Conv2 explanation for specific example

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Leo
Leo 2011-7-26
Hey Guys, i need some help for the conv2 implementation for given problem:
K>> conv2([1 2 3], [1 0 0], 'full')
ans =
1 2 3 0 0
Why are the zeros on the right side? Does varies with the border depending on the kernel-center?
For this example its clear:
K>> conv2([1 2 3], [1 0 1], 'full')
ans =
1 2 4 2 3
Thanks for your help, Leo

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回答（4 个）

Jan
Jan 2011-7-26
Because you have vectors, your example is equivalent to:
conv([1, 2, 3], [1, 0, 0])
The algorithm of CONV is described exhaustively in: doc conv
E.g. the last element of the result is "w(2*n-1) = u(n)*v(n)", which is 0 in your example. Perhaps your are searchng for the 'same' or 'valid' shapes?
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the cyclist
the cyclist 2011-7-26
I noticed with a bit of amusement that conv() calls conv2().
Jan
Jan 2011-7-26
Indeed?
In Matlab 6.5 it called FILTER after reordering the inputs: CONV(A, B) equals CONV(B, A), but FILTER(B, 1, A) is faster, if length(A) < length(B).
I think I should read through all of the toolbox functions again...

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the cyclist
the cyclist 2011-7-26
Isn't the convolution calculation essentially doing what I have written below? Does that make it clearer where the zeros come from?
a = [1 2 3]
b = [1 0 0]
conv_a_b = b(1)*[a 0 0] + b(2)*[0 a 0] + b(3) * [0 0 a]
Leo
Leo 2011-7-26
thank you for your really quick responses! @Jan: yeah, i read conv2, but there is no information about how MATLAB set the paddings. In my work I have to apply a shift_kernel on a matrix to center the kernel, and the shif-kernel could be a matrix too. @cyclist: not really.. should 'a' replaced by [1 2 3] -> b(1) * [1 2 3 0 0] + ... ?
best, leo
  1 个评论
the cyclist
the cyclist 2011-7-26
In response to what you said to Jan: There is no "padding". MATLAB is treating the trailing zeros in "b" exactly how it would treat them if they were non-zero, and giving you the result.
In response of what you said to me: Yes. What I typed is functional code, using the "a" that you defined.

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Leo
Leo 2011-8-2
Hey guys,
thanks for your answers. I had an error in reasoning. I forgot that the kernel should be mirrored.
Best!

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