# How to multiply a 3x3 kernel to a gray scale image (uint8) ?

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theblueeyeswhitedragon 2018 年 8 月 1 日
コメント済み: Rik 2018 年 8 月 3 日
Gx = [-1 0 1;-2 0 2;-1 0 1];
Gy = [-1 -2 -1; 0 0 0; 1 2 1];
[rws, cls] = size(img);
mag = zeros(rws, cls);
for i = 1:rws - 2
for j = 1 : cls - 2
S1 = sum(sum(Gx.*img(i:i+2, j:j+2))); *
S2 = sum(sum(Gy.*img(i:i+2, j:j+2)));
mag(i+1, j+1) = sqrt(S1^2 + S2^2);
end
end
• I get an error on this line saying: "Error using .* Integers can only be combined with integers of the same class,or scalar doubles." Converting img to double solves this problem, but it also makes the image white for some reason. Is there a way to multiply uint8 gray scale images to a 3x3 matrix?

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### 採用された回答

Rik 2018 年 8 月 1 日

You should indeed cast your image to double to avoid underflow and overflow. Another way to do this is to simply use the conv function family to do this convolution.
The reason for your image looking white is that a double is expected to have a value range from 0 to 1 and not 0 to 255. Casting back to uint8 will fix that, as would explicitly setting the caxis.
##### 4 件のコメント表示非表示 3 件の古いコメント
Rik 2018 年 8 月 3 日
The internal convolution method is about 250 times faster than a nested loop by the way.

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R2017b

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