Many of you are concerned about images file sizes when posting to the web.  This is not as important as it once was thanks to user access to websites with improved connections.  For those of you who are posting large images, regardless of my point of view, or are placing a large number of images onto a single page, have to pay attention to size or risk tempting your audience to leave if it takes too long to load.

For this post I am going to assume that you have read my previous blogs about compression and focus on showing you why some image file sizes get so much larger than others when watermarked.

As many of you know, the digital watermark needs to “hide” itself into image data.  A typical photographic image like these 2 copies of a Peruvian salt mine have lots of details that the watermark can use to embed itself into.  The first has a no watermark, the second does and typically we might see at most a 5% change in the file size or even no difference in file sizes when saved using the same format and settings.

 

 

As far as my computer knows, these two images appear to be identical in size, which is a pretty standard result.
  
Sometimes a “standard result” is not the case and images might gain more than 5% in file size, these "special cases" can increase in size by 30-50%.  Once in the last 10 years I've seen a file almost double in size.  There are 3 common items that usually show up in a "special case" image that increases by more than 5% in file size.

1. The image has large areas of flat tints.
2. The image is at a higher resolution, often a 150 dpi or more so the image will look better when printed.
3. The final format is a lossless compression such as PNG or LZW Tiff

The following image sample is not watermarked.   I created it at 150 dpi with a large flat tint area and saved as a PNG file, “Watermark_flat-tints_noDFI.png”.

  

As you can see in this screen shot the file size changes quite a bit as the format changes and whether the image has a watermark within it.

My naming convention and file formats:

• At 1.3MB is Watermark_flat-tints_DFI.png file saved as 24bit and has a Digimarc for Images (DFI) watermark
• At 924KB is Watermark_flat-tints_noDFI.png file saved as 24bit and has no DFI watermark
• At 768KB Watermark_flat-tints_DFI.jpg file saved at 100% max image quality and a DFI watermark
• At 576KB is Watermark_flat-tints_noDFI.jpg file saved at 100% max image quality and no DFI watermark

The lossless PNG format with a watermark is around 40% larger than the PNG without a watermark and the lossy JPEG is about 33% larger.
If the JPEG image quality was reduced to 80%, the unmarked image would be 328KB and the watermarked version would be 392KB or only about a 20% difference.  JPEG compression can be a good solution for file size issues, but you do need to test the watermark strength in the JPEG results.  The PNG file’s watermark strength is just shy of the highest result

 

and at 80% image quality the JPEG’s watermark strength detection is a solid medium.

 

Both levels are quite acceptable.

My conclusion/solution for this post deals with the reality of the watermark adding data to an image.  If you use large areas of a flat tint, need extra image resolution for printing and want to keep your type clean by avoiding lossy compression, be prepared for larger files after watermarking.  If you can live with some JPEG compression, I’d recommend bumping up the watermark durability setting to 3 or 4 and testing your images after saving as JPEG.

Last, but not least, yes I really do enjoy making garish layer affects for sample images. 

  
It’s not often I can apply 3-6 effects to an image or text for fun.

P.S. You'll get to see me talk about this "in person" very soon... we're putting the finishing touches on another video tutorial related to this subject.