internal/graphicsdriver/directx: add EBITENGINE_DIRECTX_FEATURE_LEVEL

A fix for #2447 was a breaking-change as the DirectX driver was no longer
available for some old graphics card.

To support such old cards, provide a new environment variable
EBITENGINE_DIRECTX_FEATURE_LEVEL to specify a feature level.
The possible values are 11_0, 11_1, 12_0, 12_1, and 12_2. The default
value is 12_0.

Closes #2466

doc.go

@@ -81,6 +81,10 @@
 //	"warp":  Use WARP (i.e. software rendering).
 //	"debug": Use a debug layer.
 //
+// `EBITENGINE_DIRECTX_FEATURE_LEVEL` environment variable specifies DirectX feature level.
+// The possible values are "11_0", "11_1", "12_0", "12_1", and "12_2".
+// The default value is "12_0".
+//
 // `EBITENGINE_OPENGL` environment variable specifies various parameters for OpenGL.
 // You can specify multiple values separated by a comma. The default value is empty (i.e. no parameters).
 //

internal/graphicsdriver/directx/api_windows.go

@@ -73,7 +73,11 @@ const (
 type _D3D_FEATURE_LEVEL int32
 
 const (
+	_D3D_FEATURE_LEVEL_11_0 _D3D_FEATURE_LEVEL = 0xb000
+	_D3D_FEATURE_LEVEL_11_1 _D3D_FEATURE_LEVEL = 0xb100
 	_D3D_FEATURE_LEVEL_12_0 _D3D_FEATURE_LEVEL = 0xc000
+	_D3D_FEATURE_LEVEL_12_1 _D3D_FEATURE_LEVEL = 0xc100
+	_D3D_FEATURE_LEVEL_12_2 _D3D_FEATURE_LEVEL = 0xc200
 )
 
 type _D3D_PRIMITIVE_TOPOLOGY int32

internal/graphicsdriver/directx/graphics_windows.go

@@ -173,6 +173,24 @@ func (g *Graphics) initialize() (ferr error) {
 		}
 	}
 
+	// Ebitengine itself doesn't require the features level 12 and 11 should be enough,
+	// but some old cards don't work well (#2447). Specify the level 12 by default.
+	var featureLevel _D3D_FEATURE_LEVEL = _D3D_FEATURE_LEVEL_12_0
+	if env := os.Getenv("EBITENGINE_DIRECTX_FEATURE_LEVEL"); env != "" {
+		switch env {
+		case "11_0":
+			featureLevel = _D3D_FEATURE_LEVEL_11_0
+		case "11_1":
+			featureLevel = _D3D_FEATURE_LEVEL_11_1
+		case "12_0":
+			featureLevel = _D3D_FEATURE_LEVEL_12_0
+		case "12_1":
+			featureLevel = _D3D_FEATURE_LEVEL_12_1
+		case "12_2":
+			featureLevel = _D3D_FEATURE_LEVEL_12_2
+		}
+	}
+
 	// Initialize not only a device but also other members like a fence.
 	// Even if initializing a device succeeds, initializing a fence might fail (#2142).
 
@@ -181,15 +199,15 @@ func (g *Graphics) initialize() (ferr error) {
 			return err
 		}
 	} else {
-		if err := g.initializeDesktop(useWARP, useDebugLayer); err != nil {
+		if err := g.initializeDesktop(useWARP, useDebugLayer, featureLevel); err != nil {
 			return err
 		}
 	}
 
 	return nil
 }
 
-func (g *Graphics) initializeDesktop(useWARP bool, useDebugLayer bool) (ferr error) {
+func (g *Graphics) initializeDesktop(useWARP bool, useDebugLayer bool, featureLevel _D3D_FEATURE_LEVEL) (ferr error) {
 	if err := d3d12.Load(); err != nil {
 		return err
 	}
@@ -256,9 +274,7 @@ func (g *Graphics) initializeDesktop(useWARP bool, useDebugLayer bool) (ferr err
 				continue
 			}
 			// Test D3D12CreateDevice without creating an actual device.
-			// Ebitengine itself doesn't require the features level 12 and 11 should be enough,
-			// but some old cards don't work well (#2447). Specify the level 12 here.
-			if _, err := _D3D12CreateDevice(unsafe.Pointer(a), _D3D_FEATURE_LEVEL_12_0, &_IID_ID3D12Device, false); err != nil {
+			if _, err := _D3D12CreateDevice(unsafe.Pointer(a), featureLevel, &_IID_ID3D12Device, false); err != nil {
 				continue
 			}
 
@@ -271,7 +287,7 @@ func (g *Graphics) initializeDesktop(useWARP bool, useDebugLayer bool) (ferr err
 		return errors.New("directx: DirectX 12 is not supported")
 	}
 
-	d, err := _D3D12CreateDevice(unsafe.Pointer(adapter), _D3D_FEATURE_LEVEL_12_0, &_IID_ID3D12Device, true)
+	d, err := _D3D12CreateDevice(unsafe.Pointer(adapter), featureLevel, &_IID_ID3D12Device, true)
 	if err != nil {
 		return err
 	}