Add RHI feature check for screen space derivatives

Fragment shader functions like fwidth() are useful for antialiasing
distance field text in the case of perspective projections. In order
to enable this as an alternative code path, we need to detect support.

- OpenGL: Supported with GL_OES_standard_derivatives or GLES3 and up
- Direct 3D: Supported for ps_2_x, so always supported on Direct3D 11
- Vulkan/Metal: Always supported

Task-number: QTBUG-84695
Change-Id: I5e3fa8014c808a9a2d639305c5e90ec25d44655c
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/gui/rhi/qrhigles2.cpp b/src/gui/rhi/qrhigles2.cpp
index ebe66f302b..632a5154e7 100644
--- a/src/gui/rhi/qrhigles2.cpp
+++ b/src/gui/rhi/qrhigles2.cpp
@@ -522,6 +522,7 @@ bool QRhiGles2::create(QRhi::Flags flags)
 
     caps.texelFetch = caps.ctxMajor >= 3; // 3.0 or ES 3.0
     caps.uintAttributes = caps.ctxMajor >= 3; // 3.0 or ES 3.0
+    caps.screenSpaceDerivatives = f->hasOpenGLExtension(QOpenGLExtensions::StandardDerivatives);
 
     if (!caps.gles) {
         f->glEnable(GL_VERTEX_PROGRAM_POINT_SIZE);
@@ -872,6 +873,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return caps.nonBaseLevelFramebufferTexture;
     case QRhi::UIntAttributes:
         return caps.uintAttributes;
+    case QRhi::ScreenSpaceDerivatives:
+        return caps.screenSpaceDerivatives;
     default:
         Q_UNREACHABLE();
         return false;