Add TriangleFan pipeline topology

Triangle fan drawing is only supported on some platforms. The feature
flag QRhi::TriangleFanTopology is set accordingly.

In general, TriangleStrip topology will be more efficient on most GPUs.
However, if polygon data is already stored as triangle fans, the CPU
savings may be more significant.

Change-Id: I9704fad751d2119eac8791074f58942dd9315601
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

6 files changed, 20 insertions, 2 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index e4f6da6d46..c8ec8c7410 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -563,7 +563,10 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     \value BaseInstance Indicates that instanced draw commands support the \c
     firstInstance argument. When reported as not supported, the firstInstance
     value is ignored and the instance ID starts from 0.
- */
+
+    \value TriangleFanTopology Indicates that QRhiGraphicsPipeline::setTopology()
+    supports QRhiGraphicsPipeline::TriangleFan.
+*/
 
 /*!
     \enum QRhi::BeginFrameFlag
@@ -3123,6 +3126,7 @@ QDebug operator<<(QDebug dbg, const QRhiShaderResourceBindings &srb)
 
     \value Triangles (default)
     \value TriangleStrip
+    \value TriangleFan (only available if QRhi::TriangleFanTopology is supported)
     \value Lines
     \value LineStrip
     \value Points
diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index 95e9213cac..5b371af727 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -1017,6 +1017,7 @@ public:
     enum Topology {
         Triangles,
         TriangleStrip,
+        TriangleFan,
         Lines,
         LineStrip,
         Points
@@ -1426,7 +1427,8 @@ public:
         WideLines,
         VertexShaderPointSize,
         BaseVertex,
-        BaseInstance
+        BaseInstance,
+        TriangleFanTopology
     };
 
     enum BeginFrameFlag {
diff --git a/src/gui/rhi/qrhid3d11.cpp b/src/gui/rhi/qrhid3d11.cpp
index 36c95f2306..f011d68ada 100644
--- a/src/gui/rhi/qrhid3d11.cpp
+++ b/src/gui/rhi/qrhid3d11.cpp
@@ -469,6 +469,8 @@ bool QRhiD3D11::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::BaseInstance:
         return true;
+    case QRhi::TriangleFanTopology:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index 17b40777b3..277cf12fd2 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -732,6 +732,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return caps.baseVertex;
     case QRhi::BaseInstance:
         return false; // not in ES 3.2, so won't bother
+    case QRhi::TriangleFanTopology:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;
@@ -1539,6 +1541,8 @@ static inline GLenum toGlTopology(QRhiGraphicsPipeline::Topology t)
         return GL_TRIANGLES;
     case QRhiGraphicsPipeline::TriangleStrip:
         return GL_TRIANGLE_STRIP;
+    case QRhiGraphicsPipeline::TriangleFan:
+        return GL_TRIANGLE_FAN;
     case QRhiGraphicsPipeline::Lines:
         return GL_LINES;
     case QRhiGraphicsPipeline::LineStrip:
diff --git a/src/gui/rhi/qrhimetal.mm b/src/gui/rhi/qrhimetal.mm
index 344be44d48..f9d9cdc01a 100644
--- a/src/gui/rhi/qrhimetal.mm
+++ b/src/gui/rhi/qrhimetal.mm
@@ -550,6 +550,8 @@ bool QRhiMetal::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::BaseInstance:
         return true;
+    case QRhi::TriangleFanTopology:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;
diff --git a/src/gui/rhi/qrhivulkan.cpp b/src/gui/rhi/qrhivulkan.cpp
index c7d28a4f18..cb32aa08f3 100644
--- a/src/gui/rhi/qrhivulkan.cpp
+++ b/src/gui/rhi/qrhivulkan.cpp
@@ -3727,6 +3727,8 @@ bool QRhiVulkan::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::BaseInstance:
         return true;
+    case QRhi::TriangleFanTopology:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;
@@ -4572,6 +4574,8 @@ static inline VkPrimitiveTopology toVkTopology(QRhiGraphicsPipeline::Topology t)
         return VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST;
     case QRhiGraphicsPipeline::TriangleStrip:
         return VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP;
+    case QRhiGraphicsPipeline::TriangleFan:
+        return VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN;
     case QRhiGraphicsPipeline::Lines:
         return VK_PRIMITIVE_TOPOLOGY_LINE_LIST;
     case QRhiGraphicsPipeline::LineStrip: