internal/graphicsdriver/metal: invert Y at the vertex shader

Instead of using a negative height in the viewport, invert the Y direction
at the vertex shader. This is a little more readable as a negative height
is hacky.

This is a preparation for DirectX 12. DirectX 12's coodination system
is very similar to Metal, but doesn't treat a negative height in its
viewport unfortunately.

Updates #1007

internal/graphicsdriver/graphics.go

@@ -55,6 +55,7 @@ type Graphics interface {
 	SetVsyncEnabled(enabled bool)
 	SetFullscreen(fullscreen bool)
 	FramebufferYDirection() YDirection
+	NDCYDirection() YDirection
 	NeedsRestoring() bool
 	NeedsClearingScreen() bool
 	IsGL() bool

internal/graphicsdriver/metal/graphics_darwin.go

@@ -72,11 +72,13 @@ vertex VertexOut VertexShader(
   const device VertexIn* vertices [[buffer(0)]],
   constant float2& viewport_size [[buffer(1)]]
 ) {
+  // In Metal, the NDC's Y direction (upward) and the framebuffer's Y direction (downward) don't
+  // match. Then, the Y direction must be inverted.
   float4x4 projectionMatrix = float4x4(
     float4(2.0 / viewport_size.x, 0, 0, 0),
-    float4(0, 2.0 / viewport_size.y, 0, 0),
+    float4(0, -2.0 / viewport_size.y, 0, 0),
     float4(0, 0, 1, 0),
-    float4(-1, -1, 0, 1)
+    float4(-1, 1, 0, 1)
   );
 
   VertexIn in = vertices[vid];
@@ -829,14 +831,12 @@ func (g *Graphics) draw(rps mtl.RenderPipelineState, dst *Image, dstRegion graph
 
 	g.rce.SetRenderPipelineState(rps)
 
-	// In Metal, the NDC's Y direction (upward) and the framebuffer's Y direction (downward) don't
-	// match. Then, the Y direction must be inverted.
 	w, h := dst.internalSize()
 	g.rce.SetViewport(mtl.Viewport{
 		OriginX: 0,
-		OriginY: float64(h),
+		OriginY: 0,
 		Width:   float64(w),
-		Height:  -float64(h),
+		Height:  float64(h),
 		ZNear:   -1,
 		ZFar:    1,
 	})
@@ -1051,6 +1051,10 @@ func (g *Graphics) FramebufferYDirection() graphicsdriver.YDirection {
 	return graphicsdriver.Downward
 }
 
+func (g *Graphics) NDCYDirection() graphicsdriver.YDirection {
+	return graphicsdriver.Upward
+}
+
 func (g *Graphics) NeedsRestoring() bool {
 	return false
 }

internal/graphicsdriver/opengl/graphics.go

@@ -385,6 +385,10 @@ func (g *Graphics) FramebufferYDirection() graphicsdriver.YDirection {
 	return graphicsdriver.Upward
 }
 
+func (g *Graphics) NDCYDirection() graphicsdriver.YDirection {
+	return graphicsdriver.Upward
+}
+
 func (g *Graphics) NeedsRestoring() bool {
 	return g.context.needsRestoring()
 }

internal/ui/ui.go

@@ -67,3 +67,7 @@ const (
 	WindowResizingModeOnlyFullscreenEnabled
 	WindowResizingModeEnabled
 )
+
+func NeedsInvertY() bool {
+	return graphics().FramebufferYDirection() != graphics().NDCYDirection()
+}

shader.go

@@ -26,6 +26,7 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/internal/mipmap"
 	"github.com/hajimehoshi/ebiten/v2/internal/shader"
 	"github.com/hajimehoshi/ebiten/v2/internal/shaderir"
+	"github.com/hajimehoshi/ebiten/v2/internal/ui"
 )
 
 var shaderSuffix string
@@ -105,17 +106,6 @@ func imageSrc%[1]dAt(pos vec2) vec4 {
 }
 `, i, pos)
 	}
-
-	shaderSuffix += `
-func __vertex(position vec2, texCoord vec2, color vec4) (vec4, vec2, vec4) {
-	return mat4(
-		2/__imageDstTextureSize.x, 0, 0, 0,
-		0, 2/__imageDstTextureSize.y, 0, 0,
-		0, 0, 1, 0,
-		-1, -1, 0, 1,
-	) * vec4(position, 0, 1), texCoord, color
-}
-`
 }
 
 // Shader represents a compiled shader program.
@@ -136,6 +126,29 @@ func NewShader(src []byte) (*Shader, error) {
 	var buf bytes.Buffer
 	buf.Write(src)
 	buf.WriteString(shaderSuffix)
+	if ui.NeedsInvertY() {
+		buf.WriteString(`
+func __vertex(position vec2, texCoord vec2, color vec4) (vec4, vec2, vec4) {
+	return mat4(
+		2/__imageDstTextureSize.x, 0, 0, 0,
+		0, -2/__imageDstTextureSize.y, 0, 0,
+		0, 0, 1, 0,
+		-1, 1, 0, 1,
+	) * vec4(position, 0, 1), texCoord, color
+}
+`)
+	} else {
+		buf.WriteString(`
+func __vertex(position vec2, texCoord vec2, color vec4) (vec4, vec2, vec4) {
+	return mat4(
+		2/__imageDstTextureSize.x, 0, 0, 0,
+		0, 2/__imageDstTextureSize.y, 0, 0,
+		0, 0, 1, 0,
+		-1, -1, 0, 1,
+	) * vec4(position, 0, 1), texCoord, color
+}
+`)
+	}
 
 	fs := token.NewFileSet()
 	f, err := parser.ParseFile(fs, "", buf.Bytes(), parser.AllErrors)