rhi: gl: Add support for importing an existing renderbuffer object

Normally we only allow creating wrappers for texture objects. These
can then be used with a QRhiTextureRenderTarget to allow rendering into
an externally created texture.

With OpenGL (ES), there are additional, special cases, especially on
embedded. Consider EGLImages for example. An EGLImageKHR can be bound to
a renderbuffer object (glEGLImageTargetRenderbufferStorageOES), which
can then be associated with a framebuffer object to allow rendering into
the external buffer represented by the EGLImage. To implement the same
via QRhi one needs a way to create a wrapping QRhiRenderBuffer for the
native OpenGL renderbuffer object.

Here we add a createFrom() to QRhiRenderBuffer, while providing a dummy,
default implementation. The only real implementation is in the OpenGL
backend, which simply takes a renderbuffer id, without taking ownership.

Task-number: QTBUG-92116
Change-Id: I4e68e665fb35a7d7803b7780db901c8bed5740e2
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

7 files changed, 94 insertions, 2 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 8c24f11079..242f244c1d 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -642,6 +642,15 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     supported (which can happen when the underlying API is OpenGL ES 2.0 without
     support for GL_UNPACK_ROW_LENGTH),
     QRhiTextureSubresourceUploadDescription::setDataStride() must not be used.
+
+    \value RenderBufferImport Indicates that QRhiRenderBuffer::createFrom() is
+    supported. For most graphics APIs this is not sensible because
+    QRhiRenderBuffer encapsulates texture objects internally, just like
+    QRhiTexture. With OpenGL however, renderbuffer object exist as a separate
+    object type in the API, and in certain environments (for example, where one
+    may want to associated a renderbuffer object with an EGLImage object) it is
+    important to allow wrapping an existing OpenGL renderbuffer object with a
+    QRhiRenderBuffer.
  */
 
 /*!
@@ -2306,6 +2315,44 @@ QRhiResource::Type QRhiRenderBuffer::resourceType() const
  */
 
 /*!
+    Similar to create() except that no new native renderbuffer objects are
+    created. Instead, the native renderbuffer object specified by \a src is
+    used.
+
+    This allows importing an existing renderbuffer object (which must belong to
+    the same device or sharing context, depending on the graphics API) from an
+    external graphics engine.
+
+    \note This is currently applicable to OpenGL only. This function exists
+    solely to allow importing a renderbuffer object that is bound to some
+    special, external object, such as an EGLImageKHR. Once the application
+    performed the glEGLImageTargetRenderbufferStorageOES call, the renderbuffer
+    object can be passed to this function to create a wrapping
+    QRhiRenderBuffer, which in turn can be passed in as a color attachment to
+    a QRhiTextureRenderTarget to enable rendering to the EGLImage.
+
+    \note pixelSize(), sampleCount(), and flags() must still be set correctly.
+    Passing incorrect sizes and other values to QRhi::newRenderBuffer() and
+    then following it with a createFrom() expecting that the native
+    renderbuffer object alone is sufficient to deduce such values is \b wrong
+    and will lead to problems.
+
+    \note QRhiRenderBuffer does not take ownership of the native object, and
+    destroy() will not release that object.
+
+    \note This function is only implemented when the QRhi::RenderBufferImport
+    feature is reported as \l{QRhi::isFeatureSupported()}{supported}. Otherwise,
+    the function does nothing and the return value is \c false.
+
+    \return \c true when successful, \c false when not supported.
+ */
+bool QRhiRenderBuffer::createFrom(NativeRenderBuffer src)
+{
+    Q_UNUSED(src);
+    return false;
+}
+
+/*!
     \fn QRhiTexture::Format QRhiRenderBuffer::backingFormat() const
 
     \internal
diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index b43d490ea9..c915f8df0d 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -918,6 +918,10 @@ public:
     };
     Q_DECLARE_FLAGS(Flags, Flag)
 
+    struct NativeRenderBuffer {
+        quint64 object;
+    };
+
     QRhiResource::Type resourceType() const override;
 
     Type type() const { return m_type; }
@@ -933,6 +937,7 @@ public:
     void setFlags(Flags h) { m_flags = h; }
 
     virtual bool create() = 0;
+    virtual bool createFrom(NativeRenderBuffer src);
 
     virtual QRhiTexture::Format backingFormat() const = 0;
 
@@ -1534,7 +1539,8 @@ public:
         ScreenSpaceDerivatives,
         ReadBackAnyTextureFormat,
         PipelineCacheDataLoadSave,
-        ImageDataStride
+        ImageDataStride,
+        RenderBufferImport
     };
 
     enum BeginFrameFlag {
diff --git a/src/gui/rhi/qrhid3d11.cpp b/src/gui/rhi/qrhid3d11.cpp
index 46851ad612..8d15f2622f 100644
--- a/src/gui/rhi/qrhid3d11.cpp
+++ b/src/gui/rhi/qrhid3d11.cpp
@@ -544,6 +544,8 @@ bool QRhiD3D11::isFeatureSupported(QRhi::Feature feature) const
         return false;
     case QRhi::ImageDataStride:
         return true;
+    case QRhi::RenderBufferImport:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index 43569d3752..8a5361b29a 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -1011,6 +1011,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return caps.programBinary;
     case QRhi::ImageDataStride:
         return !caps.gles || caps.ctxMajor >= 3;
+    case QRhi::RenderBufferImport:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;
@@ -4287,7 +4289,8 @@ void QGles2RenderBuffer::destroy()
     stencilRenderbuffer = 0;
 
     QRHI_RES_RHI(QRhiGles2);
-    rhiD->releaseQueue.append(e);
+    if (owns)
+        rhiD->releaseQueue.append(e);
     QRHI_PROF;
     QRHI_PROF_F(releaseRenderBuffer(this));
     rhiD->unregisterResource(this);
@@ -4375,6 +4378,34 @@ bool QGles2RenderBuffer::create()
         break;
     }
 
+    owns = true;
+    rhiD->registerResource(this);
+    return true;
+}
+
+bool QGles2RenderBuffer::createFrom(NativeRenderBuffer src)
+{
+    if (!src.object)
+        return false;
+
+    if (renderbuffer)
+        destroy();
+
+    QRHI_RES_RHI(QRhiGles2);
+    samples = rhiD->effectiveSampleCount(m_sampleCount);
+
+    if (m_flags.testFlag(UsedWithSwapChainOnly))
+        qWarning("RenderBuffer: UsedWithSwapChainOnly is meaningless when importing an existing native object");
+
+    if (!rhiD->ensureContext())
+        return false;
+
+    renderbuffer = src.object;
+
+    QRHI_PROF;
+    QRHI_PROF_F(newRenderBuffer(this, false, false, samples));
+
+    owns = false;
     rhiD->registerResource(this);
     return true;
 }
diff --git a/src/gui/rhi/qrhigles2_p_p.h b/src/gui/rhi/qrhigles2_p_p.h
index 3d9fba80dc..cb6cd241ff 100644
--- a/src/gui/rhi/qrhigles2_p_p.h
+++ b/src/gui/rhi/qrhigles2_p_p.h
@@ -100,11 +100,13 @@ struct QGles2RenderBuffer : public QRhiRenderBuffer
     ~QGles2RenderBuffer();
     void destroy() override;
     bool create() override;
+    bool createFrom(NativeRenderBuffer src) override;
     QRhiTexture::Format backingFormat() const override;
 
     GLuint renderbuffer = 0;
     GLuint stencilRenderbuffer = 0; // when packed depth-stencil not supported
     int samples;
+    bool owns = true;
     friend class QRhiGles2;
 };
 
diff --git a/src/gui/rhi/qrhimetal.mm b/src/gui/rhi/qrhimetal.mm
index 850b3a91aa..384e23598c 100644
--- a/src/gui/rhi/qrhimetal.mm
+++ b/src/gui/rhi/qrhimetal.mm
@@ -595,6 +595,8 @@ bool QRhiMetal::isFeatureSupported(QRhi::Feature feature) const
         return false;
     case QRhi::ImageDataStride:
         return true;
+    case QRhi::RenderBufferImport:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhivulkan.cpp b/src/gui/rhi/qrhivulkan.cpp
index e7e31b1b2d..73288cb4a1 100644
--- a/src/gui/rhi/qrhivulkan.cpp
+++ b/src/gui/rhi/qrhivulkan.cpp
@@ -4243,6 +4243,8 @@ bool QRhiVulkan::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ImageDataStride:
         return true;
+    case QRhi::RenderBufferImport:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;