Change default clear color to black

[rdrpenguin04]

Objective

Black is an easier color to fill for GPUs (if by a small margin), and the choice of clear color really isn't necessary for most games, where 3D games will choose skyboxes and 2D games will choose background images (and most 2D games ideally wouldn't use rgb(0.4, 0.4, 0.4) as their background color of choice anyways). Thus, choosing black would enable a small optimization.

Solution

Adjusted ClearColor::default

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Changelog

Changed: the default clear color is now black.

Migration Guide

Any applications relying on the default clear color being the original shade of dark grey should change them. Meanwhile, anyone who set the clear color to black, either as an optimization or for a space-related game, can now change their color back to the default.

[alice-i-cecile]

I don't think I agree with this decision. The performance argument isn't convincing at all to me without data, and this will make quick experiments and examples less useful since you won't be able to see black lines that are drawn.

[rdrpenguin04]

Not being able to see black lines isn't a convincing argument to me, since white-on-black is easier to see than black-on-dark-grey or white-on-dark-grey.

On the performance side, check https://gpuopen.com/learn/rdna-performance-guide/#clears; while I don't have performance metrics, I was directed here.

[JMS55]

While I agree with this in principal, I think we should delay it until we add procedural skyboxes. Probably sometime in 0.13 assuming I don't run into major issues.

[rdrpenguin04]

The thing is, if someone wanted a non-black background, chances are they set it manually anyway. The default color is almost certainly not used in production ever except by accident. I don't think procedural skyboxes are needed for a simple change that affects almost nobody; if someone wants to use a non-black clear color, there's nothing stopping them from doing so. All this does is change the default.

[superdump]

I also agree with this in principal and I think @JMS55 point about skyboxes is a good one.

I tend to think that hidden costs are much more difficult to be aware of / discover than things that just don't work.

When lots of things will use some kind of skybox or background of some form which will replace all the clear colour, it would be difficult to be aware of the lost performance from slow-path clearing of textures and from what I recall from @HackerFoo 's work on the Noumenal mobile app, things like clearing buffers has significant impact on mobile in particular. AMD/NVIDIA also recommend using fast paths for clearing/initialising textures if possible.

On the other hand, not being able to see black lines on a black background is immediate and clear friction that is actionable.

[rdrpenguin04]

I'm still somewhat failing to see the problem with black lines on a black background; if the background is obviously black, why isn't it trivial to make the lines white?

Edit: Oh right, that's your point: it is trivial to fix, but clear color is less obvious -- oh, that makes more sense, and I completely agree.

[alice-i-cecile]

Yep :) Note that this will have an impact on more complex rendering examples too: things like the examples that demonstrate the effects of lighting and PBR materials will look quite different on a black rather than neutral-gray background!

I'm fine to defer to the tastes of the rendering experts here, but we should verify that all of the visual examples still look fine as part of this PR and manually set the clear color (or change the line etc. color) as needed.

[rdrpenguin04]

Are there automated tests to check against an existing screenshot?

[dmyyy]

I think @mockersf was working on something like that here: #9220