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Given the objective function:

$$ \frac{1}{2} {\left\| h \ast x - y \right\|}_{2}^{2} $$

Where $ h $ is the 2D convolution kernel and $ x $ is the 2D convolution image and $ y $ is a given 2D image.

What would be:

$$ \frac{\mathrm{d} \frac{1}{2} {\left\| h \ast x - y \right\|}_{2}^{2} }{\mathrm{d} h}, \; \frac{\mathrm{d} \frac{1}{2} {\left\| h \ast x - y \right\|}_{2}^{2} }{\mathrm{d} x} $$

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The easiest approach would be writing each case using Matrix Form of the convolution.
In this answer we assume the discrete convolution is applied only on valid support (Matching MATLAB's valid parameter for the convolution).
Namely, given $ x \in \mathbb{R}^{m \times n} $ and $ h \in \mathbb{R}^{k \times l} $ then $ h \ast x \in \mathbb{R}^{ \left( m - k + 1 \right) \times \left( n - l + 1 \right) } $. Needless to say $ m \geq k $ and $ n \geq l $ as otherwise the operation isn't well defined. Pay attention that this form of convolution isn't commutative.

Remark
For full convolution the problem is easier as the operation is commutative hence no difference between the gradient with respect to $ h $ or $ x $ as one could switch them.

Gradient with Respect to Convolution Kernel $ h $

The matrix form is given by:

$$ f \left( h \right) = \frac{1}{2} {\left\| X h - y \right\|}_{2}^{2} $$

Where $ X $ is the 2D Convolution Matrix Form of the image. Then:

$$ \frac{\mathrm{d} f \left( h \right) }{\mathrm{d} h} = {X}^{T} \left( X h - y \right) $$

The $ {X}^{T} $ forms a correlation (Versus Convolution) with full support of the operation (Equivalent of the full convolution shape in MATLAB syntax).

Hence we have:

$$ \frac{\mathrm{d} \frac{1}{2} {\left\| h \ast x - y \right\|}_{2}^{2} }{\mathrm{d} h} = x \star \left( h \ast x - y \right) $$

In MATLAB Code:

conv2(mX(end:-1:1, end:-1:1), (conv2(mX, mH, CONVOLUTION_MODE_VALID) - mY), CONVOLUTION_MODE_VALID);

Gradient with Respect to Convolution Image $ x $

The matrix form is given by:

$$ f \left( x \right) = \frac{1}{2} {\left\| H x - y \right\|}_{2}^{2} $$

Where $ H $ is the 2D Convolution Matrix Form of the kernel. Then:

$$ \frac{\mathrm{d} f \left( x \right) }{\mathrm{d} x} = {H}^{T} \left( H x - y \right) $$

In MATLAB Code:

conv2((conv2(mX, mH, CONVOLUTION_MODE_VALID) - mY), mH(end:-1:1, end:-1:1), CONVOLUTION_MODE_FULL);

MATLAB Code

The full code is available on my StackExchange Signal Processing Q59089 GitHub Repository.

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My (lazy) option is to dig into the Matrix cookbook (pdf), by Kaare Brandt Petersen and Michael Syskind Pedersen, namely section 2.4 Derivatives of Matrices, Vectors and Scalar Forms. They have a whole section on complex matrices as well.

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    $\begingroup$ Why dig if you have the answer above :-)? The tricky part is the valid use. See my answer. $\endgroup$ – Royi Jun 24 at 12:09
  • $\begingroup$ Why ask if you already had an answer :-)? I was pondering on the notion of "the easiest approach" $\endgroup$ – Laurent Duval Jun 24 at 16:36
  • $\begingroup$ The idea of the site, as I see it, is create a knowledge resource. Many times I have seen on different forums people looking for the gradients of those equations. Hence I thought it would be great to have them properly described accompanied by code. $\endgroup$ – Royi Jun 24 at 16:39
  • $\begingroup$ The great thing will be others adding their own approach to solve it. $\endgroup$ – Royi Jun 24 at 16:39

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