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conv2 'valid' implementation

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Alla on 2 Nov 2018
Commented: Savannah Quinn on 13 Sep 2020
I need to implement the conv2 function for a the HDL coder since it's not supported, I believe that I have the main function down, what I'm struggling with is the implementation of conv2 'valid' which is described in the documentation with — Returns only parts of the convolution that are computed without zero-padded edges-
The code for conv2 is the one I'm sharing, any help would be appreciated.
function B = convolve(A, k)
[r, c] = size(A);
[m, n] = size(k);
h = rot90(k, 2);
center = floor((size(h)+1)/2);
left = center(2) - 1;
right = n - center(2);
top = center(1) - 1;
bottom = m - center(1);
Rep = zeros(r + top + bottom, c + left + right);
for x = 1 + top : r + top
for y = 1 + left : c + left
Rep(x,y) = A(x - top, y - left);
B = zeros(r , c);
for x = 1 : r
for y = 1 : c
for i = 1 : m
for j = 1 : n
q = x - 1;
w = y -1;
B(x, y) = B(x, y) + (Rep(i + q, j + w) * h(i, j));


Juan Pereira
Juan Pereira on 7 Oct 2019
Useful, thank you.
Savannah Quinn
Savannah Quinn on 13 Sep 2020
Hello, I am trying to use this code to implement various image filters however I keep getting the an index out of bounds error due to h(i,j). Any ideas?

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Accepted Answer

Lin Bai
Lin Bai on 9 Jan 2019
as mentioned by Bharath, it is more efficient to use Simulink model instead of the MATLAB code to implement the conv2d function. Here I post the link of the Simulink model, which uses the image filer from Vision HDL toolbox to implement conv2 function.
In this model, image filter uses 'same' instead since it is more widely used in deep learning.


Show 4 older comments
Alla on 17 Jan 2019
I'm still trying to use the Pixel Control Bus creator to adjust the dimensions of some of my outputs, I'm unsure of how to feed my own control singals or modify the exisiting ones to take only parts of a matrix, for example remove the last column or row.
Lin Bai
Lin Bai on 18 Jan 2019
Here I attached the model using pixel control bus to resize the image, remove the last column and remove the last row separately. please check. alough the funtions mentioned above are achieved, the total time(clock cycles) for one image trnasmit is not changed at all.
You could also check this example "Image Pyramid for FPGA" in Matlab Help. this really achieves the resize function.
Alla on 21 Jan 2019
Tremendous thanks to you Lin Bai, you have been a great help. there are not may people here who can help with these things specefically so your input is immensly valuable.

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More Answers (2)

Bharath Venkataraman
Bharath Venkataraman on 2 Nov 2018
Edited: Bharath Venkataraman on 2 Nov 2018
If you are looking for conv2 for image filtering, you can use the ImageFilter block and System object in Vision HDL Toolbox.
The Image Processing Toolbox has an example that shows how to do this .

  1 Comment

Alla on 20 Nov 2018
Bharath Venkataraman Thank you for your answer, I'm working with matlab code and not simulink, and I plan on using conv2 for operations in a convolutional neural netowrk.

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Bharath Venkataraman
Bharath Venkataraman on 20 Nov 2018
You can use the MATLAB workflow in the example for image filtering.


Alla on 29 Nov 2018
I have tried using the Imagefilter block and code on a pixel stream of my input while using my own coefficients, I compared the outputs with conv2(in, filter, 'valid') and they're different. The source code for the filt2d function is understandbly inaccessible so I can't modify it to fit my requirements.
Bharath Venkataraman
Bharath Venkataraman on 29 Nov 2018
conv2 with 'valid' returns a smaller image (without padding), so please make sure you are only comparing the central part of the image.
You may find it closer to matching if you use 'same' instead of 'valid'.

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