## What is Saliency?

Suppose that all the training images of bird class contains a tree with leaves. How do we know whether the CNN is using bird-related pixels, as opposed to some other features such as the tree or leaves in the image? This actually happens more often than you think and you should be especially suspicious if you have a small training set.

Saliency maps was first introduced in the paper: Deep Inside Convolutional Networks: Visualising Image Classification Models and Saliency Maps

The idea is pretty simple. We compute the gradient of output category with respect to input image. This should tell us how output category value changes with respect to a small change in input image pixels. All the positive values in the gradients tell us that a small change to that pixel will increase the output value. Hence, visualizing these gradients, which are the same shape as the image should provide some intuition of attention.

The idea behind saliency is pretty simple in hindsight. We compute the gradient of output category with respect to input image.

This should tell us how the output value changes with respect to a small change in inputs. We can use these gradients to highlight input regions that cause the most change in the output. Intuitively this should highlight salient image regions that most contribute towards the output.

## Usage

There are two APIs exposed to visualize saliency.

1. visualize_saliency: This is the general purpose API for visualizing saliency.
2. visualize_saliency_with_losses: This is intended for research use-cases where some custom weighted loss can be used.

See examples/ for code examples.

### Scenarios

The API is very general purpose and can be used in a wide variety of scenarios. We will list the most common use-cases below:

#### Categorical Dense layer visualization

By setting layer_idx to final Dense layer, and filter_indices to the desired output category, we can visualize parts of the seed_input that contribute most towards activating the corresponding output nodes,

• For multi-class classification, filter_indices can point to a single class.
• For multi-label classifier, simply set the appropriate filter_indices.

#### Regression Dense layer visualization

For regression outputs, we could visualize attention over input that

• increases
• decreases
• maintains

the regressed filter_indices output. For example, consider a self driving model with continuous regression steering output. One could visualize parts of the seed_input that contributes towards increase, decrease or maintenance of predicted output.

By default, saliency tells us how to increase the output activations. For the self driving car case, this only tells us parts of the input image that contribute towards steering angle increase. Other use cases can be visualized by using grad_modifier option. As the name suggests, it is used to modify the gradient of losses with respect to inputs.

• To visualize decrease in output, use grad_modifier='negate'. By default, ActivationMaximization loss yields positive gradients for inputs regions that increase the output. By setting grad_modifier='negate' you can treat negative gradients (which indicate the decrease) as positive and therefore visualize decrease use case.

• To visualize what contributed to the predicted output, we want to consider gradients that have very low positive or negative values. This can be achieved by performing grads = abs(1 / grads) to magnifies small gradients. Equivalently, you can use grad_modifier='small_values', which does the same thing. gradient_modifiers are very powerful and show up in other visualization APIs as well.

You can see a practical application for this in the self diving car example.

#### Guided / rectified saliency

Zieler et al. has the idea of clipping negative gradients in the backprop step. i.e., only propagate positive gradient information that communicates the increase in output. We call this rectified or deconv saliency. Details can be found in the paper: Visualizing and Understanding Convolutional Networks.

In guided saliency, the backprop step is modified to only propagate positive gradients for positive activations. For details see the paper: String For Simplicity: The All Convolutional Net.

For both these cases, we can use backprop_modifier='relu' and backprop_modifier='guided' respectively. You can also implement your own backprop_modifier to try your crazy research idea :)

#### Conv filter saliency

By pointing layer_idx to Conv layer, you can visualize parts of the image that influence the filter. This might help you discover what a filter cares about. Here, filter_indices refers to the index of the Conv filter within the layer.