Calibrating Camera Response Curve http://gl.ict.usc.edu/HDRShop/tutorial/tutorial2.html

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Camera Curve Calibration

A lot of digital cameras are sufficiently different from a gamma 2.2 mapping that we need to explicitly derive their“Camera Response Curves”. We do this through a process called “Camera Response Curve Calibration”. It’s prettysimple really. You take several images of a scene at known exposures. The closer the exposures the better. I’drecommend ½ to ¼ stop increments, though it works ok with the standard 1 stop increments on most cameras. If youuse a good exposure (that has both saturated regions, black regions, and stuff in between), the calibration can workwith as little as two images, but in most cases you probably want to get four or five images to get a good curve. Oncewe have these images, we start up the Curve Calibrator in HDR Shop, which is under the Create menu as "Calibrate Camera Curve":

To specify your calibration images, click on the "Select Image Sequence" button. The "Select Calibration ImageSequence" dialog box will open:

Calibrating Camera Response Curve http://gl.ict.usc.edu/HDRShop/tutorial/tutorial2.html

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Click on the Load Images button to select files to add to your sequence. Loaded files are sorted automatically based onthe "sort" column, which initially stores the average pixel value. You can change the values in that column torearrange the files if they are not in the correct order.

Now you need to enter an exposure for each image in the sequence. You can enter the data in four different formats,corresponding to the four different columns. "Abs. Stops" for example, is the exposure of each image in absolutestops. "Rel. Stops" is the exposure difference in stops relative to the previous image. The scale columns store values interms of their linear scale multiplier. If the entire sequence is taken at a certain stop increment, you can choose one ofthe "Preset Scale Increments".

If you accidentally select and open the wrong files you can simply highlight them and click the Remove button.

Calibrating Camera Response Curve http://gl.ict.usc.edu/HDRShop/tutorial/tutorial2.html

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By default, you are entering values for all three channels (Red, Green and Blue) simultaneously. If you want to enterdifferent values for each channel, you can select R, G, or B in the "Select Channel(s)" box to enter values there. This ismainly useful if you are using filters to control the exposure level, as even supposedly "Neutral Density" filters are notalways entirely neutral (that is, they tend to block more blue light than red light).

Once all the information on the images that make up the sequence is correct press the Ok button. The image data willbe loaded and you will be returned to the Calibrate Camera Response Curve dialog box. At this point, you can start theiterative solver by pressing the "Go" button.

The blue crosshairs in the middle allow you to control the scale on the curve. It has no effect on the solver, it just letsyou inspect different areas of the curve.

You can choose the valid pixel range by moving the blue arrows at the bottom of the graph. In this case, the camera isclamping pixels at a very low value of around 160, which is confusing the solver somewhat. If we move the rightarrow down to ignore values greater than 160, the curve collapses into a good solution:

Calibrating Camera Response Curve http://gl.ict.usc.edu/HDRShop/tutorial/tutorial2.html

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If need be, you can apply a regularization factor (i.e. smoothing) to the curve to get rid of any bumps that you thinkare not part of the actual curve. Simply click the 'regularize' checkbox to turn on smoothing, and adjust the degree ofregularization with the slider:

Calibrating Camera Response Curve http://gl.ict.usc.edu/HDRShop/tutorial/tutorial2.html

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Once you have a curve you like, you can stop the solver and save the curve to a file for later reference.

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