update docs

docs/en/advanced_guides/codecs.md

@@ -10,6 +10,8 @@ Here is a diagram to show where the `Codec` is:
 
 ![pose_estimator_en](https://github.com/open-mmlab/mmpose/assets/13503330/0764baab-41c7-4a1d-ab64-5d7f9dfc8eec)
 
+## Basic Concepts
+
 A typical codec consists of two parts:
 
 - Encoder
@@ -225,3 +227,225 @@ test_pipeline = [
     dict(type='PackPoseInputs')
 ]
 ```
+
+## Supported Codecs
+
+Supported codecs are in [$MMPOSE/mmpose/codecs/](https://github.com/open-mmlab/mmpose/tree/dev-1.x/mmpose/codecs). Here is a list:
+
+- [RegressionLabel](#RegressionLabel)
+- [IntegralRegressionLabel](#IntegralRegressionLabel)
+- [MSRAHeatmap](#MSRAHeatmap)
+- [UDPHeatmap](#UDPHeatmap)
+- [MegviiHeatmap](#MegviiHeatmap)
+- [SPR](#SPR)
+- [SimCC](#SimCC)
+- [DecoupledHeatmap](#DecoupledHeatmap)
+- [ImagePoseLifting](#ImagePoseLifting)
+- [VideoPoseLifting](#VideoPoseLifting)
+- [MotionBERTLabel](#MotionBERTLabel)
+
+### RegressionLabel
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/regression_label.py#L12)
+
+The `RegressionLabel` codec is used to generate normalized coordinates as the regression targets.
+
+**Input**
+
+- Encoding keypoints from input image space to normalized space.
+
+**Output**
+
+- Decoding normalized coordinates from normalized space to input image space.
+
+Related works:
+
+- [DeepPose](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#deeppose-cvpr-2014)
+- [RLE](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#rle-iccv-2021)
+
+### IntegralRegressionLabel
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/integral_regression_label.py)
+
+The `IntegralRegressionLabel` codec is used to generate normalized coordinates as the regression targets.
+
+**Input**
+
+- Encoding keypoints from input image space to normalized space, and generate Gaussian heatmaps as well.
+
+**Output**
+
+- Decoding normalized coordinates from normalized space to input image space.
+
+Related works:
+
+- [IPR](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#ipr-eccv-2018)
+- [DSNT](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#dsnt-2018)
+- [Debias IPR](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#debias-ipr-iccv-2021)
+
+### MSRAHeatmap
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/msra_heatmap.py)
+
+The `MSRAHeatmap` codec is used to generate Gaussian heatmaps as the targets.
+
+**Input**
+
+- Encoding keypoints from input image space to output space as 2D Gaussian heatmaps.
+
+**Output**
+
+- Decoding 2D Gaussian heatmaps from output space to input image space as coordinates.
+
+Related works:
+
+- [SimpleBaseline2D](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#simplebaseline2d-eccv-2018)
+- [CPM](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#cpm-cvpr-2016)
+- [HRNet](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#hrnet-cvpr-2019)
+- [DARK](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#darkpose-cvpr-2020)
+
+### UDPHeatmap
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/udp_heatmap.py)
+
+The `UDPHeatmap` codec is used to generate Gaussian heatmaps as the targets.
+
+**Input**
+
+- Encoding keypoints from input image space to output space as 2D Gaussian heatmaps.
+
+**Output**
+
+- Decoding 2D Gaussian heatmaps from output space to input image space as coordinates.
+
+Related works:
+
+- [UDP](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#udp-cvpr-2020)
+
+### MegviiHeatmap
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/megvii_heatmap.py)
+
+The `MegviiHeatmap` codec is used to generate Gaussian heatmaps as the targets, which is usually used in Megvii's works.
+
+**Input**
+
+- Encoding keypoints from input image space to output space as 2D Gaussian heatmaps.
+
+**Output**
+
+- Decoding 2D Gaussian heatmaps from output space to input image space as coordinates.
+
+Related works:
+
+- [MSPN](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#mspn-arxiv-2019)
+- [RSN](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#rsn-eccv-2020)
+
+### SPR
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/spr.py)
+
+The `SPR` codec is used to generate Gaussian heatmaps of instances' center, and offsets as the targets.
+
+**Input**
+
+- Encoding keypoints from input image space to output space as 2D Gaussian heatmaps and offsets.
+
+**Output**
+
+- Decoding 2D Gaussian heatmaps and offsets from output space to input image space as coordinates.
+
+Related works:
+
+- [DEKR](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#dekr-cvpr-2021)
+
+### SimCC
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/simcc_label.py)
+
+The `SimCC` codec is used to generate 1D Gaussian representations as the targets.
+
+**Input**
+
+- Encoding keypoints from input image space to output space as 1D Gaussian representations.
+
+**Output**
+
+- Decoding 1D Gaussian representations from output space to input image space as coordinates.
+
+Related works:
+
+- [SimCC](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#simcc-eccv-2022)
+- [RTMPose](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#rtmpose-arxiv-2023)
+
+### DecoupledHeatmap
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/decoupled_heatmap.py)
+
+The `DecoupledHeatmap` codec is used to generate Gaussian heatmaps as the targets.
+
+**Input**
+
+- Encoding human center points and keypoints from input image space to output space as 2D Gaussian heatmaps.
+
+**Output**
+
+- Decoding 2D Gaussian heatmaps from output space to input image space as coordinates.
+
+Related works:
+
+- [CID](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#cid-cvpr-2022)
+
+### ImagePoseLifting
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/image_pose_lifting.py)
+
+The `ImagePoseLifting` codec is used for image 2D-to-3D pose lifting.
+
+**Input**
+
+- Encoding 2d keypoints from input image space to normalized 3d space.
+
+**Output**
+
+- Decoding 3d keypoints from normalized space to input image space.
+
+Related works:
+
+- [SimpleBaseline3D](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#simplebaseline3d-iccv-2017)
+
+### VideoPoseLifting
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/video_pose_lifting.py)
+
+The `VideoPoseLifting` codec is used for video 2D-to-3D pose lifting.
+
+**Input**
+
+- Encoding 2d keypoints from input image space to normalized 3d space.
+
+**Output**
+
+- Decoding 3d keypoints from normalized space to input image space.
+
+Related works:
+
+- [VideoPose3D](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo_papers/algorithms.html#videopose3d-cvpr-2019)
+
+### MotionBERTLabel
+
+[\[Github\]](https://github.com/open-mmlab/mmpose/blob/dev-1.x/mmpose/codecs/motionbert_label.py)
+
+The `MotionBERTLabel` codec is used for video 2D-to-3D pose lifting.
+
+**Input**
+
+- Encoding 2d keypoints from input image space to normalized 3d space.
+
+**Output**
+
+- Decoding 3d keypoints from normalized space to input image space.
+
+Related works:
+
+- [MotionBERT](https://mmpose.readthedocs.io/zh_CN/dev-1.x/model_zoo/body_3d_keypoint.html#pose-lift-motionbert-on-h36m)