gpu: saturate instead of overflowing depth buffer

Use greater-than-or-equal test and saturate the z depth buffer
when more than 65k objects are drawn.

Fixes gio#127

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/d3d11/backend_windows.go

@@ -605,6 +605,8 @@ func (b *Backend) prepareDraw(mode backend.DrawMode) {
 		switch b.depthState.fn {
 		case backend.DepthFuncGreater:
 			desc.DepthFunc = _D3D11_COMPARISON_GREATER
+		case backend.DepthFuncGreaterEqual:
+			desc.DepthFunc = _D3D11_COMPARISON_GREATER_EQUAL
 		default:
 			panic("unsupported depth func")
 		}

app/internal/d3d11/d3d11_windows.go

@@ -621,7 +621,8 @@ const (
 
 	_D3D11_DEPTH_WRITE_MASK_ALL = 1
 
-	_D3D11_COMPARISON_GREATER = 5
+	_D3D11_COMPARISON_GREATER       = 5
+	_D3D11_COMPARISON_GREATER_EQUAL = 7
 
 	_D3D11_BLEND_OP_ADD        = 1
 	_D3D11_BLEND_ONE           = 2

gpu/backend/backend.go

@@ -156,6 +156,7 @@ type Texture interface {
 
 const (
 	DepthFuncGreater DepthFunc = iota
+	DepthFuncGreaterEqual
 )
 
 const (

gpu/gl/backend.go

@@ -399,6 +399,8 @@ func (b *Backend) DepthFunc(f backend.DepthFunc) {
 	switch f {
 	case backend.DepthFuncGreater:
 		glfunc = GREATER
+	case backend.DepthFuncGreaterEqual:
+		glfunc = GEQUAL
 	default:
 		panic("unsupported depth func")
 	}

gpu/gl/gl.go

@@ -35,6 +35,7 @@ const (
 	INFO_LOG_LENGTH                       = 0x8B84
 	INVALID_INDEX                         = ^uint(0)
 	GREATER                               = 0x204
+	GEQUAL                                = 0x206
 	LINEAR                                = 0x2601
 	LINK_STATUS                           = 0x8b82
 	LUMINANCE                             = 0x1909

gpu/gpu.go

@@ -71,7 +71,7 @@ type drawState struct {
 	t     op.TransformOp
 	cpath *pathOp
 	rect  bool
-	z     int
+	z     uint16
 
 	matType materialType
 	// Current paint.ImageOp
@@ -335,7 +335,8 @@ func (g *GPU) BeginFrame() {
 		g.zopsTimer.begin()
 	}
 	g.ctx.BindFramebuffer(g.defFBO)
-	g.ctx.DepthFunc(backend.DepthFuncGreater)
+	// Equal because of depth saturation.
+	g.ctx.DepthFunc(backend.DepthFuncGreaterEqual)
 	g.ctx.ClearDepth(0.0)
 	g.ctx.Clear(g.drawOps.clearColor.Float32())
 	g.ctx.Viewport(0, 0, viewport.X, viewport.Y)
@@ -680,7 +681,7 @@ func (d *drawOps) newPathOp() *pathOp {
 	return &d.pathOpCache[len(d.pathOpCache)-1]
 }
 
-func (d *drawOps) collectOps(r *ops.Reader, state drawState) int {
+func (d *drawOps) collectOps(r *ops.Reader, state drawState) uint16 {
 	var aux []byte
 	var auxKey ops.Key
 loop:
@@ -749,10 +750,10 @@ loop:
 				d.clearColor = mat.color.Opaque()
 				continue
 			}
-			state.z++
-			if state.z != int(uint16(state.z)) {
-				// TODO(eliasnaur) gioui.org/issue/127.
-				panic("more than 65k paint objects not supported")
+			// It's ok to saturate the depth value because we're using
+			// the >= depth comparison function.
+			if state.z < 0xffff {
+				state.z++
 			}
 			// Assume 16-bit depth buffer.
 			const zdepth = 1 << 16