gpu,gpu/gl: introduce InputLayout and use shader reflection data

InputLayout is the abstraction for the mapping between vertex data and
shader inputs. The mapping is implicit in OpenGL but explicit in
Direct3D.

Infer the attribute name location index using shader reflection data,
and get rid of a parameter to NewProgram.

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

gpu/gl/backend.go

@@ -34,6 +34,14 @@ type glstate struct {
 	nattr    int
 	prog     *gpuProgram
 	texUnits [2]*gpuTexture
+	layout   *gpuInputLayout
+	buffer   bufferBinding
+}
+
+type bufferBinding struct {
+	buf    *gpuBuffer
+	offset int
+	stride int
 }
 
 type gpuTimer struct {
@@ -52,9 +60,9 @@ type gpuFramebuffer struct {
 }
 
 type gpuBuffer struct {
-	funcs Functions
-	obj   Buffer
-	typ   Enum
+	backend *Backend
+	obj     Buffer
+	typ     Enum
 }
 
 type gpuProgram struct {
@@ -63,6 +71,12 @@ type gpuProgram struct {
 	nattr   int
 }
 
+type gpuInputLayout struct {
+	backend *Backend
+	inputs  []gpu.InputLocation
+	layout  []gpu.InputDesc
+}
+
 // textureTriple holds the type settings for
 // a TexImage2D call.
 type textureTriple struct {
@@ -159,7 +173,7 @@ func (b *Backend) NewBuffer(typ gpu.BufferType, data []byte) gpu.Buffer {
 	default:
 		panic("unsupported buffer type")
 	}
-	buf := &gpuBuffer{funcs: b.funcs, obj: obj, typ: gltyp}
+	buf := &gpuBuffer{backend: b, obj: obj, typ: gltyp}
 	buf.Bind()
 	b.funcs.BufferData(gltyp, data, STATIC_DRAW)
 	return buf
@@ -232,10 +246,12 @@ func (b *Backend) SetBlend(enable bool) {
 }
 
 func (b *Backend) DrawElements(mode gpu.DrawMode, off, count int) {
+	b.setupVertexArrays()
 	b.funcs.DrawElements(toGLDrawMode(mode), count, UNSIGNED_SHORT, off)
 }
 
 func (b *Backend) DrawArrays(mode gpu.DrawMode, off, count int) {
+	b.setupVertexArrays()
 	b.funcs.DrawArrays(toGLDrawMode(mode), off, count)
 }
 
@@ -284,7 +300,27 @@ func (b *Backend) DepthFunc(f gpu.DepthFunc) {
 	b.funcs.DepthFunc(glfunc)
 }
 
-func (b *Backend) NewProgram(vssrc, fssrc gpu.ShaderSources, attr []string) (gpu.Program, error) {
+func (b *Backend) NewInputLayout(vs gpu.ShaderSources, layout []gpu.InputDesc) (gpu.InputLayout, error) {
+	if len(vs.Inputs) != len(layout) {
+		return nil, fmt.Errorf("NewInputLayout: got %d inputs, expected %d", len(layout), len(vs.Inputs))
+	}
+	for i, inp := range vs.Inputs {
+		if exp, got := inp.Size, layout[i].Size; exp != got {
+			return nil, fmt.Errorf("NewInputLayout: data size mismatch for %q: got %d expected %d", inp.Name, got, exp)
+		}
+	}
+	return &gpuInputLayout{
+		backend: b,
+		inputs:  vs.Inputs,
+		layout:  layout,
+	}, nil
+}
+
+func (b *Backend) NewProgram(vssrc, fssrc gpu.ShaderSources) (gpu.Program, error) {
+	attr := make([]string, len(vssrc.Inputs))
+	for _, inp := range vssrc.Inputs {
+		attr[inp.Location] = inp.Name
+	}
 	p, err := CreateProgram(b.funcs, vssrc.GLES2, fssrc.GLES2, attr)
 	if err != nil {
 		return nil, err
@@ -322,29 +358,45 @@ func (p *gpuProgram) UniformFor(uniform string) gpu.Uniform {
 	return GetUniformLocation(f, p.obj, uniform)
 }
 
-func (b *Backend) SetupVertexArray(slot int, size int, dataType gpu.DataType, stride, offset int) {
-	var gltyp Enum
-	switch dataType {
-	case gpu.DataTypeFloat:
-		gltyp = FLOAT
-	case gpu.DataTypeShort:
-		gltyp = SHORT
-	default:
-		panic("unsupported data type")
-	}
-	b.funcs.VertexAttribPointer(Attrib(slot), size, gltyp, false, stride, offset)
-}
-
 func (p *gpuProgram) Release() {
 	p.backend.funcs.DeleteProgram(p.obj)
 }
 
 func (b *gpuBuffer) Release() {
-	b.funcs.DeleteBuffer(b.obj)
+	b.backend.funcs.DeleteBuffer(b.obj)
+}
+
+func (b *gpuBuffer) BindVertex(stride, offset int) {
+	if b.typ != ARRAY_BUFFER {
+		panic("not a vertex buffer")
+	}
+	b.backend.state.buffer = bufferBinding{buf: b, stride: stride, offset: offset}
+}
+
+func (b *Backend) setupVertexArrays() {
+	layout := b.state.layout
+	if layout == nil {
+		panic("no input layout is current")
+	}
+	buf := b.state.buffer
+	b.funcs.BindBuffer(ARRAY_BUFFER, buf.buf.obj)
+	for i, inp := range layout.inputs {
+		l := layout.layout[i]
+		var gltyp Enum
+		switch l.Type {
+		case gpu.DataTypeFloat:
+			gltyp = FLOAT
+		case gpu.DataTypeShort:
+			gltyp = SHORT
+		default:
+			panic("unsupported data type")
+		}
+		b.funcs.VertexAttribPointer(Attrib(inp.Location), l.Size, gltyp, false, buf.stride, buf.offset+l.Offset)
+	}
 }
 
 func (b *gpuBuffer) Bind() {
-	b.funcs.BindBuffer(b.typ, b.obj)
+	b.backend.funcs.BindBuffer(b.typ, b.obj)
 }
 
 func (f *gpuFramebuffer) IsComplete() error {
@@ -437,6 +489,12 @@ func (t *gpuTimer) Duration() (time.Duration, bool) {
 	return time.Duration(nanos), true
 }
 
+func (l *gpuInputLayout) Bind() {
+	l.backend.state.layout = l
+}
+
+func (l *gpuInputLayout) Release() {}
+
 // floatTripleFor determines the best texture triple for floating point FBOs.
 func floatTripleFor(f Functions, ver [2]int, exts []string) (textureTriple, error) {
 	var triples []textureTriple