Noise the Compression Killer.
In an earlier post we talked about the effect noise has on compression ratios. For this post I have conducted some simple experiments in Photoshop to provide some test results for you to compare and see what effect the noise has had on the resultant file size.
For all the tests I have used an image size of 800 x 640 RGB pixels which works out to be 1.46 M bytes of uncompressed data. Each image was saved using Photoshop CS2 JPEG compression level 12 ( max image quality - minimum compression.)
The first test was to create a 50% grey image using the colour picker and fill commands. The image saved down to 53,114 bytes.
A compression ratio of 29:1. Would of been better if the image area was bigger but we are suffering the overheads of the JPEG format.
This image contains virtually no information other than there are 512000 RGB pixels of a value of 127 in a 800 x 640 matrix. A more intelligent compressor would describe all that is needed to recreate the image in a dozen bytes or so.
I repeated the exercise but this time created a bright red image which saved down to 53,386 bytes. This was simply to compare a RGB image with no colour to one with a bright colour.
I next created another 50% grey and added 0.5% Gaussian noise which saved down to 185,054 bytes
The noise level was increase to 1% for the next test which saved down to 524,146 bytes
The grain effect (noise) was barely noticeable in the two images above and typical of a low noise image straight from the camera.
The last noise introduction test was with a noise level of 5% which saved down to 933,222 bytes In this case the grain was really noticeable and typical of low light photography using a high ISO setting.
The last test was to see how effective a noise reduction program such as Neat Image was at reducing the file size. A crop of 800 x 640 pixels was taken from a straight out of camera file shot at ISO 400. This saved down to 374,052 bytes. The same crop was passed to Neat Image for some very basic processing. This time it saved down to 293,126 bytes. A reduction of 20% in file size. My general experience has been that you get typically a 20% to 30% reduction in file size using a noise reduction program.
From the results above you can see it is very important to keep noise under control when trying to keep files sizes small.
Tips to avoid unwanted noise (grain effect)
- Don't shoot at a higher ISO(film sensitivity) setting than necessary.
- When choosing a new digital camera avoid cameras that have a very small sensor.
- Take care not to vastly underexpose shots. The camera sensor like all electronic devices will produce a certain amount of electrical noise. It is therefore important that the signal (light in this case) is much stronger than the noise.
- Take care in Post processing not to unduly increase noise levels when increasing the contrast or sharpening.
Noise levels can be reduced by using a noise reduction program or plug-in. However, care is needed as over processing can produce unrealistic results by removing image detail along with the noise.
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