Don't clear depth buffer after rendering

[FinleyPan]

Hey. There are two views and one render target, I want to draw the 2nd views on top of the 1st view:

filament::Scene *first_scene = nullptr;
filament::Scene *second_scene = nullptr;
filament::View* first_view     = nullptr;
filament::View* second_view    = nullptr;
filament::Renderer *renderer = nullptr;
filament::RenderTarget *target = nullptr;
//codes to initialize views, render and target
...

first_view->setScene(first_scene);
second_view->setScene(second_scene);
first_view->setRenderTarget(target);
second_view->setRenderTarget(target);
....
renderer->render(first_view);
renderer->render(second_view);

target is created with color and depth texture. I want to keep the depth buffer after the 1st rendering. However, I find filament always clear depth buffer in function FRenderer::initializeClearFlags:

void FRenderer::initializeClearFlags() {
    // We always discard and clear the depth+stencil buffers -- we don't allow sharing these
    // across views (clear implies discard)
    mDiscardStartFlags = ((mClearOptions.discard || mClearOptions.clear) ?
                          TargetBufferFlags::COLOR : TargetBufferFlags::NONE)
                         | TargetBufferFlags::DEPTH_AND_STENCIL;

    mClearFlags = (mClearOptions.clear ? TargetBufferFlags::COLOR : TargetBufferFlags::NONE)
            | TargetBufferFlags::DEPTH_AND_STENCIL;
}

Is there a way to overcome it?

[romainguy]

@pixelflinger

[FinleyPan]

Hey @romainguy @pixelflinger I modify method FRenderer::initializeClearFlags as:

void FRenderer::initializeClearFlags() {
    // We always discard and clear the depth+stencil buffers -- we don't allow sharing these
    // across views (clear implies discard)
    mDiscardStartFlags = ((mClearOptions.discard || mClearOptions.clear) ?
                          TargetBufferFlags::COLOR : TargetBufferFlags::NONE)
                         | TargetBufferFlags::STENCIL;

    mClearFlags = (mClearOptions.clear ? TargetBufferFlags::COLOR : TargetBufferFlags::NONE)
            | TargetBufferFlags::STENCIL;
}

Then it works out for my problem, but I don't know whether it will cause any bugs.

[pixelflinger]

This would impact performance, because the depth buffer would always be written to memory, when in fact it's almost never needed.

[FinleyPan]

This would impact performance, because the depth buffer would always be written to memory, when in fact it's almost never needed.

Hi. Is the memory you say device(GPU) memory or host memory?

[romainguy]

We're talking about off-tile memory, which on mobile devices is unified memory shared between the GPU and CPU.

[pixelflinger]

I believe if you set the SAMPLEABLE flag on the depth texture of your custom rendertarget, we will preserve it.
This is not supported for non-custom render targets.

[FinleyPan]

@pixelflinger Hey, is there any progress :)? I found a similar issue #3962, but the fix does not work for my case.

[pixelflinger]

Ah yes, I didn't realize that you where not redrawing the object each frame -- this use case is not supported right now.

What should work is to have a several views using the same RenderTarget and have the depth buffer preserved (if marked as SAMPLEABLE) within the same beginFrame/endFrame. Currently buffers are cleared no matter what at the beginning of a frame.

I would file an "enhancement" issue at this point.

[FinleyPan]

Ah yes, I didn't realize that you where not redrawing the object each frame -- this use case is not supported right now.

Can you illustrate "redrawing object each frame" concretely？

[FinleyPan]

What should work is to have a several views using the same RenderTarget and have the depth buffer preserved (if marked as SAMPLEABLE) within the same beginFrame/endFrame. Currently buffers are cleared no matter what at the beginning of a frame.

I think my demo conforms to this. The only difference is times of calling filament::Renderer::render (once and twice) in each beginFrame/endFrame

[FinleyPan]

Hey @pixelflinger, is there any updates?

[lucasleandro1805]

Hi, any fixes for that?
I`m using official AAR for android