gpu: replace Backend.DefaultFramebuffer by Backend.CurrentFramebuffer

DefaultFramebuffer was set up at Backend creation time, which is
difficult to predict. Instead, let GPU query and cache the current
FBO when created.

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

gpu/backend.go

@@ -19,7 +19,7 @@ type Backend interface {
 	// are valid at the point of call.
 	IsTimeContinuous() bool
 	NewTexture(format TextureFormat, width, height int, minFilter, magFilter TextureFilter, bindings BufferBinding) (Texture, error)
-	DefaultFramebuffer() Framebuffer
+	CurrentFramebuffer() Framebuffer
 	NewFramebuffer(tex Texture, depthBits int) (Framebuffer, error)
 	NewImmutableBuffer(typ BufferBinding, data []byte) (Buffer, error)
 	NewBuffer(typ BufferBinding, size int) (Buffer, error)

gpu/gl/backend.go

@@ -15,8 +15,7 @@ import (
 
 // Backend implements gpu.Backend.
 type Backend struct {
-	funcs  Functions
-	defFBO *gpuFramebuffer
+	funcs Functions
 
 	state glstate
 
@@ -59,10 +58,11 @@ type gpuTexture struct {
 }
 
 type gpuFramebuffer struct {
-	backend *Backend
+	backend  *Backend
 	obj      Framebuffer
 	hasDepth bool
 	depthBuf Renderbuffer
+	foreign  bool
 }
 
 type gpuBuffer struct {
@@ -127,14 +127,12 @@ func NewBackend(f Functions) (*Backend, error) {
 	if err != nil {
 		return nil, err
 	}
-	defFBO := Framebuffer(f.GetBinding(FRAMEBUFFER_BINDING))
 	b := &Backend{
 		funcs:       f,
 		floatTriple: floatTriple,
 		alphaTriple: alphaTripleFor(ver),
 		srgbaTriple: srgbaTriple,
 	}
-	b.defFBO = &gpuFramebuffer{backend: b, obj: defFBO}
 	if hasExtension(exts, "GL_EXT_disjoint_timer_query_webgl2") || hasExtension(exts, "GL_EXT_disjoint_timer_query") {
 		b.feats.Features |= gpu.FeatureTimers
 	}
@@ -202,8 +200,9 @@ func (b *Backend) NewFramebuffer(tex gpu.Texture, depthBits int) (gpu.Framebuffe
 	return fbo, nil
 }
 
-func (b *Backend) DefaultFramebuffer() gpu.Framebuffer {
-	return b.defFBO
+func (b *Backend) CurrentFramebuffer() gpu.Framebuffer {
+	fboID := Framebuffer(b.funcs.GetBinding(FRAMEBUFFER_BINDING))
+	return &gpuFramebuffer{backend: b, obj: fboID, foreign: true}
 }
 
 func (b *Backend) NewTexture(format gpu.TextureFormat, width, height int, minFilter, magFilter gpu.TextureFilter, binding gpu.BufferBinding) (gpu.Texture, error) {
@@ -612,6 +611,9 @@ func (f *gpuFramebuffer) Invalidate() {
 }
 
 func (f *gpuFramebuffer) Release() {
+	if f.foreign {
+		panic("cannot release framebuffer created by CurrentFramebuffer")
+	}
 	f.backend.funcs.DeleteFramebuffer(f.obj)
 	if f.hasDepth {
 		f.backend.funcs.DeleteRenderbuffer(f.depthBuf)

gpu/gpu.go

@@ -30,6 +30,7 @@ type GPU struct {
 	pathCache *opCache
 	cache     *resourceCache
 
+	defFBO                                            Framebuffer
 	profile                                           string
 	timers                                            *timers
 	frameStart                                        time.Time
@@ -275,7 +276,9 @@ const (
 )
 
 func New(ctx Backend) (*GPU, error) {
+	defFBO := ctx.CurrentFramebuffer()
 	g := &GPU{
+		defFBO:    defFBO,
 		pathCache: newOpCache(),
 		cache:     newResourceCache(),
 	}
@@ -332,6 +335,7 @@ func (g *GPU) BeginFrame() {
 	if g.drawOps.profile {
 		g.zopsTimer.begin()
 	}
+	g.ctx.BindFramebuffer(g.defFBO)
 	g.ctx.DepthFunc(DepthFuncGreater)
 	g.ctx.ClearColor(g.drawOps.clearColor[0], g.drawOps.clearColor[1], g.drawOps.clearColor[2], 1.0)
 	g.ctx.ClearDepth(0.0)
@@ -347,6 +351,7 @@ func (g *GPU) BeginFrame() {
 	g.renderer.intersect(g.drawOps.imageOps)
 	g.stencilTimer.end()
 	g.coverTimer.begin()
+	g.ctx.BindFramebuffer(g.defFBO)
 	g.ctx.Viewport(0, 0, viewport.X, viewport.Y)
 	g.renderer.drawOps(g.drawOps.imageOps)
 	g.ctx.SetBlend(false)
@@ -502,7 +507,6 @@ func (r *renderer) stencilClips(pathCache *opCache, ops []*pathOp) {
 		data, _ := pathCache.get(p.pathKey)
 		r.pather.stencilPath(p.clip, p.off, p.place.Pos, data.(*pathData))
 	}
-	r.pather.end()
 }
 
 func (r *renderer) intersect(ops []imageOp) {
@@ -526,7 +530,6 @@ func (r *renderer) intersect(ops []imageOp) {
 		r.ctx.Viewport(img.place.Pos.X, img.place.Pos.Y, img.clip.Dx(), img.clip.Dy())
 		r.intersectPath(img.path, img.clip)
 	}
-	r.pather.stenciler.endIntersect()
 }
 
 func (r *renderer) intersectPath(p *pathOp, clip image.Rectangle) {

gpu/path.go

@@ -56,9 +56,8 @@ type coverUniforms struct {
 }
 
 type stenciler struct {
-	ctx    Backend
-	defFBO Framebuffer
-	prog   struct {
+	ctx  Backend
+	prog struct {
 		prog     *program
 		uniforms struct {
 			vert struct {
@@ -138,7 +137,6 @@ func newCoverer(ctx Backend) *coverer {
 }
 
 func newStenciler(ctx Backend) *stenciler {
-	defFBO := ctx.DefaultFramebuffer()
 	// Allocate a suitably large index buffer for drawing paths.
 	indices := make([]uint16, pathBatchSize*6)
 	for i := 0; i < pathBatchSize; i++ {
@@ -173,7 +171,6 @@ func newStenciler(ctx Backend) *stenciler {
 	}
 	st := &stenciler{
 		ctx:      ctx,
-		defFBO:   defFBO,
 		indexBuf: indexBuf,
 	}
 	prog, err := ctx.NewProgram(shader_stencil_vert, shader_stencil_frag)
@@ -281,10 +278,6 @@ func (p *pather) begin(sizes []image.Point) {
 	p.stenciler.begin(sizes)
 }
 
-func (p *pather) end() {
-	p.stenciler.end()
-}
-
 func (p *pather) stencilPath(bounds image.Rectangle, offset f32.Point, uv image.Point, data *pathData) {
 	p.stenciler.stencilPath(bounds, offset, uv, data)
 }
@@ -299,14 +292,9 @@ func (s *stenciler) beginIntersect(sizes []image.Point) {
 	s.ctx.BindProgram(s.iprog.prog.prog)
 }
 
-func (s *stenciler) endIntersect() {
-	s.ctx.BindFramebuffer(s.defFBO)
-}
-
 func (s *stenciler) invalidateFBO() {
 	s.intersections.invalidate(s.ctx)
 	s.fbos.invalidate(s.ctx)
-	s.ctx.BindFramebuffer(s.defFBO)
 }
 
 func (s *stenciler) cover(idx int) stencilFBO {
@@ -347,10 +335,6 @@ func (s *stenciler) stencilPath(bounds image.Rectangle, offset f32.Point, uv ima
 	}
 }
 
-func (s *stenciler) end() {
-	s.ctx.BindFramebuffer(s.defFBO)
-}
-
 func (p *pather) cover(z float32, mat materialType, col [4]float32, scale, off, uvScale, uvOff, coverScale, coverOff f32.Point) {
 	p.coverer.cover(z, mat, col, scale, off, uvScale, uvOff, coverScale, coverOff)
 }