Hello everyone,
This patch-set introduces new ioctls to V4L2 API. The new method for
configuration of cropping and composition is presented.
This is the third version of extended crop/compose RFC. List of applied
changes:
- reduced number of hints and its semantics to be more practical and less
restrictive
- combined EXTCROP and COMPOSE ioctls into VIDIOC_{S/G}_SELECTION
- introduced crop and compose targets
- introduced try flag that prevents passing configuration to a hardware
- added usage examples
----------------
RFC
----------------
1. Introduction
There is some confusion in understanding of cropping in current version of
V4L2. In a case of Capture Devices, cropping refers to choosing only a part of
input data stream, and processing it, and storing it in a memory buffer. The
buffer is fully filled by data. There is now generic API to choose only a part
of an image buffer for being updated by hardware.
In case of OUTPUT devices, the whole content of a buffer is passed to hardware
and to output display. Cropping means selecting only a part of an output
display/signal. It is not possible to choose only a part of the image buffer to
be processed.
The overmentioned flaws in cropping API were discussed in post:
http://article.gmane.org/gmane.linux.drivers.video-input-infrastructure/28945
A solution was proposed during brainstorming session in Warsaw.
At first, the distinction between cropping and composing was stated. The
cropping operation means choosing only part of input data by bounding it by a
cropping rectangle. All data outside cropping area must be discarded. On the
other hand, composing means pasting processed data into rectangular part of
data sink. The sink may be output device, user buffer, etc.
Two concepts were introduced:
Cropping rectangle: a) for input devices, a part of input data selected by
hardware from input stream and pasted to an image buffer b) for output devices,
a part of image buffer to be passed by hardware to output stream
Composing rectangle: a) for input devices, a part of a image buffer that is
filled by hardware b) for output devices, an area on output device where image
is inserted
The configuration of composing/cropping areas is the subject of this document.
2. Data structures.
The structure v4l2_crop used by current API lacks any place for further
extensions. Therefore new and more generic structure is proposed.
struct v4l2_selection {
u32 type;
u32 target;
u32 flags;
struct v4l2_rect r;
u32 reserved[9];
};
Where,
type - type of buffer queue: V4L2_BUF_TYPE_VIDEO_CAPTURE,
V4L2_BUF_TYPE_VIDEO_OUTPUT, etc.
target - choose one of cropping/composing rectangles
flags - control over coordinates adjustments
r - selection rectangle
reserved - place for further extensions, adjust struct size to 64 bytes
3. Crop/Compose ioctl.
New ioctls are added to V4L2.
Name
VIDIOC_G_SELECTION - get crop/compose rectangles from a driver
Synopsis
int ioctl(fd, VIDIOC_G_SELECTION, struct v4l2_selection *s)
Description:
The ioctl is used to query selection rectangles. Currently, it involves
only cropping and composing ones. To query cropping rectangle application must
fill selection::type with respective stream type from V4L2_BUF_TYPE_VIDEO_*
family. Next, v4l2_selection::target must be field with desired target type.
Please refer to section Target for details. On success the rectangle is
returned in v4l2_selection::r field. Field v4l2_selection::flags and
v4l2_selection::reserved are ignored and they must be filled with zeros.
Return value
On success 0 is returned, on error -1 and the errno variable is set
appropriately:
EINVAL - incorrect buffer type, incorrect/not supported target
-----------------------------------------------------------------
Name
VIDIOC_S_SELECTION - set cropping rectangle on an input of a device
Synopsis
int ioctl(fd, VIDIOC_S_SELECTION, struct v4l2_selection *s)
Description:
The ioctl is used to configure a selection rectangle. An application
fills v4l2_selection::type field to specify an adequate stream type. Next, the
v4l2_selection::field target is filled. Basically, an application choose
between cropping or composing rectangle. Please refer to section Targets for
more details. Finally, structure v4l2-selection::r is filled with suggested
coordinates. The coordinates are expressed in driver-dependant units. The only
exception are rectangles for images in raw formats, which coordinates are
expressed in pixels.
Drivers are free to adjust selection rectangles on their own. The suggested
behaviour is to choose a rectangle with the closest coordinates to desired ones
passed in v4l2_selection::r. However, drivers are allowed to ignore suggested
it completely. A driver may even return a fixed and immutable rectangle every
call. If there is an alternative between multiple, then a driver may use hint
flags. Please refer to section Hints. Hints are optional but it is strongly
encouraged to use them.
Applications set V4L2_SEL_TRY flag in v4l2_selection::flags to prevent a driver
from applying selection configuration to hardware.
On success field v4l2_selection::r is filled with adjusted rectangle.
Return value
On success 0 is returned, on error -1 and the errno variable is set
appropriately:
EINVAL - incorrect buffer type, incorrect/not supported target
4. Flags
4.1. Hints
The field v4l2_selection::flags is used to give a driver a hint about
coordinate adjustments. The set of possible hint flags was reduced to two
entities for practical reasons. The flag V4L2_SEL_SIZE_LE is a suggestion for
the driver to decrease or keep size of a rectangle. The flags V4L2_SEL_SIZE_GE
imply keeping or increasing a size of a rectangle.
By default, lack of any hint flags indicate that driver has to choose selection
coordinates as close as possible to the ones passed in v4l2_selection::r field.
Setting both flags implies that the driver is free to adjust the rectangle. It
may return even random one however much more encouraged behaviour would be
adjusting coordinates in such a way that other stream parameters are left
intact. This means that the driver should avoid changing a format of an image
buffer and/or any other controls.
The hint flag V4L2_SEL_SIZE_GE may be useful in a following scenario. There is
a sensor with a face detection feature. An application receives information
about a position of a face via V4L2 controls. Assume that the camera's pipeline
is capable of scaling and cropping. The task it to grab only a part of an image
that contains a face. In such a case, the application would try to prevent the
driver from decreasing the rectangle, thus cutting off part of a face. It is
achieved by passing V4L2_SEL_SIZE_GE for a cropping target.
The hint V4L2_SEL_SIZE_LE is useful when data from a sensor are pasted directly
to an application window on a framebuffer. It is expected that the image would
not be inserted outside bounds of the window. Passing V4L2_SEL_SIZE_LE for a
composing target would inform driver not to exceed window's bounds.
Hints definitions:
#define V4L2_SEL_SIZE_GE 0x00000001
#define V4L2_SEL_SIZE_LE 0x00000002
Feel free to add a new flag if necessary.
4.2. Try flag.
A new flag was introduced in order to avoid adding to many new ioctl to V4L2
API. The flag name if V4L2_SEL_TRY. It is or-ed info field
v4l2_selection::flags. The flag can only be used with VIDIOC_S_SELECTION
ioctl. It implies that the driver should only adjust passed rectangle. The
rectangle must not be passed to hardware.
#define V4L2_SEL_TRY 0x80000000
Feel free to add a new flag if necessary.
5. Targets
The idea of targets was introduced for to reduce the number of new ioctls and
to increase their flexibility. Both compose and crop rectangles are types of
selection rectangles that describe a pipeline. They are very similar, the only
difference is that one touches data source, another touches data sink.
Therefore crop and compose rectangles are only distinguished by a target type.
The field v4l2_selection::target is used to choose a target.
Following targets are added:
V4L2_SEL_CROP_ACTIVE = 0
V4L2_SEL_CROP_DEFAULT = 1
V4L2_SEL_CROP_BOUNDS = 2
V4L2_SEL_COMPOSE_ACTIVE = 16 + 0
V4L2_SEL_COMPOSE_DEFAULT = 16 + 1
V4L2_SEL_COMPOSE_BOUNDS = 16 + 2
The gap between V4L2_SEL_CROP_BOUNDS and V4L2_SEL_COMPOSE_ACTIVE gives place
for further extensions.
The within cropping/composing target one may use auxiliary rectangles other
than a normal cropping/composing rectangle. Proposed auxiliary targets are:
- active - an area that is processed by hardware
- default - is the suggested active rectangle that covers the "whole picture"
- bounds - the limits that active rectangle cannot exceed
Auxiliary target default and bounds can be only used with VIDIOC_G_SELECTION.
This functionality was added to simulate VIDIOC_CROPCAP ioctl.
An application uses V4L2_SEL_CROP_ACTIVE to setup cropping rectangle. Target
V4L2_SEL_COMPOSE_ACTIVE is used to setup composing rectangle.
All cropcap fields except pixel aspect are supported in new API. I noticed that
there was discussion about pixel aspect and I am not convinced that it should
be a part of the cropping API. Please refer to the post:
http://lists-archives.org/video4linux/22837-need-vidioc_cropcap-clarification.html
Feel free to add new targets if necessary.
6. Usage examples.
6.1. Getting default rectangle of composing for video output.
struct v4l2_selection sel = {
.type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
.target = V4L2_SEL_COMPOSE_DEFAULT,
};
ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
6.2. Obtain crop from capture device and use it as composing area for an image
buffer in order to avoid scaling.
struct v4l2_selection sel = {
.type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
.target = V4L2_SEL_CROP_ACTIVE,
};
ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
... /* some error checking */
/* using input crop as a composing rectangle for the image */
sel.target = V4L2_SEL_COMPOSE_ACTIVE;
ioctl(fd, VIDIOC_S_SELECTION, &sel);
6.3. Setting a composing area on output of size of AT MOST half of limit placed
at a center of a display.
struct v4l2_selection sel = {
.type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
.target = V4L2_SEL_COMPOSE_BOUNDS,
};
struct v4l2_rect r;
ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
/* setting smaller compose rectangle */
r.width = sel.r.width / 2;
r.height = sel.r.height / 2;
r.left = sel.r.width / 4;
r.top = sel.r.height / 4;
sel.r = r;
sel.flags = V4L2_SEL_SIZE_LE;
ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
6.4. Trying to set crop 150 x 100 at coordinates (200,300) on a sensor array.
Inform the driver that the actual crop should contain desired rectangle. The
hardware configuration must not change.
struct v4l2_selection sel = {
.type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
.target = V4L2_SEL_CROP_ACTIVE,
.flags = V4L2_SEL_SIZE_GE | V4L2_SEL_TRY,
.r = {
.left = 200,
.top = 300,
.width = 150,
.height = 100,
},
};
ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
7. Possible improvements and extensions.
- add subpixel resolution
* hardware is often capable of subpixel processing by passing denominator of
rectangle's coordinates in one of v4l2_selection reserved fields
- introduce more hint
* add {RIGHT/LEFT/TOP/BOTTOM/HEIGHT/WIDTH}_{LE/GE} flags to give more
flexibility
* split SIZE_GE to LEFT_LE | RIGHT_GE | TOP_LE | BOTTOM_GE
* split SIZE_LE to LEFT_GE | RIGHT_LE | TOP_GE | BOTTOM_LE
- implement VIDIOC_S_MULTISELECTION
* allow to configure multiple rectangles simultaneously
- add image size of new target
* remove setup of width and height of image buffer from S_FMT, use S_SELECTION
with targets:
+ V4L2_SEL_FORMAT_ACTIVE - active format
+ V4L2_SEL_FORMAT_BOUNDS - maximal resolution of an image
+ V4L2_SEL_FORMAT_DEFAULT - optimal suggestion about
* resolution of an image combined with support for VIDIOC_S_MULTISELECTION
allows to pass a triple format/crop/compose sizes in a single ioctl
What it your opinion about proposed solutions?
Looking for a reply,
Best regards,
Tomasz Stanislawski
Tomasz Stanislawski (2):
v4l: add support for extended crop/compose API
v4l: simulate old crop API using extended crop/compose API
drivers/media/video/v4l2-compat-ioctl32.c | 2 +
drivers/media/video/v4l2-ioctl.c | 113 ++++++++++++++++++++++++++---
include/linux/videodev2.h | 26 +++++++
include/media/v4l2-ioctl.h | 4 +
4 files changed, 135 insertions(+), 10 deletions(-)
--
1.7.5
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