Re: width and height of JPEG compressed images

[Sylwester Nawrocki <sylvester.nawrocki@gmail.com>]

Hi Thomas,

Cc: Sakari and Laurent

On 07/05/2013 10:22 AM, Thomas Vajzovic wrote:
> Hello,
>
> I am writing a driver for the sensor MT9D131.  This device supports
> digital zoom and JPEG compression.
>
> Although I am writing it for my company's internal purposes, it will
>be made open-source, so I would like to keep the API as portable as
> possible.
>
> The hardware reads AxB sensor pixels from its array, resamples them
>to CxD image pixels, and then compresses them to ExF bytes.
>
> The subdevice driver sets size AxB to the value it receives from
>v4l2_subdev_video_ops.s_crop().
>
> To enable compression then v4l2_subdev_video_ops.s_mbus_fmt() is
>called with fmt->code=V4L2_MBUS_FMT_JPEG_1X8.
>
> fmt->width and fmt->height then ought to specify the size of the
>compressed image ExF, that is, the size specified is the size in the
>format specified (the number of JPEG_1X8), not the size it would be
>in a raw format.

In VIDIOC_S_FMT 'sizeimage' specifies size of the buffer for the
compressed frame at the bridge driver side. And width/height should
specify size of the re-sampled (binning, skipping ?) frame - CxD,
if I understand what  you are saying correctly.

I don't quite what transformation is done at CxD -> ExF. Why you are
using ExF (two numbers) to specify number of bytes ? And how can you
know exactly beforehand what is the frame size after compression ?
Does the sensor transmit fixed number of bytes per frame, by adding
some padding bytes if required to the compressed frame data ?

Is it something like:

sensor matrix (AxB pixels) -> binning/skipping (CxD pixels) ->
-> JPEG compresion (width = C, height = D, sizeimage ExF bytes)

?
> This allows the bridge driver to be compression agnostic.  It gets
>told how many bytes to allocate per buffer and it reads that many
>bytes.  It doesn't have to understand that the number of bytes isn't
>directly related to the number of pixels.
>
> So how does the user tell the driver what size image to capture
>before compression, CxD?

I think you should use VIDIOC_S_FMT(width = C, height = D, sizeimage = ExF)
for that. And s_frame_desc sudev op could be used to pass sizeimage to the
sensor subdev driver.

> (or alternatively, if you disagree and think CxD should be specified
>by s_fmt(), then how does the user specify ExF?)

Regards,
Sylwester