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What do frequencies mean in an image?

If an image has large values at high frequency components then the data is changing rapidly on a short distance scale. e.g. a page of text

If the image has large low frequency components then the large scale features of the picture are more important. e.g. a single fairly simple object which occupies most of the image.

For colour images, The measure (now a 2D matrix) of the frequency content is with regard to colour/chrominance: this shows if values are changing rapidly or slowly. Where the fraction, or value in the frequency matrix is low, the colour is changing gradually. Now the human eye is insensitive to gradual changes in colour and sensitive to intensity. So we can ignore gradual changes in colour and throw away data without the human eye noticing, we hope.

Dave Marshall