gpu,gpu/backend: don't assume constant output framebuffer

Return the output framebuffer from BeginFrame, to make it clear that
it may change between frames. Delete CurrentFramebuffer which is no
longer needed.

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

app/internal/d3d11/backend_windows.go

@@ -274,7 +274,8 @@ func NewBackend(d *Device) (*Backend, error) {
 	return b, nil
 }
 
-func (b *Backend) BeginFrame() {
+func (b *Backend) BeginFrame() backend.Framebuffer {
+	return b.currentFramebuffer()
 }
 
 func (b *Backend) EndFrame() {
@@ -368,13 +369,13 @@ func (b *Backend) NewTexture(format backend.TextureFormat, width, height int, mi
 	return &Texture{backend: b, format: d3dfmt, tex: tex, sampler: sampler, resView: resView, bindings: bindings, width: width, height: height}, nil
 }
 
-func (b *Backend) CurrentFramebuffer() backend.Framebuffer {
+func (b *Backend) currentFramebuffer() backend.Framebuffer {
 	renderTarget := b.dev.ctx.OMGetRenderTargets()
 	if renderTarget != nil {
 		// Assume someone else is holding on to it.
 		_IUnknownRelease(unsafe.Pointer(renderTarget), renderTarget.vtbl.Release)
 	}
-	if renderTarget == b.fbo.renderTarget {
+	if b.fbo != nil && renderTarget == b.fbo.renderTarget {
 		return b.fbo
 	}
 	return &Framebuffer{dev: b.dev, renderTarget: renderTarget, foreign: true}
@@ -823,7 +824,7 @@ func (f *Framebuffer) Invalidate() {
 
 func (f *Framebuffer) Release() {
 	if f.foreign {
-		panic("cannot release Framebuffer from CurrentFramebuffer")
+		panic("framebuffer not created by NewBuffer")
 	}
 	if f.renderTarget != nil {
 		_IUnknownRelease(unsafe.Pointer(f.renderTarget), f.renderTarget.vtbl.Release)

gpu/backend/backend.go

@@ -12,7 +12,7 @@ import (
 // APIs such as OpenGL, Direct3D useful for rendering Gio
 // operations.
 type Device interface {
-	BeginFrame()
+	BeginFrame() Framebuffer
 	EndFrame()
 	Caps() Caps
 	NewTimer() Timer
@@ -20,7 +20,6 @@ type Device interface {
 	// are valid at the point of call.
 	IsTimeContinuous() bool
 	NewTexture(format TextureFormat, width, height int, minFilter, magFilter TextureFilter, bindings BufferBinding) (Texture, error)
-	CurrentFramebuffer() Framebuffer
 	NewFramebuffer(tex Texture, depthBits int) (Framebuffer, error)
 	NewImmutableBuffer(typ BufferBinding, data []byte) (Buffer, error)
 	NewBuffer(typ BufferBinding, size int) (Buffer, error)

gpu/compute.go

@@ -29,7 +29,6 @@ type compute struct {
 	cache         *resourceCache
 	maxTextureDim int
 
-	defFBO   backend.Framebuffer
 	programs struct {
 		elements   backend.Program
 		tileAlloc  backend.Program
@@ -197,7 +196,6 @@ func newCompute(ctx backend.Device) (*compute, error) {
 	}
 	g := &compute{
 		ctx:           ctx,
-		defFBO:        ctx.CurrentFramebuffer(),
 		cache:         newResourceCache(),
 		maxTextureDim: maxDim,
 		conf:          new(config),
@@ -290,7 +288,7 @@ func (g *compute) Frame() error {
 		Y: (viewport.Y + tileHeightPx - 1) / tileHeightPx,
 	}
 
-	g.ctx.BeginFrame()
+	defFBO := g.ctx.BeginFrame()
 	defer g.ctx.EndFrame()
 
 	if err := g.uploadImages(g.drawOps.allImageOps); err != nil {
@@ -305,6 +303,7 @@ func (g *compute) Frame() error {
 	if err := g.render(tileDims); err != nil {
 		return err
 	}
+	g.ctx.BindFramebuffer(defFBO)
 	g.blitOutput(viewport)
 	g.cache.frame()
 	g.drawOps.pathCache.frame()
@@ -331,7 +330,6 @@ func (g *compute) Profile() string {
 // shader can only write to RGBA textures, but since we actually render in sRGB
 // format we can't use glBlitFramebuffer, because it does sRGB conversion.
 func (g *compute) blitOutput(viewport image.Point) {
-	g.ctx.BindFramebuffer(g.defFBO)
 	g.ctx.Viewport(0, 0, viewport.X, viewport.Y)
 	g.ctx.BindTexture(0, g.output.image)
 	g.ctx.BindProgram(g.output.blitProg)

gpu/gl/backend.go

@@ -167,9 +167,10 @@ func NewBackend(ctx Context) (*Backend, error) {
 	return b, nil
 }
 
-func (b *Backend) BeginFrame() {
+func (b *Backend) BeginFrame() backend.Framebuffer {
 	// Assume GL state is reset between frames.
 	b.state = glstate{}
+	return b.currentFramebuffer()
 }
 
 func (b *Backend) EndFrame() {
@@ -228,7 +229,7 @@ func (b *Backend) NewFramebuffer(tex backend.Texture, depthBits int) (backend.Fr
 	return fbo, nil
 }
 
-func (b *Backend) CurrentFramebuffer() backend.Framebuffer {
+func (b *Backend) currentFramebuffer() backend.Framebuffer {
 	fboID := glimpl.Framebuffer(b.funcs.GetBinding(glimpl.FRAMEBUFFER_BINDING))
 	return &gpuFramebuffer{backend: b, obj: fboID, foreign: true}
 }
@@ -802,7 +803,7 @@ func (f *gpuFramebuffer) Invalidate() {
 
 func (f *gpuFramebuffer) Release() {
 	if f.foreign {
-		panic("cannot release framebuffer created by CurrentFramebuffer")
+		panic("framebuffer not created by NewBuffer")
 	}
 	f.backend.funcs.DeleteFramebuffer(f.obj)
 	if f.hasDepth {

gpu/gpu.go

@@ -48,7 +48,6 @@ type GPU interface {
 type gpu struct {
 	cache *resourceCache
 
-	defFBO                                            backend.Framebuffer
 	profile                                           string
 	timers                                            *timers
 	frameStart                                        time.Time
@@ -395,10 +394,8 @@ func New(ctx backend.Device) (GPU, error) {
 }
 
 func newGPU(ctx backend.Device) (*gpu, error) {
-	defFBO := ctx.CurrentFramebuffer()
 	g := &gpu{
-		defFBO: defFBO,
-		cache:  newResourceCache(),
+		cache: newResourceCache(),
 	}
 	g.drawOps.pathCache = newOpCache()
 	if err := g.init(ctx); err != nil {
@@ -443,7 +440,7 @@ func (g *gpu) Collect(viewport image.Point, frameOps *op.Ops) {
 }
 
 func (g *gpu) Frame() error {
-	g.ctx.BeginFrame()
+	defFBO := g.ctx.BeginFrame()
 	defer g.ctx.EndFrame()
 	viewport := g.renderer.blitter.viewport
 	for _, img := range g.drawOps.imageOps {
@@ -452,7 +449,7 @@ func (g *gpu) Frame() error {
 	if g.drawOps.profile {
 		g.zopsTimer.begin()
 	}
-	g.ctx.BindFramebuffer(g.defFBO)
+	g.ctx.BindFramebuffer(defFBO)
 	g.ctx.DepthFunc(backend.DepthFuncGreater)
 	// Note that Clear must be before ClearDepth if nothing else is rendered
 	// (len(zimageOps) == 0). If not, the Fairphone 2 will corrupt the depth buffer.
@@ -472,13 +469,13 @@ func (g *gpu) Frame() error {
 	g.renderer.intersect(g.drawOps.imageOps)
 	g.stencilTimer.end()
 	g.coverTimer.begin()
-	g.ctx.BindFramebuffer(g.defFBO)
+	g.ctx.BindFramebuffer(defFBO)
 	g.ctx.Viewport(0, 0, viewport.X, viewport.Y)
 	g.renderer.drawOps(g.cache, g.drawOps.imageOps)
 	g.ctx.SetBlend(false)
 	g.renderer.pather.stenciler.invalidateFBO()
 	g.coverTimer.end()
-	g.ctx.BindFramebuffer(g.defFBO)
+	g.ctx.BindFramebuffer(defFBO)
 	g.cleanupTimer.begin()
 	g.cache.frame()
 	g.drawOps.pathCache.frame()