Re: [libcamera-devel] [PATCH v2] drm/fourcc: Add bayer formats and modifiers

[Tomasz Figa <tfiga@chromium.org>]

Hi Niklas,

On Fri, May 22, 2020 at 01:52:01AM +0200, Niklas Söderlund wrote:
> Bayer formats are used with cameras and contain green, red and blue
> components, with alternating lines of red and green, and blue and green
> pixels in different orders. For each block of 2x2 pixels there is one
> pixel with a red filter, two with a green filter, and one with a blue
> filter. The filters can be arranged in different patterns.
> 
> Add DRM fourcc formats to describe the most common Bayer formats. Also
> add a modifiers to describe the custom packing layouts used by the Intel
> IPU3 and in the MIPI (Mobile Industry Processor Interface) CSI-2
> specification.
> 
> Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
> ---
> * Changes since v1
> - Rename the defines from DRM_FORMAT_SRGGB8 to DRM_FORMAT_BAYER_RGGB8.
> - Update the fourcc codes passed to fourcc_code() to avoid a conflict.
> - Add diagrams for all Bayer formats memory layout.
> - Update documentation.
> ---
>  include/uapi/drm/drm_fourcc.h | 205 ++++++++++++++++++++++++++++++++++
>  1 file changed, 205 insertions(+)
> 
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 8bc0b31597d80737..d07dd24b49bde6c1 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -285,6 +285,73 @@ extern "C" {
>  #define DRM_FORMAT_YUV444	fourcc_code('Y', 'U', '2', '4') /* non-subsampled Cb (1) and Cr (2) planes */
>  #define DRM_FORMAT_YVU444	fourcc_code('Y', 'V', '2', '4') /* non-subsampled Cr (1) and Cb (2) planes */
>  
> +/*
> + * Bayer formats
> + *
> + * Bayer formats contain green, red and blue components, with alternating lines
> + * of red and green, and blue and green pixels in different orders. For each
> + * block of 2x2 pixels there is one pixel with a red filter, two with a green
> + * filter, and one with a blue filter. The filters can be arranged in different
> + * patterns.
> + *
> + * For example, RGGB:
> + *	row0: RGRGRGRG...
> + *	row1: GBGBGBGB...
> + *	row2: RGRGRGRG...
> + *	row3: GBGBGBGB...
> + *	...
> + *

I wonder if we're operating on the right level of abstraction within this
proposal.

The sensor itself transfers only sequential pixels, as read
out from its matrix. Whether a given pixel corresponds to a red, green
or blue color filter actually depends on the filter layer, which could
actually vary between integrations of the same sensor. (See Fujifilm
X-Trans, which uses regular Sony sensors with their own filter pattern
[1].)

Moreover, the sensor resolution is specified as the number of pixels
horizontally and the number of lines horizontally, without considering
the color pattern.

If we consider that, wouldn't the data stream coming from the sensor be
essentially DRM_FORMAT_R8/R10/R12/etc.?

Then, on top of that, we would have the packing, which I believe is
defined well in this document +/- being entangled with the Bayer
pattern.

What do you think?

[1] https://en.wikipedia.org/wiki/Fujifilm_X-Trans_sensor

Best regards,
Tomasz
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