Thursday, 18 April 2013

Web Publishing - Assignment 2

The purpose of this assignment is to experiment with the JPEG quality setting and the GIF 'no. of colours' setting in Photoshop. We are to explore the relationship between quality and file size and number of colours and file size using different images. The version of Photoshop that I'm using for this assignment is CS6.

These are the images that I'll be experimenting with:



The original images are in PNG format. Photoshop gives you a variety of settings when you go to save an image as a different format.



To explore the relationship between quality and file size for JPEG I set the images at quality levels 2, 4, 6, 8, 10, and 12. I then took note of the corresponding file size for each of these settings. The bar chart representing this data is below.

Bar chart showing the relationship between quality and file size
I find that the best value quality setting is 8 as it is difficult to see a difference between an image of this quality and an image of quality 12, yet as you can see in the bar chart it's approximately a third of the file size across all images. JPEG images are mainly used for digital cameras and web displays and the majority of people wouldn't see a difference between an 8 and a 12 in these instances.

Quality: 8

Quality: 12
I find that a quality setting of 2 is when the image becomes unacceptable which I find surprising as the differences from an 8 to a 6 to a 4 are only slight but going from a 4 to a 2 is quite jarring. The difference in quality is very noticeable and details in the image start to become hard to identify.

Quality: 2

To explore the relationship between the number of colours and file size for GIF I set the number of colours to 8, 16, 32, 64, 128 and 256. I then took note of the corresponding file size for each of these settings. I did this twice, once with dithering on (pattern) and once with dithering off. The line charts representing the data are below.


You can see the gradual increase in file size as the number of colours increases. You can also see that having dithering means a larger file size. Is dithering worth the larger file size? Let's compare two of the same images, one with dithering and one without.

With dithering
Without dithering
The two images above use 64 colours. You can see the difference with the dithering on but whether or not it's worth the extra file size, I believe, is personal opinion. I find dithering in this case to be more of a different quality rather than a better quality and I personally prefer the image without dithering.

With dithering, I find that 128 colours provides the best value as it's hard to see a difference between 256 colours and 128. When you drop down to 64 the image starts to become noticeably grainy and isn't really acceptable, unless maybe for an 'image preview' function or something of that nature.

256 colours would be the best value as there isn't too much of a difference in file size between that and 128 colours yet it's quality is noticeably better. In my opinion, 64 colours is the lowest acceptable number of colours as once you go down to 32 colours the image becomes really grainy and blotchy and the slightly lower file size isn't worth it.



After experimenting with the images and observing the data it's easy to see the direct correlation between image quality and file size and 'no. of colours' and file size. There are some really obvious differences, especially between the lowest quality and highest quality. The higher the quality and number of colours get, the harder it is to see a difference and most people probably won't perceive the differences at the higher end of the spectrum.

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