Hi,
I need to implement some controls for my driver and would like to understand
the control ioctl framework available today. I am not very sure how the control
ioctls are to be implemented and it is not well defined in the specification. I
have provided below my understanding of the below set of controls. I would like
to hear what you think about the same.
I see following controls defined for adjusting brightness, contrast etc.
V4L2_CID_BRIGHTNESS integer Picture brightness, or more precisely, the
black level.
V4L2_CID_CONTRAST integer Picture contrast or luma gain.
V4L2_CID_SATURATION integer Picture color saturation or chroma gain.
V4L2_CID_HUE integer Hue or color balance.
I think these controls refer to the YUV color space. Y (luma) and UV (chroma)
signals will be modified by above controls.
V4L2_CID_DO_WHITE_BALANCE button This is an action control. When set
(the value is ignored), the device will do a white balance and then hold the
current setting. Contrast this with the boolean V4L2_CID_AUTO_WHITE_BALANCE,
which, when activated, keeps adjusting the white balance.
V4L2_CID_RED_BALANCE integer Red chroma balance.
V4L2_CID_BLUE_BALANCE integer Blue chroma balance.
My understanding is these controls are applied to RGB color space.
V4L2_CID_AUTO_WHITE_BALANCE is applicable where hardware is capable of
adjusting the wb automatically. But V4L2_CID_DO_WHITE_BALANCE is used in
conjunction with V4L2_CID_RED_BALANCE & V4L2_CID_BLUE_BALANCE. i.e application
set these values and they take effect when V4L2_CID_DO_WHITE_BALANCE is issued.
So driver hold onto the current values until another set of above commands are
issued.
But one question I have is (if the above is correct), why there is no
V4L2_CID_GREEN_BALANCE ??
I don't see any control IDs available for Bayer RGB color space.
In our video hardware, there is a set of Gain values that can be applied to the
Bayer RGB data. We can apply them individually to R, Gr, Gb or B color
components. So I think we need to have 4 more controls defined for doing white
balancing in the Bayer RGB color space that is applicable for sensors (like
MT9T031) and image tuning hardware like the VPFE CCDC & IPIPE.
Define following new controls for these in Bayer RGB color space White Balance
(WB) controls??
V4L2_CID_BAYER_RED_BALANCE integer Bayer Red balance.
V4L2_CID_BAYER_BLUE_BALANCE integer Bayer Blue balance.
V4L2_CID_BAYER_GREEN_R_BALANCE integer Bayer Gr balance.
V4L2_CID_BAYER_GREEN_B_BALANCE integer Bayer Gb balance.
There is also an offset value defined per color which is like adjusting the
black level in the video image data. It is subtracted from the image byte.
What you call this ? Should we define a new control, V4l2_CID_BAYER_OFFSET ??
In my experience, all these values (except offset) have a sign bit which means
the nominal value is zero and it can be changed with positive or negative
values.
Then for image tuning hardware like, IPIPE (Image Pipe) of Texas Instruments,
there are additional controls that are applicable. They are mostly applicable
for devices that captures Bayer RGB data from sensors. Some of these are given
below...
Defect Pixel correction - Correct dead pixels in the captured image data.
Color Space conversion - Convert between Bayer RGB pattern and others
Data Formatter - Allow reading of different arrangement of R, Gr, Gb, B color
filters in the sensor.
Black Clamp - Adjust blackness in the image data either automatically using
black area pixels or using manual controls
RGB to RGB gain control - After converting from Bayer RGB to RGB data, these
are applied
RGB to YUV gain control - Applied after YUV conversion
Noise filters - Noise filters to remove noise from the image data
VPFE hardware can do above processing on the image sensor data and how do we
implement them. Do we implement them through following extended control IOCTLs ?
#define VIDIOC_G_EXT_CTRLS _IOWR('V', 71, struct v4l2_ext_controls)
#define VIDIOC_S_EXT_CTRLS _IOWR('V', 72, struct v4l2_ext_controls)
#define VIDIOC_TRY_EXT_CTRLS _IOWR('V', 73, struct v4l2_ext_controls)
Currently they are implemented using proprietary ioctls. But if other hardware
supports similar features, then it is worth standardizing these control IDs.
But configuring them may still require proprietary structures. Does extended
control structure will allow this?
Following are the structures available for extended controls:-
struct v4l2_ext_control {
__u32 id;
__u32 reserved2[2];
union {
__s32 value;
__s64 value64;
void *reserved;
};
} __attribute__ ((packed));
struct v4l2_ext_controls {
__u32 ctrl_class;
__u32 count;
__u32 error_idx;
__u32 reserved[2];
struct v4l2_ext_control *controls;
};
If I have to use v4l2_ext_control to configure the above modules in the
hardware, I might have to use reserved field to pass the control parameter
structure ptr to the driver. In that case it is better to rename the reserved
field as to accept a ptr to configuration structure as :-
void *config
Finally, what is the criteria used for defining control classes? Currently we
have USER, MPEG and CAMERA control classes. Do I need to define a new control
class for the Bayer RGB color space WB and other controls mentioned here.
Please let me know what your thoughts are....
Murali Karicheri
Software Design Engineer
Texas Instruments Inc.
Germantown, MD 20874
email: [email protected]
Murali Karicheri
Software Design Engineer
Texas Instruments Inc.
Germantown, MD 20874
Phone : 301-515-3736
email: [email protected]
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