Читать книгу Multimedia Security, Volume 1 - William Puech - Страница 22

White balance aims to adjust values obtained by the sensors, so that they match the colors perceived by the observer by adjusting the gain values of each channel. White balance adjusts the output using characteristics of the light source, so that achromatic objects in the real scene are rendered as such (Losson and Dinet 2012).

For example, white balance can be achieved by multiplying the value of each channel, so that a pixel that has a maximum value in each channel is found to have the same maximum value 255 in all channels.

Then, the image goes through what is known as gamma correction. The charge accumulated by the sensor is proportional to the number of photons incident on the device during the exposure time. However, human perception is not linear with the signal intensity (Fechner 1860). Therefore, the image is processed to accurately represent human vision by applying a concave function of the form , where γ typically varies between 1.8 and 2.2. The idea behind this procedure is not only to enhance the contrast of the image but also to encode more precisely the information in the dark areas, which are too dark in the raw image.

Nevertheless, commercial cameras generally do not apply this simple function, but rather a tone curve. Tone curves allow image intensities to be mapped according to precomputed tables that simulate the nonlinearity present in human vision.

Figure 1.4. JPEG compression pipeline