Auto Exposure

 

Unfortunately it is not possible to manually control the exposure of the Kinect.

 

The Kinect development team did not want any one application to be able to control the exposure. So the camera’s gain, exposure time, frame rate, and white balance are all automatically controlled by the Kinect itself.

 

To make a bad situation worse, the Kinect’s auto exposure algorithm is extremely sensitive and “exposes to the right”.

 

This leads to inconsistent exposure, flickering, over exposure, and noisy images in low light.

 

Fortunately NextStage has an option to “compensate” for the automatic exposure, which alleviates a lot of these issues. By reading the exposure values for a given frame, NextStage can reverse the Kinect’s exposure and set it to a custom value.

 

This is far from an ideal solution, but it does suppress flickering and gives you some amount of creative control.

 

Exposure Value

 

Since we can’t adjust the actual values that affect exposure like gain and shutter speed, they’ve been combined into a single “exposure value” for simplicity.

 

The top slider shows us what the current “exposure value” is. The color tells us if the image is “well lit” and what the noise level is.

 

If the value is green then there will be little to no noise.

 

If it’s orange the lower values in the image will likely appear noisy.

 

If the value is red then we are dangerously close to being underexposed.

 

Below the exposure slider we can enable compensation, and use the second slider to set our target exposure. Moving it to the right will brighten the image, and moving it to the left will darken it.

 

Framerate Drop

 

There is one more big thing too look out for, and that’s underexposure. In HD mode, if the Kinect “feels” that the scene is too dark, it will drop the framerate down to 15 frames per second.

 

When this happens the “exposure value” will change from red to black, and drop all the way to the left. You will also see the image up above begin to stutter, and NextStage will temporarily turn off exposure compensation.

 

When recording at 15 frames per second NextStage treats every other frame as a dropped frame. So it will be up to you whether these missing frames are interpolated, or ignored.

 

If the exposure value hovers around “red” The Kinect might be right on the brink of dropping the framerate.

 

Other Modes

 

This is only in HD mode when you are using the Kinect’s color camera.

 

In Sync and Stream modes the framerate will never automatically drop. Since the tracking only uses the infrared camera and emitter, NextStage will track in a completely dark room.

 

This is an extremely frustrating limitation that we have no control over. Hopefully at some point Microsoft will fix this issue, though it does not seem likely.

 

With all of that in mind the onboard camera is capable of creating a very nice, uncompressed HD image. As long as you use a decent amount of light and avoid shooting high contrast scenes you should be able to get good footage natively.

 

 

Exposure Compensation