Camera Gamma Estimation
We had to determine whether or not the camera was performing some sort of gamma correction to its images. Gamma values in general, are the same for all three color channels. Thus, we made the assumption that the camera, if it does perform any type of gamma correction, would behave similarly.
To start out, I generated a gray image of varying intensities. The intensities vary linearly, with values going from 0.1 to 1.0, in increments of 0.1. This image is then gamma corrected to make sure that the monitor is displaying this GreyStrip image with the correct intensity values and varys the intensities linearly. We then took a picture of this image with our digital camera. The image is show below:
Monitor GreyStrip Image
The above image was generated using the script, varGray.m.
After the camera process the image, we downloaded the image and took sample values within a window of 200 x 50 pixels for each of the ten strips and averaged them. These average values were then used to represent the intensity values for each strip. This corrected camera image is shown below:
Corrected Camera GreyStrip Image
The next step was to take the values of the camera output and perform curve fitting to obtain the gamma value. The equation for gamma is of the form:
Y = X ^ gamma;
Thus, if we take the log of both sides, we get:
log(Y) = gamma * log(X);
The log of the values can be estimated by a straight line, with gamma being the slope of the line. The plot below shows our results after performing curve fitting on the camera data.
The fit is not an exact fit but close enough. The discrepancies seen at higher intensity values could be attributed to measurement error and averaging error of the camera output.
From the data above, we found that the camera does in fact perform gamma correction and the value of gamma is 1.8004.
The MATLAB script used to generate and evaluate the estimated Camera output image is loadImages2.m.
Camera Settings
All images are assumed to be taken with the lights turned off, unless otherwise specified. The camera settings are specified below: