On: Corrected "real world analysis of Google’s webp ve...

[jmillikin]

After further testing, it appears that the "Save EXIF data" and "Save XMP data" options are very expensive at these resolutions/file sizes. Disabling them nets a few dozen KiB. Additionally, setting "Smoothing" to full makes the image smaller at the expense of blurriness.

Updated version: < http://imgur.com/8aEPJ.jpg >

"Save for web" is probably a shortcut to remove those extra data. I wonder if Photoshop is also adding additional smoothing, beyond GIMP's maximum? That would explain why the PS image's background is smoother, but the inner details (like specular highlights) are not as distinct.

[jjcm → jmillikin]

The EXIF/XMP data is highly insightful, I didn't even think about how much space those would take up. I updated the original blog post with the information you provided here, and cited you for it. Hope you don't mind.

[Terretta → jjcm]

I mentioned in comments on your original article: I wasn't able to appreciably beat the BMW or portrait images.

Metadata would help explain this, since as a percentage of file size, it'd be a lot more detrimental on the first image at 45K than the BMW at 575K.

But sure seems there's something else going on with the first image's circular gradient; PS seems to be dithering its blocks in a moire pattern while GIMP is showing strong banding even on files w/o metadata.

[oozcitak → Terretta]

Metadata is most certainly the case here. The breakdown of metadata for the original image (http://fc07.deviantart.net/fs21/f/2007/275/f/4/f4e91d2565442...):

  EXIF (APP1) -> 5328 bytes
  Photoshop IRB (APP13) -> 5896 bytes
  Adobe XMP (APP1) -> 26284 bytes
  Adobe (APP14) -> 12 bytes 
  Total -> 37520 bytes (36.6 kB)

That's 36 kB of metadata, leaving only 8 kB for compressed image data. This explains the banding seen in the English Hard post. The BMW image on the other hand (http://fc01.deviantart.net/fs51/f/2009/313/1/c/Jesus_Mobile_...):

  Photoshop Ducky (APP12) -> 15 bytes 
  Adobe (APP14) -> 12 bytes
  Total -> 27 bytes

The existence of the "Ducky" section means that the image was "saved for web" in Photoshop.

EDIT: Added numbers for the BMW image.

[Terretta → oozcitak]

I got GIMP, and used the "Save as JPEG" output to see if it's including metadata.

According to jhead[1], GIMP is not including metadata in the saved JPEG.

At quality "13", the file was 46302 bytes. Using the "purejpg" option on jhead, it dropped to 45592, or -710 bytes.

This 45592 bytes is exactly, to the byte, what the original article found.

[1] http://www.sentex.net/~mwandel/jhead/usage.html