rhi: Update the docs and manual test for geometry shaders

Task-number: QTBUG-137521
Change-Id: Ic9256eaaa55aef20c622429058fda9235c1f73c1
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/gui/rhi/qrhi.cpp

@@ -936,16 +936,17 @@ Q_LOGGING_CATEGORY(QRHI_LOG_RUB, "qt.rhi.rub")
     avoided as it will not be supported by all backends. The maximum patch
     control point count portable between backends is 32.
 
-    \value GeometryShader Indicates that the geometry shader stage is
+    \value GeometryShader Indicates that the geometry shader stage is supported.
-    supported. When supported, a geometry shader can be specified in the
+    When supported, a geometry shader can be specified in the QRhiShaderStage
-    QRhiShaderStage list. Geometry Shaders are considered an experimental
+    list. Geometry Shaders are considered an experimental feature in QRhi and
-    feature in QRhi and can only be expected to be supported with Vulkan,
+    can only be expected to be supported with Vulkan, Direct 3D 11 and 12,
-    Direct 3D, OpenGL (3.2+) and OpenGL ES (3.2+), assuming the implementation
+    OpenGL (3.2+) and OpenGL ES (3.2+), assuming the implementation reports it
-    reports it as supported at run time. Geometry shaders have portability
+    as supported at run time. Starting with Qt 6.11 geometry shaders are
-    issues between APIs, and therefore no guarantees can be given for a
+    automatically translated to HLSL, and therefore no injection of handwritten
-    universal solution. They will never be supported with Metal. Whereas with
+    HLSL geometry shaders is necessary anymore (but note that gl_in and
-    Direct 3D a handwritten HLSL geometry shader must be injected into each
+    expressions such as gl_in[0].gl_Position are not supported; rather, pass the
-    QShader for the geometry stage since qsb cannot generate this from SPIR-V.
+    position as an output variable from the vertex shader). Geometry shaders are
+    not supported with Metal.
 
     \value TextureArrayRange Indicates that for
     \l{QRhi::newTextureArray()}{texture arrays} it is possible to specify a

tests/manual/rhi/geometryshader/buildshaders.bat

@@ -1,6 +1,5 @@
 :: Copyright (C) 2024 The Qt Company Ltd.
 :: SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
 qsb --glsl 320es,410 --hlsl 50 test.vert -o test.vert.qsb
-qsb --glsl 320es,410 test.geom -o test.geom.qsb
+qsb --glsl 320es,410 --hlsl 50 test.geom -o test.geom.qsb
-qsb -r hlsl,50,test_geom.hlsl test.geom.qsb
 qsb --glsl 320es,410 --hlsl 50 test.frag -o test.frag.qsb

tests/manual/rhi/geometryshader/test.geom

@@ -4,6 +4,7 @@
 
 layout(points) in;
 layout(line_strip, max_vertices = 7) out;
+layout(location = 0) in vec4 v_position[];
 
 layout(std140, binding = 0) uniform buf {
     float radius;
@@ -16,7 +17,7 @@ void main(void)
     {
         float theta = float(i) / 6.0f * 2.0 * M_PI;
 
-        gl_Position = gl_in[0].gl_Position;
+        gl_Position = v_position[0];
         gl_Position.xy += radius * vec2(cos(theta), sin(theta));
 
         EmitVertex();

tests/manual/rhi/geometryshader/test.vert

@@ -1,8 +1,10 @@
 #version 440
 
 layout(location = 0) in vec3 position;
+layout(location = 0) out vec4 v_position;
 
 void main()
 {
-    gl_Position = vec4(position, 1.0);
+    v_position = vec4(position, 1.0);
+    gl_Position = v_position;
 }

tests/manual/rhi/geometryshader/test_geom.hlsl

@@ -1,26 +0,0 @@
-struct VertexOutput
-{
-    float4 position : SV_Position;
-};
-
-struct PixelInput
-{
-    float4 position : SV_POSITION;
-};
-
-cbuffer buf : register(b0)
-{
-    float radius : packoffset(c0);
-};
-
-[maxvertexcount(7)]
-void main(point VertexOutput input[1], inout LineStream<PixelInput> OutputStream)
-{
-    PixelInput output;
-    for (int i = 0; i < 7; ++i) {
-        float theta = float(i) / 6.0f * 2.0 * 3.14159265;
-        output.position = input[0].position;
-        output.position.xy += radius * float2(cos(theta), sin(theta));
-        OutputStream.Append(output);
-    }
-}