rhi: Introduce multiview starting with OpenGL (ES)

Fixes: QTBUG-114770
Change-Id: Ibb1ced7f19d15a5116c60e95fd3e6b86ace63155
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

10 files changed, 124 insertions, 5 deletions

diff --git a/src/gui/opengl/qopenglextensions_p.h b/src/gui/opengl/qopenglextensions_p.h
index fdb9b51f06..58231545c6 100644
--- a/src/gui/opengl/qopenglextensions_p.h
+++ b/src/gui/opengl/qopenglextensions_p.h
@@ -59,7 +59,9 @@ public:
         StandardDerivatives     = 0x02000000,
         ASTCTextureCompression  = 0x04000000,
         ETC2TextureCompression  = 0x08000000,
-        HalfFloatVertex         = 0x10000000
+        HalfFloatVertex         = 0x10000000,
+        MultiView               = 0x20000000,
+        MultiViewExtended       = 0x40000000
     };
     Q_DECLARE_FLAGS(OpenGLExtensions, OpenGLExtension)
 
diff --git a/src/gui/opengl/qopenglfunctions.cpp b/src/gui/opengl/qopenglfunctions.cpp
index ee76987566..0b760eb2dd 100644
--- a/src/gui/opengl/qopenglfunctions.cpp
+++ b/src/gui/opengl/qopenglfunctions.cpp
@@ -348,6 +348,10 @@ static int qt_gl_resolve_extensions()
         extensions |= QOpenGLExtensions::StandardDerivatives;
     if (extensionMatcher.match("GL_ARB_half_float_vertex"))
         extensions |= QOpenGLExtensions::HalfFloatVertex;
+    if (extensionMatcher.match("GL_OVR_multiview"))
+        extensions |= QOpenGLExtensions::MultiView;
+    if (extensionMatcher.match("GL_OVR_multiview2"))
+        extensions |= QOpenGLExtensions::MultiViewExtended;
 
     if (ctx->isOpenGLES()) {
         if (format.majorVersion() >= 2)
diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 16c508b36c..6b3a4bd7c3 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -967,6 +967,22 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     \value ThreeDimensionalTextureMipmaps Indicates that generating 3D texture
     mipmaps are supported. In practice this feature will be unsupported with
     Direct 3D 12.
+
+    \value MultiView Indicates that multiview, see e.g.
+    \l{https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_KHR_multiview.html}{VK_KHR_multiview}
+    is supported. With OpenGL ES 2.0, Direct 3D 11, and OpenGL (ES)
+    implementations without \c{GL_OVR_multiview2} this feature will not be
+    supported. With Vulkan 1.1 and newer, and Direct 3D 12 multiview is
+    typically supported. When reported as supported, creating a
+    QRhiTextureRenderTarget with a QRhiColorAttachment that references a texture
+    array and has \l{QRhiColorAttachment::setMultiViewCount()}{multiViewCount}
+    set enables recording a render pass that uses multiview rendering. Note that
+    multiview is only available in combination with 2D texture arrays. It cannot
+    be used to optimize the rendering into individual textures (e.g. two, for
+    the left and right eyes). Rather, the target of a multiview render pass is
+    always a texture array, automatically rendering to the layer (array element)
+    corresponding to each view. Therefore this feature implies \l TextureArrays
+    as well. This enum value has been introduced in Qt 6.7.
  */
 
 /*!
@@ -2297,6 +2313,70 @@ QRhiColorAttachment::QRhiColorAttachment(QRhiRenderBuffer *renderBuffer)
  */
 
 /*!
+    \fn int QRhiColorAttachment::multiViewCount() const
+
+    \return the currently set number of views. Defaults to 0 which indicates
+    the render target with this color attachment is not going to be used with
+    multiview rendering.
+
+    \since 6.7
+ */
+
+/*!
+    \fn void QRhiColorAttachment::setMultiViewCount(int count)
+
+    Sets the view \a count. Setting a value larger than 1 indicates that the
+    render target with this color attachment is going to be used with multiview
+    rendering. The default value is 0. Values smaller than 2 indicate no
+    multiview rendering.
+
+    When \a count is set to \c 2 or greater, the color attachment must be
+    associated with a 2D texture array. layer() and multiViewCount() together
+    define the range of texture array elements that are targeted during
+    multiview rendering.
+
+    For example, if \c layer is \c 0 and \c multiViewCount is \c 2, the texture
+    array must have 2 (or more) elements, and the multiview rendering will
+    target elements 0 and 1. The \c{gl_ViewIndex} variable in the shaders has a
+    value of \c 0 or \c 1 then, where view \c 0 corresponds to the texture array
+    element \c 0, and view \c 1 to the array element \c 1.
+
+    \note Setting a \a count larger than 1, using a texture array as texture(),
+    and calling \l{QRhiCommandBuffer::beginPass()}{beginPass()} on a
+    QRhiTextureRenderTarget with this color attachment implies multiview
+    rendering for the entire render pass. multiViewCount() should not be set
+    unless multiview rendering is wanted. Multiview cannot be used with texture
+    types other than 2D texture arrays. (although 3D textures may work,
+    depending on the graphics API and backend; applications are nonetheless
+    advised not to rely on that and only use 2D texture arrays as the render
+    targets of multiview rendering)
+
+    See
+    \l{https://registry.khronos.org/OpenGL/extensions/OVR/OVR_multiview.txt}{GL_OVR_multiview}
+    for more details regarding multiview rendering. Do note that Qt requires
+    \l{https://registry.khronos.org/OpenGL/extensions/OVR/OVR_multiview2.txt}{GL_OVR_multiview2}
+    as well, when running on OpenGL (ES).
+
+    Multiview rendering is available only when the
+    \l{QRhi::MultiView}{MultiView} feature is reported as supported from
+    \l{QRhi::isFeatureSupported()}{isFeatureSupported()}.
+
+    \note For portability, be aware of limitations that exist for multiview
+    rendering with some of the graphics APIs. For example, OpenGL disallows
+    tessellation or geometry shaders with multiview. With other APIs, e.g.
+    Vulkan, some of these are optional features, the actual support depending
+    on the implementation. It is therefore recommended that multiview render
+    passes do not rely on any of the features that
+    \l{https://registry.khronos.org/OpenGL/extensions/OVR/OVR_multiview.txt}{GL_OVR_multiview}
+    declares as unsupported. The one exception is shader stage outputs other
+    than \c{gl_Position} depending on \c{gl_ViewIndex}: that can be relied on
+    (even with OpenGL) because QRhi never reports multiview as supported without
+    \c{GL_OVR_multiview2} also being present.
+
+    \since 6.7
+ */
+
+/*!
     \class QRhiTextureRenderTargetDescription
     \inmodule QtGui
     \since 6.6
diff --git a/src/gui/rhi/qrhi.h b/src/gui/rhi/qrhi.h
index eb300f5873..585bead7b3 100644
--- a/src/gui/rhi/qrhi.h
+++ b/src/gui/rhi/qrhi.h
@@ -592,6 +592,9 @@ public:
     int resolveLevel() const { return m_resolveLevel; }
     void setResolveLevel(int level) { m_resolveLevel = level; }
 
+    int multiViewCount() const { return m_multiViewCount; }
+    void setMultiViewCount(int count) { m_multiViewCount = count; }
+
 private:
     QRhiTexture *m_texture = nullptr;
     QRhiRenderBuffer *m_renderBuffer = nullptr;
@@ -600,6 +603,7 @@ private:
     QRhiTexture *m_resolveTexture = nullptr;
     int m_resolveLayer = 0;
     int m_resolveLevel = 0;
+    int m_multiViewCount = 0;
 };
 
 Q_DECLARE_TYPEINFO(QRhiColorAttachment, Q_RELOCATABLE_TYPE);
@@ -1822,7 +1826,8 @@ public:
         OneDimensionalTextureMipmaps,
         HalfAttributes,
         RenderToOneDimensionalTexture,
-        ThreeDimensionalTextureMipmaps
+        ThreeDimensionalTextureMipmaps,
+        MultiView
     };
 
     enum BeginFrameFlag {
diff --git a/src/gui/rhi/qrhid3d11.cpp b/src/gui/rhi/qrhid3d11.cpp
index feda195c51..f8284462ee 100644
--- a/src/gui/rhi/qrhid3d11.cpp
+++ b/src/gui/rhi/qrhid3d11.cpp
@@ -588,6 +588,8 @@ bool QRhiD3D11::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ThreeDimensionalTextureMipmaps:
         return true;
+    case QRhi::MultiView:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhid3d12.cpp b/src/gui/rhi/qrhid3d12.cpp
index 7c55ff9f5c..55e2d626e0 100644
--- a/src/gui/rhi/qrhid3d12.cpp
+++ b/src/gui/rhi/qrhid3d12.cpp
@@ -638,6 +638,8 @@ bool QRhiD3D12::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ThreeDimensionalTextureMipmaps:
         return false; // we generate mipmaps ourselves with compute and this is not implemented
+    case QRhi::MultiView:
+        return false;
     }
     return false;
 }
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index 0a4bc8d01c..af5f07d6de 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -1009,6 +1009,15 @@ bool QRhiGles2::create(QRhi::Flags flags)
 
     caps.halfAttributes = f->hasOpenGLExtension(QOpenGLExtensions::HalfFloatVertex);
 
+    // We always require GL_OVR_multiview2 for symmetry with other backends.
+    caps.multiView = f->hasOpenGLExtension(QOpenGLExtensions::MultiView)
+                     && f->hasOpenGLExtension(QOpenGLExtensions::MultiViewExtended);
+    if (caps.multiView) {
+        glFramebufferTextureMultiviewOVR =
+            reinterpret_cast<void(QOPENGLF_APIENTRYP)(GLenum, GLenum, GLuint, GLint, GLint, GLsizei)>(
+                ctx->getProcAddress(QByteArrayLiteral("glFramebufferTextureMultiviewOVR")));
+    }
+
     nativeHandlesStruct.context = ctx;
 
     contextLost = false;
@@ -1371,6 +1380,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return caps.texture1D;
     case QRhi::ThreeDimensionalTextureMipmaps:
         return caps.texture3D;
+    case QRhi::MultiView:
+        return caps.multiView && caps.maxTextureArraySize > 0;
     default:
         Q_UNREACHABLE_RETURN(false);
     }
@@ -5554,8 +5565,13 @@ bool QGles2TextureRenderTarget::create()
             QGles2Texture *texD = QRHI_RES(QGles2Texture, texture);
             Q_ASSERT(texD->texture && texD->specified);
             if (texD->flags().testFlag(QRhiTexture::ThreeDimensional) || texD->flags().testFlag(QRhiTexture::TextureArray)) {
-                rhiD->f->glFramebufferTextureLayer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + uint(attIndex), texD->texture,
-                                                   colorAtt.level(), colorAtt.layer());
+                if (it->multiViewCount() < 2) {
+                    rhiD->f->glFramebufferTextureLayer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + uint(attIndex), texD->texture,
+                                                       colorAtt.level(), colorAtt.layer());
+                } else {
+                    rhiD->glFramebufferTextureMultiviewOVR(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + uint(attIndex), texD->texture,
+                                                           colorAtt.level(), colorAtt.layer(), colorAtt.multiViewCount());
+                }
             } else if (texD->flags().testFlag(QRhiTexture::OneDimensional)) {
                 rhiD->glFramebufferTexture1D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + uint(attIndex),
                                              texD->target + uint(colorAtt.layer()), texD->texture,
diff --git a/src/gui/rhi/qrhigles2_p.h b/src/gui/rhi/qrhigles2_p.h
index 76779fdca8..fecb15698f 100644
--- a/src/gui/rhi/qrhigles2_p.h
+++ b/src/gui/rhi/qrhigles2_p.h
@@ -909,6 +909,8 @@ public:
                                                        GLsizei, const GLvoid *) = nullptr;
     void(QOPENGLF_APIENTRYP glFramebufferTexture1D)(GLenum, GLenum, GLenum, GLuint,
                                                     GLint) = nullptr;
+    void(QOPENGLF_APIENTRYP glFramebufferTextureMultiviewOVR)(GLenum, GLenum, GLuint, GLint,
+                                                              GLint, GLsizei) = nullptr;
 
     uint vao = 0;
     struct Caps {
@@ -962,7 +964,8 @@ public:
               geometryShader(false),
               texture1D(false),
               hasDrawBuffersFunc(false),
-              halfAttributes(false)
+              halfAttributes(false),
+              multiView(false)
         { }
         int ctxMajor;
         int ctxMinor;
@@ -1016,6 +1019,7 @@ public:
         uint texture1D : 1;
         uint hasDrawBuffersFunc : 1;
         uint halfAttributes : 1;
+        uint multiView : 1;
     } caps;
     QGles2SwapChain *currentSwapChain = nullptr;
     QSet<GLint> supportedCompressedFormats;
diff --git a/src/gui/rhi/qrhimetal.mm b/src/gui/rhi/qrhimetal.mm
index 699e7a09be..e47ee9118a 100644
--- a/src/gui/rhi/qrhimetal.mm
+++ b/src/gui/rhi/qrhimetal.mm
@@ -833,6 +833,8 @@ bool QRhiMetal::isFeatureSupported(QRhi::Feature feature) const
         return false;
     case QRhi::ThreeDimensionalTextureMipmaps:
         return true;
+    case QRhi::MultiView:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhivulkan.cpp b/src/gui/rhi/qrhivulkan.cpp
index 4b7af8a89e..ae0e705106 100644
--- a/src/gui/rhi/qrhivulkan.cpp
+++ b/src/gui/rhi/qrhivulkan.cpp
@@ -4437,6 +4437,8 @@ bool QRhiVulkan::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ThreeDimensionalTextureMipmaps:
         return true;
+    case QRhi::MultiView:
+        return false;
     default:
         Q_UNREACHABLE_RETURN(false);
     }