compressed images results

Web Performance Discuss Test Results
1

Hello all,
The compressed images results are showing up as a warning or failed
for the jpg images. As for the png, gift, etc. they show up as compressed (gzipped). I’m pretty sure images are compressed by default. Please correct me if I"m wrong.

ex)

Compress Images: 78/100
90.9 KB total in images, target size = 71.2 KB - potential savings = 19.7 KB

FAILED - (19.7 KB, compressed = 13.0 KB - savings of 6.8 KB) - http://www.qad.com/Public/Images/ERP/homePage/buttons/qad-explore-conference-2011.jpg
WARNING - (32.8 KB, compressed = 28.0 KB - savings of 4.8 KB) - http://www.qad.com/Public/Images/ERP/homePage/banner_images/qad-2011-enterprise-edition-erp.jpg
WARNING - (15.0 KB, compressed = 10.8 KB - savings of 4.2 KB) - http://www.qad.com/Public/Images/ERP/homePage/buttons/qad-on-demand-erp.jpg
WARNING - (12.5 KB, compressed = 8.6 KB - savings of 3.9 KB) - http://www.qad.com/Public/Images/ERP/homePage/buttons/qad-life-sciences-erp.jpg

Any thoughts on why this is happening?

Regards,
TRX

2

It’s telling you that the jpeg files can probably be saved at a lower quality level without looking any worse and save some space. Jpeg’s ARE always compressed but it is a “lossy” format and you can pick different quality levels which result in different file sizes.

The quality level’s aren’t consistent across tools but generally, in Photoshop you should be able to use a quality level of 50 in “save for web” mode. In most other tools that equates to around a level of 75. Ultimately a human still needs to decide at what point the images start to look bad (often times you can compress even more than the tool suggests).

3

Thanks. Does your explanation apply to failed URL also?

Regards,
TRX

4

Trx,

I think the difference between the “Failed” and 'Warning" is just a level of compression you could achieve versus your current level of compression.

The “Fail” is likely just telling you that the particular file is simply way too “big” in contrast to what it could be. If you save the file with a bit more compression, you may get just a “Warning”. If you save the file with a lot more compression, then the file may pass.

This is just my understanding of it. I haven’t paid too much attention to this area of the test as I don’t recall seeing these warnings on my site before.

5

Yep, what Marvin said. If the image can be reduced by 50% or more it will fail. 10-50% will warn.

6

How do you explain that when it fails and when the response header does NOT say “Content-Encoding: gzip” vs. fails and response header has “Content-Encoding: gzip”?

I thought images such as jpg as compressed by default by gzip. The below has warnings and failed results, but there is NO “Content-Encoding: gzip” in the response header. Thoughts on the weird results?

Compress Images:
FAILED (19.7 KB, target = 13.0 KB - savings of 6.8 KB)- http://qadpreviewdev.qad.com/Public/Explore%202011/images/buttons/qad-explore-conference-2011.jpg
WARNING (32.8 KB, target = 28.0 KB - savings of 4.8 KB)- http://qadpreviewdev.qad.com/Public/Images/ERP/banners/qad-2011-enterprise-edition-erp.jpg
WARNING (15.0 KB, target = 10.8 KB - savings of 4.2 KB)- http://qadpreviewdev.qad.com/Public/Explore%202011/images/buttons/qad-on-demand-erp.jpg
WARNING (12.5 KB, target = 8.6 KB - savings of 3.9 KB)- http://qadpreviewdev.qad.com/Public/Explore%202011/images/buttons/qad-life-sciences-erp.jpg
Image Compression score : 78
92.0 KB total in images, target size = 72.3 KB - potential savings = 19.7 KB

Regards,
TRX[hr]
ex)

HTTP/1.1 200 OK
Content-Length: 12844
Content-Type: image/jpeg
Last-Modified: Wed, 27 Apr 2011 20:57:12 GMT
Accept-Ranges: bytes
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Thu, 16 Jun 2011 23:54:57 GMT

7

It’s not about gzipping the image (that’s not actually something you want to do). It’s about the image itself. If you e-mail me the image that is failing (pmeenan@webpagetest.org) I can send you back an optimized version of it so you can better understand what I’m talking about.

Thanks,

-Pat

8

Thanks! I’ve emailed you the 2 images.

9

So below are the results of the new 2 files you’ve emailed me.
The results will show the original, opt version, and then the Photoshop version.

qad-2011-enterprise-edition-erp
Original: 32.8 KB
Gimp hand-tuned: 17.8 KB (46% savings)
Photoshop 50: 24.7 KB (25% savings)

qad-explore-conference-2011
Original: 19.7 KB
Gimp hand-tuned: 10.4 KB (47% savings)
Photoshop 50: 11.4 KB (42% savings)

New compressed files:

FAILED (19.7 KB, target = 13.0 KB - savings of 6.8 KB)- http://qadpreviewdev.qad.com/Public/Explore%202011/images/buttons/qad-explore-conference-2011.jpg
HTTP/1.1 200 OK
Content-Length: 20199
Content-Type: image/jpeg
Last-Modified: Wed, 27 Apr 2011 20:57:41 GMT
Accept-Ranges: bytes
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Tue, 21 Jun 2011 06:18:19 GMT

Compress Images:
FAILED (19.7 KB, target = 13.0 KB - savings of 6.8 KB)- http://qadpreviewdev.qad.com/Public/Explore%202011/images/buttons/qad-explore-conference-2011.jpg
Image Compression score : 65
19.7 KB total in images, target size = 13.0 KB - potential savings = 6.8 KB

http://qadpreviewdev.qad.com/Public/Explore%202011/images/buttons/opt-qad-explore-conference-2011.jpg
HTTP/1.1 200 OK
Content-Length: 10746
Content-Type: image/jpeg
Last-Modified: Tue, 21 Jun 2011 05:30:27 GMT
Accept-Ranges: bytes
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Tue, 21 Jun 2011 06:26:23 GMT

Compress Images:
Image Compression score : 100
10.5 KB total in images, target size = 10.5 KB - potential savings = 0.0 KB

http://qadpreviewdev.qad.com/Public/Explore%202011/images/buttons/ps-qad-explore-conference-2.jpg
HTTP/1.1 200 OK
Content-Length: 11725
Content-Type: image/jpeg
Last-Modified: Tue, 21 Jun 2011 05:29:41 GMT
Accept-Ranges: bytes
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Tue, 21 Jun 2011 06:32:04 GMT

Compress Images:
Image Compression score : 100
11.5 KB total in images, target size = 11.5 KB - potential savings = 0.0 KB

WARNING (32.8 KB, target = 28.0 KB - savings of 4.8 KB)- http://qadpreviewdev.qad.com/Public/Images/ERP/banners/qad-2011-enterprise-edition-erp.jpg
HTTP/1.1 200 OK
Content-Length: 33589
Content-Type: image/jpeg
Last-Modified: Wed, 27 Apr 2011 22:43:04 GMT
Accept-Ranges: bytes
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Tue, 21 Jun 2011 06:32:41 GMT

Compress Images:
WARNING (32.8 KB, target = 28.0 KB - savings of 4.8 KB)- http://qadpreviewdev.qad.com/Public/Images/ERP/banners/qad-2011-enterprise-edition-erp.jpg
Image Compression score : 85
32.8 KB total in images, target size = 28.0 KB - potential savings = 4.8 KB

http://qadpreviewdev.qad.com/Public/Images/ERP/banners/opt-qad-2011-enterprise-edition-erp.jpg

HTTP/1.1 200 OK
Content-Length: 18320
Content-Type: image/jpeg
Last-Modified: Tue, 21 Jun 2011 05:30:27 GMT
Accept-Ranges: bytes
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Tue, 21 Jun 2011 06:41:13 GMT

Compress Images:
Image Compression score : 100
17.9 KB total in images, target size = 17.9 KB - potential savings = 0.0 KB

http://qadpreviewdev.qad.com/Public/Images/ERP/banners/ps-qad-2011-enterprise-edit.jpg
HTTP/1.1 200 OK
Content-Length: 25359
Content-Type: image/jpeg
Last-Modified: Tue, 21 Jun 2011 05:32:11 GMT
Accept-Ranges: bytes
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Tue, 21 Jun 2011 06:42:29 GMT

Compress Images:
Image Compression score : 100
24.8 KB total in images, target size = 24.8 KB - potential savings = 0.0 KB

So I understand now on how a image will show a warning or a failed message, but I’m still confused on why there is no “Content-Encoding: gzip” in the response header. Any reason why you or anyone think that is the case?

Thanks in advance.

Regards,
TRX

10

You don’t gzip images, you only gzip text. Images (and video for that matter) are binary and already compressed and gzipping them would not make them any smaller. The gzip header is for additional compression that was done to transfer the file from the server to the client and just for the transfer, it doesn’t say anything about the file itself.

11

So in the response header it will say nothing about the images that are being compressed? Images are compressed by default by what layer? IIS? F5? Application?. What is the most convenient way to find out if images are being compressed without using tools such as webpagetest tool?

Thanks.

Regards,
TRX

12

No, there are no headers because the image files themselves are already compressed when they were originally saved. By definition, ALL images are compressed, the main question is if they are “compressed enough”. You can only check that with tools that were designed for checking images (WebPagetest and Zoompf are the only ones I know of that check jpeg’s for lossy compression).

If you have a publishing system then you can build checks into that but it’s usually just done by policy - telling any editors to always save at “photoshop quality level 50” for example.

13

Normally, images are compressed by image editing software (such as PhotoShop, etc) before you upload them to your server. Normally, there is no further compression done to images by the server.

14

Makes perfect sense. Thank for everyone’s help.

Regards,
TRX