在之前的增强型视觉系统 (EVS) 1.0 版本中，摄像头设备被视为只读设备，因此不存在任何可让应用更改摄像头控制参数（如缩放或亮度）的方法。
由于这可能会限制 EVS 应用的功能，因此新的 EVS 1.1 引入了新的方法，让应用能够用程序编写多个摄像头控制参数，所有这些参数都在 enum CameraParam 中定义：
/**
* EVS Camera Parameter
*/
enum CameraParam : uint32_t {
/**
* The brightness of image frames
*/
BRIGHTNESS,
/**
* The contrast of image frames
*/
CONTRAST,
/**
* Automatic gain/exposure control
*/
AUTOGAIN,
/**
* Gain control
*/
GAIN,
/**
* Automatic Whitebalance
*/
AUTO_WHITE_BALANCE,
/**
* Manual white balance setting as a color temperature in Kelvin.
*/
WHITE_BALANCE_TEMPERATURE,
/**
* Image sharpness adjustment
*/
SHARPNESS,
/**
* Auto Exposure Control modes; auto, manual, shutter priority, or
* aperture priority.
*/
AUTO_EXPOSURE,
/**
* Manual exposure time of the camera
*/
ABSOLUTE_EXPOSURE,
/**
* Set the focal point of the camera to the specified position. This
* parameter may not be effective when auto focus is enabled.
*/
ABSOLUTE_FOCUS,
/**
* Enables continuous automatic focus adjustments.
*/
AUTO_FOCUS,
/**
* Specify the objective lens focal length as an absolute value.
*/
ABSOLUTE_ZOOM,
};
方法定义如下：
/**
* Requests to be a master client.
*
* When multiple clients subscribe to a single camera hardware and one of
* them adjusts a camera parameter such as the contrast, it may disturb
* other clients' operations. Therefore, the client must call this method
* to be a master client. Once it becomes a master, it will be able to
* change camera parameters until either it dies or explicitly gives up the
* role.
*
* @return result EvsResult::OK if a master role is granted.
* EvsResult::OWNERSHIP_LOST if there is already a
* master client.
*/
setMaster() generates (EvsResult result);
/**
* Sets to be a master client forcibly.
*
* The client, which owns the display, has a high priority and can take over
* a master role from other clients without the display.
*
* @param display IEvsDisplay handle. If this is valid, the calling client
* is considered as the high priority client and therefore
* it would take over a master role.
*
* @return result EvsResult::OK if a master role is granted.
* EvsResult::OWNERSHIP_LOST if there is already a
* master client with the display.
*/
forceMaster(IEvsDisplay display) generates (EvsResult result);
/**
* Retires from a master client role.
*
* @return result EvsResult::OK if this call is successful.
* EvsResult::INVALID_ARG if the caller client is not a
* master client.
*/
unsetMaster() generates (EvsResult result);
/**
* Retrieves a list of parameters this camera supports.
*
* @return params A list of CameraParam that this camera supports.
*/
getParameterList() generates (vec<CameraParam> params);
/**
* Requests a valid value range of a camera parameter
*
* @param id The identifier of camera parameter, CameraParam enum.
*
* @return min The lower bound of the valid parameter value range.
* @return max The upper bound of the valid parameter value range.
* @return step The resolution of values in valid range.
*/
getIntParameterRange(CameraParam id)
generates (int32_t min, int32_t max, int32_t step);
/**
* Requests to set a camera parameter.
*
* @param id The identifier of camera parameter,
* CameraParam enum.
* value A desired parameter value.
* @return result EvsResult::OK if it succeeds to set a parameter.
* EvsResult::INVALID_ARG if either a requested
* parameter is not supported or a given value is out
* of bounds.
* effectiveValue A programmed parameter value. This may differ
* from what the client gives if, for example, the
* driver does not support a target parameter.
*/
setIntParameter(CameraParam id, int32_t value)
generates (EvsResult result, int32_t effectiveValue);
/**
* Retrieves a value of given camera parameter.
*
* @param id The identifier of camera parameter, CameraParam enum.
* @return result EvsResult::OK if it succeeds to read a parameter.
* EvsResult::INVALID_ARG if either a requested parameter is
* not supported.
* value A value of requested camera parameter.
*/
getIntParameter(CameraParam id) generates(EvsResult result, int32_t value);
getParameterList() 返回客户端可以读取和写入（如果客户端是主客户端）的参数列表（CameraParam 枚举），getIntParameterRange() 则传递有效的值范围和分辨率。当主客户端更改摄像头参数时，系统会通过发送带有参数 ID 和新值的 PARAMETER_CHANGED 事件通知同一摄像头硬件上的所有其他客户端。
注意：传感器驱动程序可能会以不同的方式处理无效参数值。它可能会只返回错误代码，也可能会裁剪有效范围内的值并加以应用。因此，setIntParameter() 方法会返回一个有效值，客户端可以使用此值来确认请求的处理方式。
多个摄像头客户端之间的请求仲裁
由于之前的 EVS 设计允许多个应用同时订阅一个摄像头硬件，因此一个应用有可能通过更改摄像头参数干扰其他应用的操作。此外，多个客户端可能需要以不同方式调整同一参数，因而导致运行摄像头服务时出现意外行为。
为了避免此类问题，EVS 管理器仅允许主客户端用程序编写摄像头参数。在尝试调整任何摄像头参数之前，客户端必须通过调用 setMaster() 方法成为主客户端。如果调用失败，则表示该摄像头硬件上已有活动的主客户端。在当前主客户端崩溃或通过 unsetMaster() 明确放弃主客户端角色之前，系统不会允许任何其他客户端更改摄像头参数。当主客户端退回其特权时，系统会通过 MASTER_RELEASED 事件向所有其他应用发送通知。
高优先级客户端
EVS 管理器会以高优先级处理拥有屏幕的客户端，并允许它窃取当前主客户端的角色。由于 EVS 屏幕所有权基于新近度，因此新客户端甚至可以从当前客户端接管屏幕。
高优先级客户端必须调用 IEvsCamera::forceMaster(sp<IEvsDisplay>& display) 才能获得主客户端角色。EVS 管理器会检查给定屏幕句柄的状态，当（且仅当）其状态有效且不为 DisplayState::NOT_OPEN 和 DisplayState::DEAD 时，才会替换主客户端。刚刚失去主客户端角色的客户端将通过 MASTER_RELEASED 事件获得通知，并且必须正确处理这一情况。