Human Visual System. 4c8 Handout 2. Image and Video Compression. We have already seen the need for compression We can use what we have learned in information theory to exploit spatial and temporal redundancy but it is not enough
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4c8 Handout 2
just noticeable difference in intensity
a constant defining a quantum for perceived intensity
intensity is the power of the incident visible light
The solution is
These functions are known as colour matching functions and can be used to estimate RGB for any combination of colours.
Note: exact values of weights vary
Note: It is common to scale the U and V components so that it fits inside the range 0 to 255 (add 128 to both values)
Spatial frequency is measured in cycles per degree. It can be measured at any orientation.
Grating increases in freq. Left to Right
Intensity decreases vertically.
Sensitivity is given by the perceived
height of the columns.
Bands appear to be brighter on the left than on the right. This is due to spatial filtering in the visual cortex. This phenomenon is simulated with simple filtering of an image row using Unsharp Masking.
512 x 512 x 3
= 0.64 MB
2:1 in both
Downsample the U and V chrominance channels and leave the Y channel alone
512 x 512 + 256 x 256 x 2 = 0.31 MB (1/2 bandwidth of original)
512 x 512 + 128 x 128 x 2 = 0.24 MB (1/3 bandwidth of original)
128 x 128 x 3 = 0.04 MB (1/16 bandwidth of original)
Noise harder to see in Textured areas due to reduction in contrast sensitivity at higher spatial frequencies.
Objective Measures of Quality do not in general align well with the HVS
A 100 x 100 block of noise has been added to each image at two locations. Because of activity masking it is much less visible in right image. Hence perceived quality of the right image should be higher.
However, the MSE and PSNR for both images will be the same because the variance of the noise is the same in both images.
These images show the difference between the corrupted images and the original. MSE 3.7
Conclusion: Objective measures are of limited use. Better measures exist than MSE (eg. VQM and SSIM)
Subjective Measures of Quality are necessary
5 point CCIR 500 scale
Lots of subjects required to get reliable measurements. Also subjectivity implies that there will be disagreements between subjects.