Add article on thermal camera perception and vision tasks (#211)

Introduces a new wiki page covering thermal perception, including sensor types, thermal-to-RGB mapping, and common perception tasks like depth estimation and SLAM.

---------

Co-authored-by: Abhishek-Iyer1 <iyer.abhishek18@gmail.com>
Co-authored-by: p-amyjiang <52675002+p-amyjiang@users.noreply.github.com>
Co-authored-by: Amy Jiang <p.amyjiang@gmail.com>
Co-authored-by: ranai-srivastav <ranaisrivastav@gmail.com>
Co-authored-by: Nevin Valsaraj <nevin.valsaraj32@gmail.com>

Author: 5 people

_data/navigation.yml

@@ -157,6 +157,8 @@ wiki:
         url: /wiki/sensing/apple-vision-pro/
       - title: Robotics with the Microsoft Hololens2
         url: /wiki/sensing/hololens-101/
+      - title: Perception via Thermal Imaging
+        url: /wiki/sensing/thermal-perception/
   - title: Controls & Actuation
     url: /wiki/actuation/
     children:

assets/images/foundation-stereo.png

@@ -71,6 +71,9 @@ This section dives into various sensing modalities such as GPS modules, fiducial
 - **[Thermal Cameras](/wiki/sensing/thermal-cameras/):**
   Examines the use of thermal cameras in robotics, including types of thermal cameras, calibration techniques, and debug tips.
 
+- **[Perception via Thermal Imaging](/wiki/sensing/thermal-perception/):**
+  Discusses strategies to implement key steps in a robotic perception pipeline using thermal cameras, including depth estimation and metric recovery.
+
 - **[Tracking Vehicles Using a Static Traffic Camera](/wiki/sensing/trajectory_extraction_static_camera/):**
   Describes a system for extracting vehicle trajectories using static traffic cameras, incorporating detection, tracking, and homography estimation.
 

assets/images/moge-relative-thermal.png

@@ -0,0 +1,255 @@
+---
+# Jekyll 'Front Matter' goes here. Most are set by default, and should NOT be
+# overwritten except in special circumstances. 
+# You should set the date the article was last updated like this:
+date: 2026-04-26 # YYYY-MM-DD
+# This will be displayed at the bottom of the article
+# You should set the article's title:
+title: Perception via Thermal Imaging
+# The 'title' is automatically displayed at the top of the page
+# and used in other parts of the site.
+---
+
+In this article, we discuss strategies to implement key steps in a robotic perception pipeline using thermal cameras.
+Specifically, we discuss the conditions under which a thermal camera provides more utility than an RGB camera, followed
+by implementation details to perform camera calibration, dense depth estimation and odometry using thermal cameras.
+
+## Why Thermal Cameras?
+
+Thermal cameras are useful in key situations where normal RGB cameras fail - notably, in perceptual degradation like
+smoke and darkness.
+Furthermore, unlike LiDAR and RADAR, thermal cameras do not emit any detectable radiation.
+If your robot is expected to operate in darkness and smoke-filled areas, thermal cameras are a means for your robot to
+perceive the environment in nearly the same way as visual cameras would in ideal conditions.
+
+## Why Depth is Hard in Thermal
+
+Depth perception — inferring the 3D structure of a scene — generally relies on texture-rich, high-contrast inputs.
+Thermal imagery tends to violate these assumptions:
+
+- **Low Texture**: Stereo matching algorithms depend on local patches with distinctive features. Thermal scenes often
+  lack these.
+- **High Noise**: Infrared sensors may introduce non-Gaussian noise, which confuses pixel-level correspondence.
+- **Limited Resolution**: Consumer-grade thermal cameras are often <640×480, constraining disparity accuracy.
+- **Spectral Domain Shift**: Models trained on RGB datasets fail to generalize directly to the thermal domain.
+
+_________________________
+
+## Calibration
+
+Calibration is the process by which we can estimate the internal and external parameters of a camera. Usually, the
+camera intrinsics matrix has the following numbers
+
+- fx, fy - This the focal length of the camera in the x and y directions **in the camera's frame**. px/distance_unit
+- cx, cy OR px, py - The principal point or the optical center of the image
+- distortion coefficients (2 - 6 numbers depending on distortion model used)
+
+Additionally, we must also estimate camera extrinsics which is the pose of the camera relative to another sensor - the
+body frame of a robot is defined to be the same as the IMU, or another camera in the case of multi-camera system
+
+- This will be in the form of series of 12 numbers - 9 for the rotation matrix and 3 for the translation
+- *NOTE*: BE VERY CAREFUL OF COORDINATE FRAMES
+- If using more than one sensor, timesync will help you.
+
+- Calibrating thermal cameras is quite similar to calibrating any other RGB sensor. To accomplish this you must have a
+  checkerboard pattern, Aruco grid or some other calibration target.
+    - A square checkerboard is not ideal because it is symmetrical and it is hard for the algorithm to estimate if the
+      orientation of the board has changed.
+    - An aruco grid gives precise orientation and is the most reliable option but is not necessary.
+
+General tips
+
+- For a thermal camera you will need to use something with distinct hot and cold edges, eg: a thermal checkerboard
+- Ensure that the edges on the checkerboard are visible and are not fuzzy. If they are adjust the focus, wipe the lens
+  and check if there is any blurring being applied
+- Ensure the hot parts of the checkerboard are the hottest things in the picture. This will make it easier to detect the
+  checkerboard
+- Thermal cameras by default give 16bit output. You will need to convert this to an 8bit grayscale image.
+- Other than the checkerboard, the lesser things that are visible in the image, the better your calibration will be
+- If possible, preprocess your image so that other distracting features will be ignored
+
+### Camera Intrinsics
+
+- Calibrating thermal camera intrinsics will give you fx, fy, cx, cy, and the respective distortion coefficients
+
+1. Heat up the checkerboard
+2. Record a rosbag with the necessary topics
+3. Preprocess your images
+4. Run them through OpenCV or Kalibr. There are plenty of good resources online.
+
+Example output from Kalibr:
+
+```text
+  cam0:
+  cam_overlaps: []
+  camera_model: pinhole
+  distortion_coeffs: [-0.3418843277284295, 0.09554844659447544, 0.0006766728551819399, 0.00013250437150091342]
+  distortion_model: radtan
+  intrinsics: [404.9842534577856, 405.0992911907136, 313.1521147858522, 237.73982476898445]
+  resolution: [640, 512]
+  rostopic: /thermal_left/image
+```
+
+### Thermal Camera peculiarities
+
+- Thermal Cameras are extremely noisy. There are ways you can reduce this noise
+- **Camera gain calibration:** The gain values on the camera are used to reduce or increase the intensity of the noise
+  in the image.
+    - The noise is increased if you are trying to estimate the static noise and remove it from the image (FFC)
+
+- **Flat Field Correction**: FFC is used to remove any lens effects in the image such as vignetting and thermal patterns
+  in the images
+    - FFC is carried out by placing a uniform object in front of the camera and taking a picture
+    - Then the noise patterns and then vignetting effects are estimated and then removed from the cameras
+    - The FLIR thermal cameras constantly "click" which them placing a shutter in front of the sensor and taking picture
+      and correcting for any noise
+    - The FLIR documentation describes Supplemental FFC (SFFC) which is the user performing FFC manually. It is
+      recommended that this is performed when the cameras are in their operating conditions
+
+### Camera Extrinsics
+
+- Relative camera pose is necessary to perform depth estimation. Kalibr calls this a camchain
+- Camera-IMU calibration is necessary to perform sensor fusion and integrate both sensor together. This can be estimated
+  using CAD as well.
+- Time-sync is extremely important for this because the sensor readings need to be at the exact same time for the
+  algorithm to effectively estimate poses.
+- While performing extrinsics calibration, ensure that all axes are excited (up-down, left-right, fwd-back, roll, pitch,
+  yaw) sufficiently. ENSURE that you move slow enough that there is no motion blur with the calibration target but fast
+  enough to excite the axes enough.
+
+________
+
+## Our Depth Estimation Pipeline Evolution
+
+### 1. **Stereo Block Matching**
+
+We started with classical stereo techniques. Given left and right images $I_L, I_R$, stereo block matching computes
+disparity $d(x, y)$ using a sliding window that minimizes a similarity cost (e.g., sum of absolute differences):
+
+$d(x, y) = argmin_d \space Cost(x, y, d)$
+
+In broad strokes, this brute force approach compares blocks from $I_L$ and $I_R$. For each block it computes a cost
+based on the pixel to pixel similarity (using a loss between feature descriptors generally). Finally, once a block match
+is found, the disparity is found by checking how much each pixel has moved in the x direction.
+
+As you can imagine, this approach is simple and lightweight. However, it is dependent on many things such as the noise
+in your images, the contrast separation, and will struggle to find accurate matches when looking at textureless and
+colorless inputs (like a wall in a thermal image). The algorithm performed better than expected, but we chose not to go
+ahead with it.
+
+---
+
+### 2. **Monocular Relative Depth with MoGe**
+
+If you are using a single camera setup, this is called a monocular approach. One issue is that this problem is ill
+posed. For example, objects can be placed at twice the distance and be scaled to twice their size to yield the same
+image. There is a reflection of the scale ambiguity that exists in any monocular depth estimation method. Therefore,
+learning based models are employed to "guess" the right depth (based on data driven priors like the usual chairs). One
+such model is MoGe (Monocular Geometry) which estimates *relative* depth $z'$ from a single image. These estimates are
+affine-invariant,
+meaning we need to apply a scale and a shift to retrieve metric depth:
+
+$z = s \cdot z' + t$
+
+This means they look visually coherent (look at the image below on the right), but the ambiguity limits 3D metric use (
+SLAM based applications).
+
+![Relative Depth on Thermal Images](/assets/images/moge-relative-thermal.png)
+
+---
+
+### 3. **MADPose Solver for Metric Recovery**
+
+To determine global scale and shift, we incorporated a stereo system and inferred relative depth from both. We then
+utilized the MADPose solver to find the scale and shift of both relative depth images to make them align, i.e. both
+depthmaps, after being made metric, should tell us the same 3D structure. This optimizer also estimates other properties
+such as extrinsics between the cameras, solving for more unknowns than necessary. Additionally, there is no
+temporal constraint imposed (you are looking at mostly the same things between $T$ and $T+1$ timesteps). This meant that
+the metric depth that we recovered would keep changing significantly across frames, resulting in pointclouds of
+different sizes and distances across timesteps. This method, while sound in theory, did not work out ver well in
+practise.
+
+---
+
+### 4. **Monocular Metric Depth Predictors**
+
+We also tested monocular models trained to output metric depth directly. This problem would be the most ill-posed
+problem as you would definitely overfit to the baseline of your training data and the approach would fail to generalize
+to other baselines. These treat depth as a regression problem from single input $I$:
+
+$z(x, y) = f(I(x, y))$
+
+Thermal's lack of depth cues and color made the problem even harder, and the models performed poorly.
+
+---
+
+### 5. **Stereo Networks Trained on RGB (e.g., MS2, KITTI)**
+
+Alternatively, when a dual camera setup is used, we call it a stereo approach. This inherently is a much simpler problem
+to solve as you have two rays that intersect at the point of capture. I encourage looking at the following set of videos
+to understand epipolar geometry and the formualation behind the stereo camera
+setup [Link](https://www.youtube.com/watch?v=6kpBqfgSPRc).
+
+We evaluated multiple pretrained stereo disparity networks. However, there were a lot of differences between the
+datasets used for pretraining and our data distribution. These models failed to generalize due to:
+
+- Domain mismatch (RGB → thermal)
+- Texture reliance
+- Exposure to only outdoor content
+- Reduced exposure
+
+---
+
+## Final Approach: FoundationStereo
+
+Our final and most successful solution was [FoundationStereo](https://github.com/NVlabs/FoundationStereo), a foundation
+model for depth estimation that generalizes to unseen domains without retraining. It is trained on large-scale synthetic
+stereo data and supports robust zero-shot inference.
+
+### Why It Works:
+
+- **Zero-shot Generalization**: No need for thermal-specific fine-tuning.
+- **Strong Priors**: Learned over large datasets of scenes with varied geometry and lighting. (These variations helped
+  overcome RGB to thermal domain shifts and textureless cues)
+- **Robust Matching**: Confidence estimation allows the model to ignore uncertain matches rathern than hallucinate.
+- **Formulation**: Formulating the problem as dense depth matching problem also served well. This allowed generalization
+  to any baseline by constraining the output to the pixel space.
+
+Stereo rectified thermal image pairs are given to FoundationStereo, which gives us clean disparity maps (image space).
+We
+recover metric depth using the intrinsics of the camera and the baseline. Finally, we can reproject this into the 3D
+space to get consistent point clouds:
+
+$$
+z = \frac{f \cdot B}{d}
+$$
+
+Where:
+
+- $f$ = focal length,
+- $B$ = baseline between cameras,
+- $d$ = disparity at pixel.
+
+An example output is given below (thermal preprocessed on the top left, disparity is middle left, and the metric
+pointcloud is on the right).
+
+![Metric Depth using Foundation Models](/assets/images/foundation-stereo.png)
+
+## Lessons Learned
+
+1. **Texture matters**: Thermal's low detail forces the need for models that use global context.
+2. **Don’t trust pretrained RGB models**: They often don’t generalize without retraining.
+3. **Stereo > Monocular for thermal**: Even noisy stereo is better than ill-posed monocular predictions.
+4. **Foundation models are promising**: Large-scale pretrained vision backbones like FoundationStereo are surprisingly
+   effective out-of-the-box.
+
+## Conclusion
+
+Recovering depth from thermal imagery is hard — but not impossible. While classical and RGB-trained methods struggled,
+modern foundation stereo models overcame the domain gap with minimal effort. Our experience suggests that for any team
+facing depth recovery in non-traditional modalities, foundation models are a compelling place to start.
+
+## See Also
+
+- The [Thermal Cameras wiki page](https://roboticsknowledgebase.com/wiki/sensing/thermal-cameras/) goes into more depth
+  about how thermal cameras function.