Image Exposure Controller

本文最后更新于:2022年8月26日 晚上

  • 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).


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


  • image_exposure_control for RS cam [2]

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


Image Exposure Controller
Gavin Gao