[Reminder]: expose Metal's render pass store action in public API?

[septag]

Is it possible to do two default passes with sampleCount > 1 on Metal device ?

I'm using SG_ACTION_LOAD for the second pass, but after the second one, drawable contents gets wiped with purple color. This behavior does not happen with other APIs (d3d11+GL).

This is for example what I'm doing with sokol-triangle sample (with sampleCount = 4):

    action.colors[0].action = SG_ACTION_CLEAR;
    action.colors[0].value = (sg_color) {1.0f, 0, 0, 1.0f};
    sg_begin_default_pass(&action, osx_width(), osx_height());
    sg_apply_pipeline(state.pip);
    sg_apply_bindings(&state.bind);
    sg_draw(0, 3, 1);
    sg_end_pass();

    action.colors[0].action = SG_ACTION_LOAD;
    sg_begin_default_pass(&action, osx_width(), osx_height());
    sg_apply_pipeline(state.pip);
    sg_apply_bindings(&state.bind);
    sg_end_pass();

[floooh]

Hmm, I wouldn't recommend this with any backend, two default passes in one frame is basically "undefined behaviour", it might work in some situations (or backends) but not in others.

I think the reason why you're seeing a blank surface is the MTLStoreActionMultisampleResolve here in this line:

sokol/sokol_gfx.h

Line 10627 in 89b1b6b

pass_desc.colorAttachments[i].storeAction = is_msaa ? MTLStoreActionMultisampleResolve : MTLStoreActionStore;

You could try to replace this with MTLStoreActionStoreAndMultisampleResolve and see what happens, but there will be lots of other unexpected problems with two default passes in the same frame (because in the Metal backend each default pass requests a new drawable to render into if I'm not mistaken).

With offscreen passes there's probably fewer problems (but I think the MSAA case still won't work because of the Metal store action that's used). I don't want to always do MTLStoreActionStoreAndMultisampleResolve because that shouldn't be usually needed, but maybe it makes sense to expose both the load- and store-action in the API (when creating a pass object).

Ah PS: of course replacing this MTLStoreActionMultisampleResolve with MTLStoreActionStoreAndMultisampleResolve will not work in the default pass anyway, only in an offscreen pass, because in a default pass you have no control over the load- and store-action (because that's provided by MTKView in sokol_app.h).

[septag]

Ah nice! Thanks for the explanation.

I used your tip (MTLStoreActionMultisampleResolve) and modified here at line 10680 instead of the spot you mentioned (that one was for offscreen rendering):

bool is_msaa = _sg.desc.context.sample_count > 1;
pass_desc.colorAttachments[0].loadAction = _sg_mtl_load_action(action->colors[0].action);
pass_desc.colorAttachments[0].storeAction = is_msaa ? MTLStoreActionStoreAndMultisampleResolve :
                                                              MTLStoreActionStore;

It is now fixed and behaves like no msaa mode and other graphics backends.
I'm not sure if this is undefined, since I couldn't find anything in the documentation. So I can confirm that it works on the default pass. Regarding the thing you said about having no control over actions, for drawables, you are getting the descriptor (MTLRenderPassDescriptor) from [MTKView currentRenderPassDescriptor] so we could change it's behavior like offscreen ones.

And I checked, as far as I understood, you are discarding the drawable in commit and not on every pass, so it should be safe.

[floooh]

Ah yeah right. I'll keep the ticket open as reminder (and change the title). I think it's better to give the API user more control over the store action, but I need to think about this a bit more.

[septag]

Sure thing, thanks for your support.

[floooh]

Fixed via: #834