[ANNOUNCE] pixfmtdb

Sebastian Wick sebastian.wick at redhat.com
Mon Jan 23 23:54:07 UTC 2023

On Mon, Jan 23, 2023 at 11:43 PM Simon Ser <contact at emersion.fr> wrote:
> On Monday, January 23rd, 2023 at 21:25, Sebastian Wick <sebastian.wick at redhat.com> wrote:
> > Why is the TF defined for GL formats and both the primaries and TF for
> > Vulkan formats? The only exception here should be sRGB formats. Where
> > did you get the information from?
> This is what upstream dfdutils does [1]. Can you explain why you think
> it should be undefined instead of linear?

The channels have no meaning. You can put whatever you want in there.
It doesn't have to be linear, it doesn't have to be colors and
especially not colors with specific primaries. It's only when it's
used to form an image that the TF and primaries are known. Vulkan
specifically requires you to explicitly provide this information to
the WSI and YCC samplers (generally everywhere where knowing them is
required) and never assumes that certain pixel formats imply certain
TFs and primaries (exception being the sRGB variants).

(See also https://registry.khronos.org/vulkan/specs/1.3-extensions/html/vkspec.html#_issues_26,
Q 23)

The problem here seems to be that the Data Format spec describes more
than the pixel format. If you want to share an image, the TF and
primaries are essential but they are not an inherent part of the pixel
format of the image. So yeah, I think what dfdutils does is...
probably not wrong but not what you're after.

> I was wondering what to do for DRM formats regarding these. I think it
> would be worthwhile to do like Vulkan: set TF = linear, primaries =
> BT.709, pre-multiplied alpha = yes. These are the things KMS assume
> when there is no override (ie, when there is no KMS property saying
> otherwise).

Please no. All undefined is absolutely the right thing to do. Adding
any more meaning to pixel formats is a horrible idea. The KMS
properties are also not an override, they describe the image and the
description has default values.

> [1]: https://github.com/KhronosGroup/dfdutils/blob/5cd41cbdf63e80b00c085c6906a1152709e4c0f2/createdfd.c#L47

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