Exposure and autofocus properties fixed/added.

Exposure can now be manually controlled (implemented only in libv4l).
Will work only with a kernel version >= 4.5.
Autofocus can be enabled/disabled.

modules/videoio/src/cap_libv4l.cpp

@@ -1352,6 +1352,29 @@ static IplImage* icvRetrieveFrameCAM_V4L( CvCaptureCAM_V4L* capture, int) {
    return(&capture->frame);
 }
 
+static int zeroPropertyQuietly(CvCaptureCAM_V4L* capture, int property_id, int value)
+{
+  struct v4l2_control c;
+  int v4l2_min;
+  int v4l2_max;
+  //we need to make sure that the autocontrol is switch off, if available.
+  capture->control.id = property_id;
+  v4l2_min = v4l2_get_ctrl_min(capture, capture->control.id);
+  v4l2_max = v4l2_get_ctrl_max(capture, capture->control.id);
+  if ( !((v4l2_min == -1) && (v4l2_max == -1)) ) {
+    //autocontrol capability is supported, switch it off.
+    c.id    = capture->control.id;
+    c.value = value;
+    if( v4l2_ioctl(capture->deviceHandle, VIDIOC_S_CTRL, &c) != 0 ){
+      if (errno != ERANGE) {
+        fprintf(stderr, "VIDEOIO ERROR: V4L2: Failed to set autocontrol \"%d\": %s (value %d)\n", c.id, strerror(errno), c.value);
+        return -1;
+      }
+    }
+  }//lack of support should not be considerred an error.
+  return 0;
+}
+
 /* TODO: review this adaptation */
 static double icvGetPropertyCAM_V4L (CvCaptureCAM_V4L* capture,
                                      int property_id ) {
@@ -1431,32 +1454,15 @@ static double icvGetPropertyCAM_V4L (CvCaptureCAM_V4L* capture,
       break;
     case CV_CAP_PROP_EXPOSURE:
       sprintf(name, "Exposure");
-      capture->control.id = V4L2_CID_EXPOSURE;
+      capture->control.id = V4L2_CID_EXPOSURE_ABSOLUTE;
       break;
-    case CV_CAP_PROP_FOCUS: {
-      struct v4l2_control c;
-      int v4l2_min;
-      int v4l2_max;
-      //we need to make sure that the autofocus is switch off, if available.
-      capture->control.id = V4L2_CID_FOCUS_AUTO;
-      v4l2_min = v4l2_get_ctrl_min(capture, capture->control.id);
-      v4l2_max = v4l2_get_ctrl_max(capture, capture->control.id);
-      if ( !((v4l2_min == -1) && (v4l2_max == -1)) ) {
-        //autofocus capability is supported, switch it off.
-        c.id    = capture->control.id;
-        c.value = 0;//off
-        if( v4l2_ioctl(capture->deviceHandle, VIDIOC_S_CTRL, &c) != 0 ){
-          if (errno != ERANGE) {
-            fprintf(stderr, "VIDEOIO ERROR: V4L2: Failed to set control \"%d\"(FOCUS_AUTO): %s (value %d)\n", c.id, strerror(errno), c.value);
-            return -1;
-          }
-        }
-      }//lack of support should not be considerred an error.
-
+    case CV_CAP_PROP_FOCUS:
       sprintf(name, "Focus");
       capture->control.id = V4L2_CID_FOCUS_ABSOLUTE;
       break;
-    }
+    case CV_CAP_PROP_AUTOFOCUS:
+      sprintf(name, "Autofocus");
+      capture->control.id = V4L2_CID_FOCUS_AUTO;
     default:
       sprintf(name, "<unknown property string>");
       capture->control.id = property_id;
@@ -1671,30 +1677,22 @@ static int icvSetControl (CvCaptureCAM_V4L* capture, int property_id, double val
       capture->control.id = V4L2_CID_GAIN;
       break;
     case CV_CAP_PROP_EXPOSURE:
+      //we need to make sure that the autoexposure is switch off, if available.
+      zeroPropertyQuietly(capture, V4L2_CID_EXPOSURE_AUTO, V4L2_EXPOSURE_MANUAL);
+      //now get the manual exposure value
       sprintf(name, "Exposure");
-      capture->control.id = V4L2_CID_EXPOSURE;
+      capture->control.id = V4L2_CID_EXPOSURE_ABSOLUTE;
       break;
     case CV_CAP_PROP_FOCUS:
       //we need to make sure that the autofocus is switch off, if available.
-      capture->control.id = V4L2_CID_FOCUS_AUTO;
-      v4l2_min = v4l2_get_ctrl_min(capture, capture->control.id);
-      v4l2_max = v4l2_get_ctrl_max(capture, capture->control.id);
-      if ( !((v4l2_min == -1) && (v4l2_max == -1)) ) {
-        //autofocus capability is supported, switch it off.
-        c.id    = capture->control.id;
-        c.value = 0;//off
-        if( v4l2_ioctl(capture->deviceHandle, VIDIOC_S_CTRL, &c) != 0 ){
-          if (errno != ERANGE) {
-            fprintf(stderr, "VIDEOIO ERROR: V4L2: Failed to set control \"%d\"(FOCUS_AUTO): %s (value %d)\n", c.id, strerror(errno), c.value);
-            return -1;
-          }
-        }
-      }//lack of support should not be considerred an error.
-
+      zeroPropertyQuietly(capture, V4L2_CID_FOCUS_AUTO, 0 /*off*/);
       //now set the manual focus
       sprintf(name, "Focus");
       capture->control.id = V4L2_CID_FOCUS_ABSOLUTE;
       break;
+    case CV_CAP_PROP_AUTOFOCUS:
+      sprintf(name, "Autofocus");
+      capture->control.id = V4L2_CID_FOCUS_AUTO;
     default:
       sprintf(name, "<unknown property string>");
       capture->control.id = property_id;