EP0225029A2 — Image signal processing

Claim 1

Independent Claim 176 words

An image signal processing apparatus comprising: a memory (12); means (14) for generating addresses for allocation to successive words of a digital input representing an input image for mapping the image into the memory (12) in such a manner that individual portions of the mapped image stored in the memory may be compressed and/or rotated with respect to corresponding portions of the input image; a digital filter (40) of variable bandwidth operative to effect two-dimensional filtering of the input image; local scaling factor computation means (42) operative to monitor the addresses allocated to successive sets of words of the input signal corresponding to successive local image areas and to compute from each set of addresses, for the corresponding set of words, a local scaling factor representing the extent of compression and/or rotation of the corresponding local areas; and bandwidth control means (44) responsive to the successive local scaling factors to vary the bandwidth of the digital filter (40) in a sense to minimise aliasing that would be caused by compression and/or rotation of the local areas.

Claim 2

Dependent Claim 176 words

An image signal processing apparatus comprising: a memory (12); means (14) for generating addresses for allocation to successive words of a digital input representing an input image for mapping the image into the memory (12) in such a manner that individual portions of the mapped image stored in the memory may be compressed and/or rotated with respect to corresponding portions of the input image; a digital filter (40) of variable bandwidth operative to effect two-dimensional filtering of the input image; local scaling factor computation means (42) operative to monitor the addresses allocated to successive sets of words of the input signal corresponding to successive local image areas and to compute from each set of addresses, for the corresponding set of words, a local scaling factor representing the extent of compression and/or rotation of the corresponding local areas; and bandwidth control means (44) responsive to the successive local scaling factors to vary the bandwidth of the digital filter (40) in a sense to minimise aliasing that would be caused by compression and/or rotation of the local areas.

Claim 3

Dependent Claim 94 words

Apparatus according to claim 1, wherein the local scaling factor computation means (42) is operative to calculate, for each set of addresses, both a horizontal local scaling factor (HLSF) representing the extent of compression of the corresponding local area in the horizontal direction and a vertical local scaling factor (VLSF) representing the extent of compression of the corresponding local area in the vertical direction, and the bandwidth control means (44) is operative to vary the horizontal and vertical bandwidths of the filter (40) in accordance with the horizontal and vertical local scaling factors, respectively.

Claim 4

Dependent Claim 89 words

Apparatus according to claim 2, wherein the local scaling factor computation means (42) is operative to monitor the addresses allocated to a set (46) of at least three words of the input signal that bear a predetermined positional relationship in the input image, the set (46) comprising at least two words that are relatively horizontally spaced in the input signal and at least two words (at least one of which may be common to the first-mentioned at least two words) that are relatively vertically spaced in the input signal.

Claim 5

Dependent Claim 88 words

Apparatus according to claim 3, wherein the local scaling factor computation means (42) is operative to compute the horizontal local scaling factor by determining the spacing between the positions (I₁,I₂) signified by the addresses allocated to the at least two words of the input signal that are relatively horizontally spaced and is operative to compute the vertical local scaling factor by determining the spacing between the positions (I₁,I₃) signified by the addresses allocated to the at least two words of the input signal that are relatively vertically spaced.

Claim 6

Dependent Claim 165 words

Apparatus according to claim 3 or claim 4, wherein the local scaling factor computation means (42) comprises: (i) means (72) for calculating the angle ϑ h to the horizontal of a line joining the positions (I₁,I₂) signified by the addresses allocated to the at least two words of the input signal that are relatively horizontally spaced; (ii) means (74) for calculating the angle ϑ v to the vertical of a line joining the positions (I₁,I₃) signified by the addresses allocated to the at least two words of the input signal that are relatively vertically spaced; (iii) means (76,78) for determining respective values F(ϑ h ) and F(ϑ v ) that indicate the likelihood of aliasing for the calculated angles ϑ h and ϑ v , respectively; and (iv) means (66,70) for multiplying the horizontal local scaling factor and the vertical local scaling factor by F(ϑ h ) and F(ϑ v ), respectively, before the scaling factors (HLSF,VLSF) are applied to the bandwidth control means (44).

Claim 7

Dependent Claim 76 words

Apparatus according to claim 5, wherein the means (76,78) for determining the respective values F(ϑ h ) and F(ϑ v ) comprises look-up tables storing different values of F(ϑ h ) and F(ϑ v ) for different values of the angles ϑ h and ϑ v and is operative to select appropriate values of F(ϑ h ) and F(ϑ v ) in accordance with the calculated values of the angles ϑ h and ϑ v .

Claim 8

Dependent Claim 66 words

Apparatus according to any one of claims 2 to 6, wherein the filter (40) comprises a finite impulse response filter and the bandwidth control means (44) is operative to apply to the filter (40), for each set of addresses, a set of weighting coefficients that will cause the filter to have horizontal and vertical bandwidths appropriate to the horizontal and vertical local scaling factors (HLSF,VLSF), respectively.

Claim 9

Dependent Claim 79 words

Apparatus according to claim 7, wherein the bandwidth control means (44) comprises a look-up table storing a plurality of sets of weighting coefficients for different values of the horizontal and vertical local scaling factors, the bandwidth control means (44) being operative, for each set of addresses, to apply to the filter (40) a set of weighting coefficients appropriate to the values of the horizontal and vertical local scaling factors (HLSF,VLSF) computed by the local scaling factor computation means (42).

Claim 10

Dependent Claim 159 words

An image signal processing method comprising: generating addresses for allocation to successive words of a digital input signal representing an input image for mapping the image into a memory (12) in such a manner that individual portions of the mapped image stored in the memory may be compressed and/or rotated with respect to corresponding portions of the input image; effecting two-dimensional filtering of the input image by means of a digital filter (40) of variable bandwidth; monitoring the addresses allocated to successive sets of words of the input signal corresponding to successive local image areas and computing from each set of addresses, for the corresponding set of words, a local scaling factor representing the extent of compression and/or rotation of the corresponding local area; and varying the bandwidth of the digital filter (40) is response to the successive local scaling factors in a sense to minimise aliasing that would be caused by compression and/or rotation of the local areas.

Claim 11

Dependent Claim 159 words

An image signal processing method comprising: generating addresses for allocation to successive words of a digital input signal representing an input image for mapping the image into a memory (12) in such a manner that individual portions of the mapped image stored in the memory may be compressed and/or rotated with respect to corresponding portions of the input image; effecting two-dimensional filtering of the input image by means of a digital filter (40) of variable bandwidth; monitoring the addresses allocated to successive sets of words of the input signal corresponding to successive local image areas and computing from each set of addresses, for the corresponding set of words, a local scaling factor representing the extent of compression and/or rotation of the corresponding local area; and varying the bandwidth of the digital filter (40) is response to the successive local scaling factors in a sense to minimise aliasing that would be caused by compression and/or rotation of the local areas.

Claim 12

Dependent Claim 86 words

A method according to claim 9, wherein the step of computing the local scaling factor comprises, for each set of addresses, calculating both a horizontal local scaling factor (HLSF) representing the extent of compression of the local area in the horizontal direction and a vertical local scaling factor (VLSF) representing the extent of compression of the local area in the vertical direction, and wherein the horizontal and vertical bandwidths of the filter (40) are varied in accordance with the horizontal and vertical local scaling factors, respectively.

Claim 13

Dependent Claim 84 words

A method according to claim 10, wherein the monitoring step comprises monitoring the addresses allocated to a set (46) of at least three words of the input signal that bear a predetermined positional relationship in the input image, the set (46) comprising at least two words that are relatively horizontally spaced in the input signal and at least two words (at least one of which may be common to the first-mentioned at least two words) that are relatively vertically spaced in the input signal.

Claim 14

Dependent Claim 80 words

A method according to claim 11, wherein the horizontal local scaling factor (HLSF) is calculated by determining the spacing between the positions (I₁,I₂) signified by the addresses allocated to the at least two words of the inputs signal that are relatively horizontally spaced and the vertical local scaling factor (VLSF) is calculated by determining the spacing between the positions (I₁,I₃) signified by the addresses allocated to the at least two words of the input signal that are relatively vertically spaced.

Claim 15

Dependent Claim 155 words

A method according to claim 11 or claim 12 wherein the computation step comprises: (i) calculating (72) the angle ϑ h to the horizontal of a line joining the positions (I₁,I₂) signified by the addresses allocated to the at least two words of the input signal that are relatively horizontally spaced; (ii) calculating (74) the angle ϑ v to the vertical of a line joining the positions (I₁,I₃) signified by the addresses allocated to the at least two words of the input signal that are relatively vertically spaced; (iii) determining (76,78) respective values F(ϑ h ) and F(ϑ v ) that indicate the likelihood of aliasing for the calculated angles ϑ h and ϑ v , respectively; and (iv) multiplying (66,70) the horizontal local scaling factor and the vertical local scaling factor by F(ϑ h ) and F(ϑ v ), respectively, before the scaling factors (HLSF,VLSF) are applied to the bandwidth of the filter (40).

Claim 16

Dependent Claim 62 words

A method according to claim 13, wherein different values of F(ϑ h ) and F(ϑ v ) for different values of the angles ϑ h and ϑ v are stored in look-up tables (76,78) and appropriate values of F(ϑ h ) and F(ϑ v ) are selected in accordance with the calculated values of the angles ϑ h and ϑ v .

Claim 17

Independent Claim 64 words

A method according to any one of claims 10 to 14, wherein the filter (40) comprises a finite impulse response filter and its bandwidth is varied by applying to the filter (40), for each set of addresses, a set of weighting coefficients that will cause the filter to have horizontal and vertical bandwidths, appropriate to the horizontal and vertical local scaling factors (HLSF,VLSF), respectively.

Claim 18

Dependent Claim 63 words

A method according to claim 15, wherein a plurality of sets of weighting coefficients for different values of the horizontal and vertical local scaling factors (HLSF,VLSF) are stored in a look-up table (44) and, for each set of addresses, a set of weighting coefficients appropriate to the calculated values of the horizontal and vertical local scaling factors are applied to the filter (40).

Image signal processing

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