Add support for stereoscopic content in QRhi::OpenGLES2

Setting the flag QSurfaceFormat::StereoBuffers does not actually do
anything, because we do not utilize the extra buffers provided. We need
to expose setting the correct buffers using glDrawBuffers between draw
calls.

Change-Id: I6a5110405e621030ac3a2886fa83df0cfe928723
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

4 files changed, 101 insertions, 12 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 701cecb3d1..20f4b4beae 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -4860,6 +4860,35 @@ QRhiResource::Type QRhiSwapChain::resourceType() const
  */
 
 /*!
+    \enum QRhiSwapChain::StereoTargetBuffer
+    Selects the backbuffer to use with a stereoscopic swapchain.
+
+    \value LeftBuffer
+    \value RightBuffer
+ */
+
+/*!
+    \return a render target that can be used with beginPass() in order to
+    render to the swapchain's left or right backbuffer. This overload should be
+    used only with stereoscopic rendering, that is, when the associated QWindow
+    is backed by two color buffers, one for each eye, instead of just one.
+
+    When stereoscopic rendering is not supported, the return value will be
+    null. For the time being the only backend and 3D API where traditional
+    stereoscopic rendering is supported is OpenGL (excluding OpenGL ES), in
+    combination with \l QSurfaceFormat::StereoBuffers, assuming it is supported
+    by the graphics and display driver stack at run time. All other backends
+    are going to return null from this overload.
+
+    \note the value must not be cached and reused between frames
+ */
+QRhiRenderTarget *QRhiSwapChain::currentFrameRenderTarget(StereoTargetBuffer targetBuffer)
+{
+    Q_UNUSED(targetBuffer);
+    return nullptr;
+}
+
+/*!
     \fn bool QRhiSwapChain::createOrResize()
 
     Creates the swapchain if not already done and resizes the swapchain buffers
diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index 1d32d08e84..5ee0d6e1fd 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -1379,6 +1379,11 @@ public:
         HDR10
     };
 
+    enum StereoTargetBuffer {
+        LeftBuffer,
+        RightBuffer
+    };
+
     QRhiResource::Type resourceType() const override;
 
     QWindow *window() const { return m_window; }
@@ -1403,6 +1408,7 @@ public:
 
     virtual QRhiCommandBuffer *currentFrameCommandBuffer() = 0;
     virtual QRhiRenderTarget *currentFrameRenderTarget() = 0;
+    virtual QRhiRenderTarget *currentFrameRenderTarget(StereoTargetBuffer targetBuffer);
     virtual QSize surfacePixelSize() = 0;
     virtual bool isFormatSupported(Format f) = 0;
     virtual QRhiRenderPassDescriptor *newCompatibleRenderPassDescriptor() = 0;
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index 1a7be9eb49..180bb7e360 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -435,6 +435,14 @@ QT_BEGIN_NAMESPACE
 #define GL_GEOMETRY_SHADER                0x8DD9
 #endif
 
+#ifndef GL_BACK_LEFT
+#define GL_BACK_LEFT                      0x0402
+#endif
+
+#ifndef GL_BACK_RIGHT
+#define GL_BACK_RIGHT                     0x0403
+#endif
+
 /*!
     Constructs a new QRhiGles2InitParams.
 
@@ -2997,24 +3005,31 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
                                 cmd.args.bindShaderResources.dynamicOffsetCount);
             break;
         case QGles2CommandBuffer::Command::BindFramebuffer:
+        {
+            QVarLengthArray<GLenum, 8> bufs;
             if (cmd.args.bindFramebuffer.fbo) {
                 f->glBindFramebuffer(GL_FRAMEBUFFER, cmd.args.bindFramebuffer.fbo);
+                const int colorAttCount = cmd.args.bindFramebuffer.colorAttCount;
+                bufs.append(colorAttCount > 0 ? GL_COLOR_ATTACHMENT0 : GL_NONE);
                 if (caps.maxDrawBuffers > 1) {
-                    const int colorAttCount = cmd.args.bindFramebuffer.colorAttCount;
-                    QVarLengthArray<GLenum, 8> bufs;
-                    for (int i = 0; i < colorAttCount; ++i)
+                    for (int i = 1; i < colorAttCount; ++i)
                         bufs.append(GL_COLOR_ATTACHMENT0 + uint(i));
-                    f->glDrawBuffers(colorAttCount, bufs.constData());
                 }
             } else {
                 f->glBindFramebuffer(GL_FRAMEBUFFER, ctx->defaultFramebufferObject());
+                if (cmd.args.bindFramebuffer.stereo && cmd.args.bindFramebuffer.stereoTarget == QRhiSwapChain::RightBuffer)
+                    bufs.append(GL_BACK_RIGHT);
+                else
+                    bufs.append(caps.gles ? GL_BACK : GL_BACK_LEFT);
             }
+            f->glDrawBuffers(bufs.count(), bufs.constData());
             if (caps.srgbCapableDefaultFramebuffer) {
                 if (cmd.args.bindFramebuffer.srgb)
                     f->glEnable(GL_FRAMEBUFFER_SRGB);
                 else
                     f->glDisable(GL_FRAMEBUFFER_SRGB);
             }
+        }
             break;
         case QGles2CommandBuffer::Command::Clear:
             f->glDisable(GL_SCISSOR_TEST);
@@ -3980,6 +3995,9 @@ QGles2RenderTargetData *QRhiGles2::enqueueBindFramebuffer(QRhiRenderTarget *rt,
             *wantsDsClear = doClearBuffers;
         fbCmd.args.bindFramebuffer.fbo = 0;
         fbCmd.args.bindFramebuffer.colorAttCount = 1;
+        fbCmd.args.bindFramebuffer.stereo = rtD->stereoTarget.has_value();
+        if (fbCmd.args.bindFramebuffer.stereo)
+            fbCmd.args.bindFramebuffer.stereoTarget = rtD->stereoTarget.value();
         break;
     case QRhiResource::TextureRenderTarget:
     {
@@ -3991,6 +4009,7 @@ QGles2RenderTargetData *QRhiGles2::enqueueBindFramebuffer(QRhiRenderTarget *rt,
             *wantsDsClear = !rtTex->m_flags.testFlag(QRhiTextureRenderTarget::PreserveDepthStencilContents);
         fbCmd.args.bindFramebuffer.fbo = rtTex->framebuffer;
         fbCmd.args.bindFramebuffer.colorAttCount = rtD->colorAttCount;
+        fbCmd.args.bindFramebuffer.stereo = false;
 
         for (auto it = rtTex->m_desc.cbeginColorAttachments(), itEnd = rtTex->m_desc.cendColorAttachments();
              it != itEnd; ++it)
@@ -5753,6 +5772,8 @@ void QGles2CommandBuffer::destroy()
 QGles2SwapChain::QGles2SwapChain(QRhiImplementation *rhi)
     : QRhiSwapChain(rhi),
       rt(rhi, this),
+      rtLeft(rhi, this),
+      rtRight(rhi, this),
       cb(rhi)
 {
 }
@@ -5779,6 +5800,16 @@ QRhiRenderTarget *QGles2SwapChain::currentFrameRenderTarget()
     return &rt;
 }
 
+QRhiRenderTarget *QGles2SwapChain::currentFrameRenderTarget(StereoTargetBuffer targetBuffer)
+{
+    if (targetBuffer == LeftBuffer)
+        return rtLeft.d.isValid() ? &rtLeft : &rt;
+    else if (targetBuffer == RightBuffer)
+        return rtRight.d.isValid() ? &rtRight : &rt;
+    else
+        Q_UNREACHABLE_RETURN(nullptr);
+}
+
 QSize QGles2SwapChain::surfacePixelSize()
 {
     Q_ASSERT(m_window);
@@ -5795,6 +5826,18 @@ QRhiRenderPassDescriptor *QGles2SwapChain::newCompatibleRenderPassDescriptor()
     return new QGles2RenderPassDescriptor(m_rhi);
 }
 
+void QGles2SwapChain::initSwapChainRenderTarget(QGles2SwapChainRenderTarget *rt)
+{
+    rt->setRenderPassDescriptor(m_renderPassDesc); // for the public getter in QRhiRenderTarget
+    rt->d.rp = QRHI_RES(QGles2RenderPassDescriptor, m_renderPassDesc);
+    rt->d.pixelSize = pixelSize;
+    rt->d.dpr = float(m_window->devicePixelRatio());
+    rt->d.sampleCount = qBound(1, m_sampleCount, 64);
+    rt->d.colorAttCount = 1;
+    rt->d.dsAttCount = m_depthStencil ? 1 : 0;
+    rt->d.srgbUpdateAndBlend = m_flags.testFlag(QRhiSwapChain::sRGB);
+}
+
 bool QGles2SwapChain::createOrResize()
 {
     // can be called multiple times due to window resizes
@@ -5813,14 +5856,14 @@ bool QGles2SwapChain::createOrResize()
         m_depthStencil->create();
     }
 
-    rt.setRenderPassDescriptor(m_renderPassDesc); // for the public getter in QRhiRenderTarget
-    rt.d.rp = QRHI_RES(QGles2RenderPassDescriptor, m_renderPassDesc);
-    rt.d.pixelSize = pixelSize;
-    rt.d.dpr = float(m_window->devicePixelRatio());
-    rt.d.sampleCount = qBound(1, m_sampleCount, 64);
-    rt.d.colorAttCount = 1;
-    rt.d.dsAttCount = m_depthStencil ? 1 : 0;
-    rt.d.srgbUpdateAndBlend = m_flags.testFlag(QRhiSwapChain::sRGB);
+    initSwapChainRenderTarget(&rt);
+
+    if (m_window->format().stereo()) {
+        initSwapChainRenderTarget(&rtLeft);
+        rtLeft.d.stereoTarget = QRhiSwapChain::LeftBuffer;
+        initSwapChainRenderTarget(&rtRight);
+        rtRight.d.stereoTarget = QRhiSwapChain::RightBuffer;
+    }
 
     frameCount = 0;
 
diff --git a/src/gui/rhi/qrhigles2_p_p.h b/src/gui/rhi/qrhigles2_p_p.h
index 69164e5ebf..41854f931d 100644
--- a/src/gui/rhi/qrhigles2_p_p.h
+++ b/src/gui/rhi/qrhigles2_p_p.h
@@ -23,6 +23,7 @@
 #include <QWindow>
 #include <QPointer>
 #include <QtCore/private/qduplicatetracker_p.h>
+#include <optional>
 
 QT_BEGIN_NAMESPACE
 
@@ -173,6 +174,8 @@ struct QGles2RenderTargetData
 {
     QGles2RenderTargetData(QRhiImplementation *) { }
 
+    bool isValid() const { return rp != nullptr; }
+
     QGles2RenderPassDescriptor *rp = nullptr;
     QSize pixelSize;
     float dpr = 1;
@@ -181,6 +184,7 @@ struct QGles2RenderTargetData
     int dsAttCount = 0;
     bool srgbUpdateAndBlend = false;
     QRhiRenderTargetAttachmentTracker::ResIdList currentResIdList;
+    std::optional<QRhiSwapChain::StereoTargetBuffer> stereoTarget;
 };
 
 struct QGles2SwapChainRenderTarget : public QRhiSwapChainRenderTarget
@@ -399,6 +403,8 @@ struct QGles2CommandBuffer : public QRhiCommandBuffer
                 GLuint fbo;
                 bool srgb;
                 int colorAttCount;
+                bool stereo;
+                QRhiSwapChain::StereoTargetBuffer stereoTarget;
             } bindFramebuffer;
             struct {
                 GLenum target;
@@ -690,6 +696,7 @@ struct QGles2SwapChain : public QRhiSwapChain
 
     QRhiCommandBuffer *currentFrameCommandBuffer() override;
     QRhiRenderTarget *currentFrameRenderTarget() override;
+    QRhiRenderTarget *currentFrameRenderTarget(StereoTargetBuffer targetBuffer) override;
 
     QSize surfacePixelSize() override;
     bool isFormatSupported(Format f) override;
@@ -697,9 +704,13 @@ struct QGles2SwapChain : public QRhiSwapChain
     QRhiRenderPassDescriptor *newCompatibleRenderPassDescriptor() override;
     bool createOrResize() override;
 
+    void initSwapChainRenderTarget(QGles2SwapChainRenderTarget *rt);
+
     QSurface *surface = nullptr;
     QSize pixelSize;
     QGles2SwapChainRenderTarget rt;
+    QGles2SwapChainRenderTarget rtLeft;
+    QGles2SwapChainRenderTarget rtRight;
     QGles2CommandBuffer cb;
     int frameCount = 0;
 };