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Text present in captured images is high frequency or low frequency component?Why?

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The edges of the words will probably contribute to a high frequency, the background of the text and the portion within the boundaries of the letters of the text will be otherwise.

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When text is more, it is a high frequency image. If the image has only one or few letters in larger font size it is a low frequency image. This is because of following reasons:

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.

In other words, High Spatial Frequencies represent abrupt spatial changes in the image, such as edges, and generally correspond to featural information and fine detail. Low spatial frequencies, on the other hand, represent global information about the shape, such as general orientation and proportions.

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The letter boundaries are high frequencies and the letter interiors are low frequencies. So, thicker fonts mean more low frequencies and thinner fonts means more high frequencies.

However, this applies only if the text color is constant and the text contrast is high (e.g., black test on white background). If the color varies for a letter, then the variation will determine the frequency response of the letter interior. And if the text has low contrast, the high frequency components of the boundary will not be as prominent as for black text on white background.

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Actually this is very general. It can also depends on the clarity of the text.

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    $\begingroup$ Can you expand more on what you mean by "clarity" in terms of signal processing? The system is flagging this as a low-quality answer... and it's a little short! $\endgroup$ – Peter K. Nov 15 '13 at 12:39

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