        }
        catch(OutOfMemoryException)
        {
            if(deletecurrupt)
                File.Delete(sourcepath);
        }
    }
    return filethumb;
}

Thanks, Jacob.

Ankur_Verma · November 13th, 2005, 09:56 AM

Its probably 'cause of the difference of the encoding of the two image files. If you specify something like jpeg of gif while saving the thumbnail file, you will get a file of smaller size.

Regards
Ankur Verma

Imar · November 13th, 2005, 11:07 AM

Hi Jacob,

GetThumbnailImage isn't really the best way to get good looking thumbnails. If I am not mistaken, it tries to retrieve an embedded thumbnail from the image when present, or otherwise it creates a custom thumb. Internally, it's not using optimized settings.

You'll get more control over the resize process if you use the Graphics class and then use DrawImage top copy one Bitmap to another. With the Graphics object, you can control the quality of the image (with InterpolationMode) and control the output type (using the Save method of the Bitmap class and passing it an ImageFormat).

HtH and if not, let me know.

Cheers,

Imar
---------------------------------------
Imar Spaanjaars
Everyone is unique, except for me.

jacob · November 13th, 2005, 01:45 PM

Thanks guys. I changed the Save line to...

Code:

thumbnail.Save(filethumb, System.Drawing.Imaging.ImageFormat.Jpeg);

... and the picture I was refering to in the first post became 9kb. Quite a difference.

About the Graphics class... i found this inefficiency with the GetHumbnailImage a few days ago when I was looking for an answer for this post. If you are interested...

http://www.devx.com/dotnet/Article/22079/1954?pf=true

Unfortunately, my knowledge about images/pictures etc. are limited, why I am pretty sure I wouldn't gain the full benefit. I don't see the drawbacks mentioned in the article in the page I am currently doing.

The web application I am doing is merely a hobby project, and I therefore have the freedom to refine as much as I want, so perhaps I should look into graphics, image processing etc. Do you know about the performance in relation to GetThumbnailImage method? Does it require that the original image is a bitmap?

Thanks, Jacob.
(BTW Imar... one more abbreviation figured out)