lecture_HVS.220.06.4up - Using the Frequency Response of...

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Hemami 220 Lec HVS_freq- 1 - 9/26/06 Using the Frequency Response of the Human Visual System to Analyze Applying DT Filtering to Digital Images Outline Understanding of “frequency” in space Human contrast sensitivity function Analysis of Image Filtering ECE 220 Fall 2006 Prof. Hemami Hemami 220 Lec HVS_freq- 2 - 9/26/06 Applying DT System Analysis to Images We know how to compute the frequency response of a DT LTI filter given the impulse response . We can describe the behavior of a filter (low-pass, high-pass, band-pass) by looking at the frequency response. For image processing when the receiver is a human, we need to understand how to relate frequencies to what we see — that is, to spatial frequencies in the image. hn () ω ˆ Hemami 220 Lec HVS_freq- 3 - 9/26/06 Spatial Frequency A frequency in time is a function of a single variable, e.g. A frequency in space is a function of two variables, x and y in the horizontal and vertical directions. 2 π 17 t cos Hemami 220 Lec HVS_freq- 4 - 9/26/06 Units of Spatial Frequency The spatial frequency unit is cycles/degree . cycles refers to the number of cycles of a sine (or cosine) degree measures the size of the field of view occupied by the stimulus 180 o field of view
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Hemami 220 Lec HVS_freq- 5 - 9/26/06 Defining Spatial Frequency 10 cycles 60 o The spatial frequency of the stimuli is 10 cycles/60 o = 0.167 cycles/degree Hemami 220 Lec HVS_freq- 6 - 9/26/06 Spatial Frequency is NOT Constant for an Image 10 cycles 45 o Same stimuli, different
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lecture_HVS.220.06.4up - Using the Frequency Response of...

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