File size / Metadata

[RScholl]

I take pictures with my Lumix GX-1. The JPG-file size is approx. 4.5 MB.
I edit the photos with Picture Window Pro 6. The saved JPG-files are approx. 6 to 10 MB.
I use ImageMetaDataStripper to strip all the Metadata. Result: filesize of approx. 1 MB (!).
I open the 1-MB-file in Picture Window, don't edit it but save it again. Result: 2.5 MB.

Why are the files saved by Picture Window Pro 6 so big?

Thanks,
Rainer

[ksinkel]

Jpeg is what is called a lossy compression system. In other words to achieve high compression, it discards data by glossing over details in the image. The amount of data it discards is controlled by the quality setting. At 100% quality it discards no (or very little) detail and thus produces the largest files. As you lower the quality setting, it discards progressively more data and achieves more compact files.

In PWP, after you save a jpeg file, the size and compression ratio is displayed. If you would like a smaller file, change the quality setting or strip comments and or profile and save again. This will overwrite the previous save and report the statistics again. Each time the save is performed from the memory image, so each save is independent of previous iterations.

Kiril

[RScholl]

Thanks. But how is it possible, that a tool that strips only metadata, reduces the file size to only 10 - 20% ?
How large is the proportion of metadata in a jpg-file saved by PWP?

Rainer

[jsachs]

Metadata is usually generated by the camera and may be relatively large (it depends on the camera model) - for the most part, PWP just passes it through to the output file. Possibly the tool you are using to strip the metadata is re-compressing the image at a lower quality setting. If the JPEG includes a color profile that can also potentially be large. It is also possible the JPEG includes a thumbnail image as part of its metadata to facilitate image browsers.

To determine the metadata size, save the same image at the same quality setting both with and without the metadata and compare the sizes of the resulting images.

[RScholl]

Ok, I opened an original file from the camera, 4.622 KB, saved it by PWP (100%): 5.001 KB.
I saved it without profile and without comments: 5.001 KB, no difference (?).
I use Image MetaData Stripper (on it's web site http://codewelt.com/stripper it says "The image quality is _not_ altered in any way."), result is 776 KB.
I save the original file with PWP at 81 %: 798 KB.
Curiously I see no difference between the original file and the file stripped by Image MetaData Stripper.
But between the 81-%-file and the original is a significant difference in picture quality.

Rainer

[jsachs]

I can't speak to what MetaData Stripper is actually doing, but the comments on their web site suggest that there may be a large thumbnail image embedded in the JPEG file that they are removing. I don't really have an explanation for the file sizes you are seeing. Did you try saving in PWP without the profile?

[gio]

I woul'nt trust metadata stripper - from the resulting file sizes I suppose that metadata stripper strips the real image and the exif-data and leaves just the preview image.

[RScholl]

I compared a stripped file and an original file again, more thoroughly. Now I saw a difference, not clearly at the edges, but foremost in homogenous colour areas. But the stripped file (776 KB) is better than a file saved by 90 % (1.222 KB).

I don't think, Metadata stripper leaves only a preview image. I saved in PWP without profile and comments (and suppose without preview image) and stripped this with the same result. And I don't think that a file of 5 MB contains a preview image of 4 MB.

If there is a preview image in a file, does PWP change or refresh this preview image after processing when saving?

I suspect Metadata stripper to compress the image with a highly efficient algorithm. I see no other explanation.

BTW: The source code of Metadata stripper is very short and published on the website (codewelt.com/stripper). Maybe someone else is able to recognize what the program does.

Rainer

[gio]

And I don't think that a file of 5 MB contains a preview image of 4 MB.

I think a file of 5 MB contains a preview image of 1 MB - and everything else is removed by metadata stripper, also the real image, thus the file sizes you see. Other programs - ie pwp - use the real 4MB image.

You should provide the author of metadata stripper a sample file and your findings.

[Dieter Mayr]

Rainer, how did you check the 2 imaged for the differences? Just by eye ?
In my opinion there is ony one way to check for such subtle differences:
Use Transformation -> Special Effects -> Difference with the images to comapre as Input- resp. Subtract Image
The result usually looks black, but now use Levels and Color on it and expand the dynamic range by clicking "Full Range", now you have a good representation of the difference in the 2 original images.

I did play around with the program you mentioned and to me it looks as it just compresses the jpg image more then it was before (or more drastically, as it should).
Have a look here:
http://www.aldeid.com/wiki/Exiftool
I have uses Exiftool to examine some of my image files here:
No embedded preview image in any of my direct from camera file, no preview image in any of my PWP-processed files.
The site abobe contains links to 2 images that have embedded previews, if you want test.
I have processed images with no preview image and no exif data with ImageMetaDataStripper and they get smaller, PWPs test image bird.jpeg goes from 171 to 83 kB.
So, for me it is clear until one proofs me the opposite that there is bigger compression in the jpeg file.
The file saved with PWP as jpeg of about the same size requires a setting of about 80%, and gives btw a similar pattern of differences the the IMDS-processed image.