Hi Nate,
On Wednesday 29 April 2009 07:06:36 Dongsoo, Nathaniel Kim wrote:
> Hello everyone.
>
> As you know the experimental V4L2 feature VIDIOC_ENUM_FRAMESIZES is
> all about frame sizes supported per pixelformat. We can get width and
> height sizes supported per every pixelformat.
> It's very useful feature to reduce dependency between device driver
> and middleware or application. This could be used to make
> application/middleware be aware of supported resolutions on runtime.
> Very flexible.
>
> (Note : From now every example is based on soc camera interface and mmap
> method)
>
> But here is the thing.
> Most of pixel formats can is based on pixel-line paradigm. VSYNC
> representing single frame, HSYNC representing a line and pixel clock
> is representing a pixel. So, we can give those informations to sync
> interface, like camera interface which could make them expecting
> proper pixel and line to sync a image frame. Not so different for
> ITU-R.601 format, it also has a sync mechanism. OK, sounds fair
> enough.
>
> But how about encoded data like JPEG? JPEG is not pixel-line based,
> just a lump of data.Synchronize interface cannot figure out how to
> sync JPEG data with VSYNC and HSYNC. According to my experience in
> camera device, all we can get from JPEG encoder in camera ISP is about
> the maximum data size can be retrieved and actual data size when
> encoding job is finished.
>
> So when we want to take a JPEG encoded picture from camera device, we
> need to mmap with big enough size for JPEG. But how can we figure out
> how big size of memory could be called enough?
> Basically if you are using JPEG encoder to encode the exact resolution
> of YUV data for preview, you may not need for bigger buffer size I
> guess. But for digital camera, we use quite smaller resolution of YUV
> image than we are expecting to capture in JPEG, like 640*480 for
> preview in YUV and 2560*1920 for still shot in JPEG. It is obvious
> that we can't use mmaped buffer for preview to capture JPEG.
>
> So here is my Idea. (Restricted example for digital camera. sorry for that)
> Let's assume that we have two thread handling v4l2 device. like thread
> A for preview and thread B for still capture.
> Here is the procedure that I have in mind.
>
> camera device : video0
> YUV thread : thread A
> JPEG thread : thread B
>
> => therad A, thread B open video0 at the same time
> => enum_framesizes
> => thread A gets discrete YUV frame sizes in resolution and thread B
> gets byte sized JPEG size
> => thread A S_FMT preview size with YUV
>
> <snip>
>
> => thread A STREAMON
> => preview working
> => JPEG capture start issued
> => thread A STREAMOFF
> => thread B S_FMT JPEG size with JPEG
>
> <snip>
>
> => thread B STREAMON => copy JPEG data
> => thread B STREAMOFF
> => thread A STREAMON
> => preview working
>
>
> To make that procedure possible, we may have a patch for videodev2.h
> You can find them in following patched lines:
>
>
> @@ -372,6 +378,7 @@
> V4L2_FRMSIZE_TYPE_DISCRETE = 1,
> V4L2_FRMSIZE_TYPE_CONTINUOUS = 2,
> V4L2_FRMSIZE_TYPE_STEPWISE = 3,
> + V4L2_FRMSIZE_TYPE_ENCODED = 4,
> };
>
> struct v4l2_frmsize_discrete {
> @@ -388,6 +395,12 @@
> __u32 step_height; /* Frame height step size [pixel] */
> };
>
> +struct v4l2_frmsize_byte {
> + __u32 width; /* decoded frame width */
> + __u32 height; /* decodec frame height */
> + __u32 max_byte; /* maximum encoded data size in byte */
> +};
> +
> struct v4l2_frmsizeenum {
> __u32 index; /* Frame size number */
> __u32 pixel_format; /* Pixel format */
> @@ -396,6 +409,7 @@
> union { /* Frame size */
> struct v4l2_frmsize_discrete discrete;
> struct v4l2_frmsize_stepwise stepwise;
> + struct v4l2_frmsize_byte encoded;
> };
>
> __u32 reserved[2]; /* Reserved space for future use */
> @@ -408,6 +422,7 @@
> V4L2_FRMIVAL_TYPE_DISCRETE = 1,
> V4L2_FRMIVAL_TYPE_CONTINUOUS = 2,
> V4L2_FRMIVAL_TYPE_STEPWISE = 3,
> + V4L2_FRMIVAL_TYPE_ENCODED = 4,
> };
>
> struct v4l2_frmival_stepwise {
> @@ -426,6 +441,7 @@
> union { /* Frame interval */
> struct v4l2_fract discrete;
> struct v4l2_frmival_stepwise stepwise;
> + struct v4l2_fract encoded;
> };
>
> __u32 reserved[2]; /* Reserved space for future use */
>
>
>
> I want to know what do you think about this. I wish I could post my
> driver too, but it's not ready yet ;-(
I'm not sure to understand why this would be needed.
The format enumeration/negotiation ioctls are meant to negotiate a video
format and a frame size between the userspace application, the kernel driver
and the device. When capturing (M)JPEG data, the frame still has a width and a
height (in pixels) even if the data transferred between the encoder and the
"bridge" (basically the next chip in the chain, usually the video input
interface of an embedded processor, a PCI chipset, ...) doesn't convey any
HSYNC/VSYNC information. A userspace application can expect the driver to
report the frame size in pixels, and to be able to select the appropriate size
if the device supports multiple sizes.
Once a frame size has been selected, computing the required buffer size is the
driver's responsibility. The application will call VIDIOC_REQBUFS and the
driver will allocate appropriately sized buffers. The size can then be queried
using VIDIOC_QUERYBUF.
Cheers,
Laurent Pinchart
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