drivers/media/video/gspca/coarse_expo_autogain.h

/*
 * Auto gain algorithm for camera's with a coarse exposure control
 *
 * Copyright (C) 2010 Hans de Goede <hdegoede@redhat.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

/* Autogain + exposure algorithm for cameras with a coarse exposure control
   (usually this means we can only control the clockdiv to change exposure)
   As changing the clockdiv so that the fps drops from 30 to 15 fps for
   example, will lead to a huge exposure change (it effectively doubles),
   this algorithm normally tries to only adjust the gain (between 40 and
   80 %) and if that does not help, only then changes exposure. This leads
   to a much more stable image then using the knee algorithm which at
   certain points of the knee graph will only try to adjust exposure,
   which leads to oscilating as one exposure step is huge.

   Note this assumes that the sd struct for the cam in question has
   exp_too_high_cnt and exp_too_high_cnt int members for use by this function.

   Returns 0 if no changes were made, 1 if the gain and or exposure settings
   where changed. */
static int gspca_coarse_grained_expo_autogain(struct gspca_dev *gspca_dev,
	int avg_lum, int desired_avg_lum, int deadzone)
{
	int i, steps, gain, orig_gain, exposure, orig_exposure;
	int gain_low, gain_high;
	const struct ctrl *gain_ctrl = NULL;
	const struct ctrl *exposure_ctrl = NULL;
	struct sd *sd = (struct sd *) gspca_dev;
	int retval = 0;

	for (i = 0; i < gspca_dev->sd_desc->nctrls; i++) {
		if (gspca_dev->ctrl_dis & (1 << i))
			continue;
		if (gspca_dev->sd_desc->ctrls[i].qctrl.id == V4L2_CID_GAIN)
			gain_ctrl = &gspca_dev->sd_desc->ctrls[i];
		if (gspca_dev->sd_desc->ctrls[i].qctrl.id == V4L2_CID_EXPOSURE)
			exposure_ctrl = &gspca_dev->sd_desc->ctrls[i];
	}
	if (!gain_ctrl || !exposure_ctrl) {
		PDEBUG(D_ERR, "Error: gspca_coarse_grained_expo_autogain "
			"called on cam without gain or exposure");
		return 0;
	}

	if (gain_ctrl->get(gspca_dev, &gain) ||
	    exposure_ctrl->get(gspca_dev, &exposure))
		return 0;

	orig_gain = gain;
	orig_exposure = exposure;
	gain_low =
		(gain_ctrl->qctrl.maximum - gain_ctrl->qctrl.minimum) / 5 * 2;
	gain_low += gain_ctrl->qctrl.minimum;
	gain_high =
		(gain_ctrl->qctrl.maximum - gain_ctrl->qctrl.minimum) / 5 * 4;
	gain_high += gain_ctrl->qctrl.minimum;

	/* If we are of a multiple of deadzone, do multiple steps to reach the
	   desired lumination fast (with the risc of a slight overshoot) */
	steps = (desired_avg_lum - avg_lum) / deadzone;

	PDEBUG(D_FRAM, "autogain: lum: %d, desired: %d, steps: %d",
		avg_lum, desired_avg_lum, steps);

	if ((gain + steps) > gain_high &&
	    sd->exposure < exposure_ctrl->qctrl.maximum) {
		gain = gain_high;
		sd->exp_too_low_cnt++;
	} else if ((gain + steps) < gain_low &&
		   sd->exposure > exposure_ctrl->qctrl.minimum) {
		gain = gain_low;
		sd->exp_too_high_cnt++;
	} else {
		gain += steps;
		if (gain > gain_ctrl->qctrl.maximum)
			gain = gain_ctrl->qctrl.maximum;
		else if (gain < gain_ctrl->qctrl.minimum)
			gain = gain_ctrl->qctrl.minimum;
		sd->exp_too_high_cnt = 0;
		sd->exp_too_low_cnt = 0;
	}

	if (sd->exp_too_high_cnt > 3) {
		exposure--;
		sd->exp_too_high_cnt = 0;
	} else if (sd->exp_too_low_cnt > 3) {
		exposure++;
		sd->exp_too_low_cnt = 0;
	}

	if (gain != orig_gain) {
		gain_ctrl->set(gspca_dev, gain);
		retval = 1;
	}
	if (exposure != orig_exposure) {
		exposure_ctrl->set(gspca_dev, exposure);
		retval = 1;
	}

	return retval;
}