Image Exposure Controller

• msckf-vio Issue: About image exposure controller #41 ^[1]

---

The auto exposure code is not open source because for one the actual implementation is ugly, and second because it ties into other software that is not open sourced.

Here a few (edited) notes that I sent to somebody who asked about how it works:

• what we implemented is super simple but works well in practice:

  \[ \text{new-shutter-time} = \text{current-shutter-time} \cdot \frac{\text{desired-brightness}}{\text{current-brightness}} \]

  You can go fancier by implementing

  \[ \text{new-shutter-time} = \text{current-shutter-time} \cdot \left(\frac{\text{desired-brightness}}{\text{current-brightness}}\right)^\alpha \]

  \(\alpha \leq 1\), but in practice \(\alpha = 1\) works just fine.

• brightness is computed from every 16th row and column, so in fact only 1 out of every 256 is used for brightness computation

• threshold on brightness:

  we only change shutter time if

  \[ |\text{desired-brightness} - \text{current-brightness}| > \text{threshold} \]

  This is so that we don't torture the ptgrey cameras with constant shutter speed changes.

• region of interest (ROI) support: we compute brighness only from the bottom X%. Typically X=70, so we will ignore the top 30% of the image, because that's often where the sun or sky are, and where we don't usually pick up features.

• configurable max shutter limit: the max shutter time is the min of the shutter time imposed by frame rate, and a configurable hard shutter limit (in case we get motion blur and decide we'd rather live with a darker image and/or gain noise than the motion blur).

• auto gain: once we hit max shutter, we turn on auto gain (and when it get's brighter again, we take the gain off first, then decrease shutter).

So

\[ \text{if} \; \left| \bar{B} - B_c \right| > B_{th}: \\ \; T_s \leftarrow T_s \cdot \frac{\bar{B}}{B_c} \\ \; \text{if} \; T_s < T_{min}: \\ \text{auto gain} \]

Implementation

• image_exposure_control for RS cam ^[2]

• vo-autoexpose ^[3] : An auto-exposure algorithm for maxing out VO performance in challenging light conditions

Reference

1. About image exposure controller #41 ↩
2. image_exposure_control (CCV) ↩
3. vo-autoexpose ↩