Adds support for property CV_CAP_PROP_FOCUS in functions cvSetCaptureProperty…

Adds support for property CV_CAP_PROP_FOCUS in functions cvSetCaptureProperty and cvGetCaptureProperty.
In compliance with the Windows (DirectDraw) version autofocus is disabled when either of the abovementioned functions is called.

modules/videoio/src/cap_libv4l.cpp

@@ -1433,6 +1433,30 @@ static double icvGetPropertyCAM_V4L (CvCaptureCAM_V4L* capture,
       sprintf(name, "Exposure");
       capture->control.id = V4L2_CID_EXPOSURE;
       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.
+
+      sprintf(name, "Focus");
+      capture->control.id = V4L2_CID_FOCUS_ABSOLUTE;
+      break;
+    }
     default:
       sprintf(name, "<unknown property string>");
       capture->control.id = property_id;
@@ -1650,6 +1674,27 @@ static int icvSetControl (CvCaptureCAM_V4L* capture, int property_id, double val
       sprintf(name, "Exposure");
       capture->control.id = V4L2_CID_EXPOSURE;
       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.
+
+      //now set the manual focus
+      sprintf(name, "Focus");
+      capture->control.id = V4L2_CID_FOCUS_ABSOLUTE;
+      break;
     default:
       sprintf(name, "<unknown property string>");
       capture->control.id = property_id;