Convolutional Pose MachinesConvolutional Pose MachinesWei, Shih-En and Ramakrishna, Varun and Kanade, Takeo and Sheikh, Yaser2016
Paper summaryaleju * They suggest a new model for human pose estimation (i.e. to lay a "skeleton" over the image of a person).
* Their model has a (more or less) recurrent architecture.
* Initial estimates of keypoint locations are refined in several steps.
* The idea of the recurrent architecture is derived from message passing, unrolled into one feed-forward model.
* They generate the end result in multiple steps, similar to a recurrent network.
* Step 1:
* Receives the image (368x368 resolution).
* Applies a few convolutions to the image in order to predict for each pixel the likelihood of belonging to a keypoint (head, neck, right elbow, ...).
* Step 2 and later:
* (Modified) Receives the image (368x368 resolution) and the previous likelihood scores.
* (Same) Applies a few convolutions to the image in order to predict for each pixel the likelihood of belonging to a keypoint (head, neck, right elbow, ...).
* (New) Concatenates the likelihoods with the likelihoods of the previous step.
* (New) Applies a few more convolutions to the concatenation to compute the final likelihood scores.
* Visualization of the architecture:
* ![Architecture](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Convolutional_Pose_Machines__architecture.jpg?raw=true "Architecture")
* Loss function
* The basic loss function is a simple mean squared error between the expected output maps per keypoint and the predicted ones.
* In the expected output maps they mark the correct positions of the keypoints using a small gaussian function.
* They apply losses after each step in the architecture, argueing that this helps against vanishing gradients (they don't seem to be using BN).
* The expected output maps of the first step actually have the positions of all keypoints of a certain type (e.g. neck) marked, i.e. if there are multiple people in the extracted image patch there might be multiple correct keypoint positions. Only at step 2 and later they reduce that to the expected person (i.e. one keypoint position per map).
* Example results:
* ![Example results](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Convolutional_Pose_Machines__results.jpg?raw=true "Example results")
* Self-correction of predictions over several timesteps:
* ![Effect of timesteps](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Convolutional_Pose_Machines__timesteps.jpg?raw=true "Effect of timesteps")
* They beat existing methods on the datasets MPII, LSP and FLIC.
* Applying a loss function after each step (instead of only once after the last step) improved their results and reduced problems related to vanishing gradients.
* The effective receptive field size of each step had a significant influence on the results. They increased it to up to 300px (about 80% of the image size) and saw continuous improvements in accuracy.
* ![Receptive field size effect](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Convolutional_Pose_Machines__rf_size.jpg?raw=true "Receptive field size effect")