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Here is a crude Matlab code. Can you test it, and if OK, i'll send a one-liner (if I can).

If a 2D $K_2$ filter kernel is of rank $0$ or $1$, it can be written as a separable product of $2$ 1D kernels $K_1^r$ and $K_1^c$ on rows and columns. As such, it can implemented by 1D convolutions, as long as one properly reshape the 2D matrices into 1D ones, and take care about "out-of-range" values, to avoid wrap-around. For instance, you can pad in every direction by the size of the filter, and make sure the convolution does not add unwanted information.

Assuming that you know that you have a separable 2D filter, the following code does the job. A one-liner would be:

xRowFull = reshape(conv(reshape(reshape( conv(x(:),s1,'same'),nRow,nCol)',nRow*nCol,1),s2,'same'),nRow,nCol)';

And the code is:

% https://dsp.stackexchange.com/questions/62115/2d-convolution-of-image-with-filter-as-successive-1d-convolutions
%% Initialization
clear all
nRow = 16;
nCol = 16;
HalfSizeCentralImageKernel = 1;
x = zeros(nRow,nCol);
x(nRow/2-HalfSizeCentralImageKernel:nRow/2+HalfSizeCentralImageKernel,nCol/2-HalfSizeCentralImageKernel:nCol/2+HalfSizeCentralImageKernel)=rand(2*HalfSizeCentralImageKernel+1);

%% Original 2D version
s1=[1,0,-1]';
s2=[1 2 1];
y = s1*s2;

%% Step by step 2x1D version
xRowFlat1 = x(:);
xRowFlat1FiltCol = conv(xRowFlat1,s1,'same');
xRowFlat2 = (reshape(xRowFlat1FiltCol,nRow,nCol))';
xRowFlat2 = xRowFlat2(:);
xRowFlat2FiltRowFlat = conv(xRowFlat2,s2,'same');
xRowFlatFilt2Row = reshape(xRowFlat2FiltRowFlat,nRow,nCol)';

%% Compact vectorized 1D version
xRowFull = reshape(conv(reshape(reshape( conv(x(:),s1,'same'),nRow,nCol)',nRow*nCol,1),s2,'same'),nRow,nCol)';

%% Display
figure(1);
imagesc(x);

figure(2);
subplot(1,3,1)
imagesc([conv2(x,y,'same')]); xlabel('Original')
subplot(1,3,2)
imagesc(xRowFlatFilt2Row); xlabel('Separable, step by step')
subplot(1,3,3)
imagesc(xRowFull); xlabel('Separable, one-liner')

diff1=conv2(x,y,'same')-conv2(conv2(x,s1,'same'),s2,'same');
disp(['Max error 1: ',num2str(max(abs(diff1(:))))]);

diff2=conv2(x,y,'same')-xRowFlatFilt2Row;
disp(['Max error 2: ',num2str(max(abs(diff2(:))))]);

[First answer]

Here is a crude Matlab code. Can you test it, and if OK, i'll send a one-liner (if I can).

Here is a crude Matlab code. Can you test i, and if OK, i'll send a one-liner (if I can).

nRow = 8;
nCol = 8;
HalfSizeCentralKernel = 1;
x = zeros(nRow,nCol);
x(nRow/2-HalfSizeCentralKernel:nRow/2+HalfSizeCentralKernel,nCol/2-HalfSizeCentralKernel:nCol/2+HalfSizeCentralKernel)=rand(2*HalfSizeCentralKernel+1);
figure(1);
imagesc(x);

% 2D version
s1=[1,0,-1]';
s2=[1 2 1];
y = s1*s2;
diff1=conv2(x,y,'same')-conv2(conv2(x,s1,'same'),s2,'same');
disp(['Max error 1: ',num2str(max(abs(diff1(:))))]);

% 1D version
xRowFlat1 = x(:);
xRowFlat1FiltCol = conv(xRowFlat1,s1,'same');
xRowFlat2 = (reshape(xRowFlat1FiltCol,nRow,nCol))';
xRowFlat2 = xRowFlat2(:);
xRowFlat2FiltRow = conv(xRowFlat2,s2,'same');
xRowFlatFilt2Row = reshape(xRowFlat2FiltRow,nRow,nCol)';

figure(2);
subplot(1,2,1)
imagesc([conv2(x,y,'same')])
subplot(1,2,2)
imagesc(xRowFlatFilt2Row)

Here is a crude Matlab code. Can you test it, and if OK, i'll send a one-liner (if I can).

nRow = 8;
nCol = 8;
HalfSizeCentralKernel = 1;
x = zeros(nRow,nCol);
x(nRow/2-HalfSizeCentralKernel:nRow/2+HalfSizeCentralKernel,nCol/2-HalfSizeCentralKernel:nCol/2+HalfSizeCentralKernel)=rand(2*HalfSizeCentralKernel+1);
figure(1);
imagesc(x);

% 2D version
s1=[1,0,-1]';
s2=[1 2 1];
y = s1*s2;
diff1=conv2(x,y,'same')-conv2(conv2(x,s1,'same'),s2,'same');
disp(['Max error 1: ',num2str(max(abs(diff1(:))))]);

% 1D version
xRowFlat1 = x(:);
xRowFlat1FiltCol = conv(xRowFlat1,s1,'same');
xRowFlat2 = (reshape(xRowFlat1FiltCol,nRow,nCol))';
xRowFlat2 = xRowFlat2(:);
xRowFlat2FiltRow = conv(xRowFlat2,s2,'same');
xRowFlatFilt2Row = reshape(xRowFlat2FiltRow,nRow,nCol)';

figure(2);
subplot(1,2,1)
imagesc([conv2(x,y,'same')])
subplot(1,2,2)
imagesc(xRowFlatFilt2Row)

Here is a crude Matlab code. Can you test i, and if OK, i'll send a on-liner (if I can).

nRow = 8;
nCol = 8;
HalfSizeCentralKernel = 1;
x = zeros(nRow,nCol);
x(nRow/2-HalfSizeCentralKernel:nRow/2+HalfSizeCentralKernel,nCol/2-HalfSizeCentralKernel:nCol/2+HalfSizeCentralKernel)=rand(2*HalfSizeCentralKernel+1);
figure(1);
imagesc(x);

% 2D version
s1=[1,0,-1]';
s2=[1 2 1];
y = s1*s2;
diff1=conv2(x,y,'same')-conv2(conv2(x,s1,'same'),s2,'same');
disp(['Max error 1: ',num2str(max(abs(diff1(:))))]);

% 1D version
xRowFlat1 = x(:);
xRowFlat1FiltCol = conv(xRowFlat1,s1,'same');
xRowFlat2 = (reshape(xRowFlat1FiltCol,nRow,nCol))';
xRowFlat2 = xRowFlat2(:);
xRowFlat2FiltRow = conv(xRowFlat2,s2,'same');
xRowFlatFilt2Row = reshape(xRowFlat2FiltRow,nRow,nCol)';

figure(2);
subplot(1,2,1)
imagesc([conv2(x,y,'same')])
subplot(1,2,2)
imagesc(xRowFlatFilt2Row)

Here is a crude Matlab code. Can you test i, and if OK, i'll send a one-liner (if I can).

nRow = 8;
nCol = 8;
HalfSizeCentralKernel = 1;
x = zeros(nRow,nCol);
x(nRow/2-HalfSizeCentralKernel:nRow/2+HalfSizeCentralKernel,nCol/2-HalfSizeCentralKernel:nCol/2+HalfSizeCentralKernel)=rand(2*HalfSizeCentralKernel+1);
figure(1);
imagesc(x);

% 2D version
s1=[1,0,-1]';
s2=[1 2 1];
y = s1*s2;
diff1=conv2(x,y,'same')-conv2(conv2(x,s1,'same'),s2,'same');
disp(['Max error 1: ',num2str(max(abs(diff1(:))))]);

% 1D version
xRowFlat1 = x(:);
xRowFlat1FiltCol = conv(xRowFlat1,s1,'same');
xRowFlat2 = (reshape(xRowFlat1FiltCol,nRow,nCol))';
xRowFlat2 = xRowFlat2(:);
xRowFlat2FiltRow = conv(xRowFlat2,s2,'same');
xRowFlatFilt2Row = reshape(xRowFlat2FiltRow,nRow,nCol)';

figure(2);
subplot(1,2,1)
imagesc([conv2(x,y,'same')])
subplot(1,2,2)
imagesc(xRowFlatFilt2Row)