gpu: allocate path draw index buffer once

Before this change, the index buffer would start empty and grow up to the
maximum size (128kb). It would never shrink. We're about to tighten the GPU
buffer API to be immutable for performance and to better match Direct3D, so
allocate the index buffer once at startup, and limit it to a reasonable size.

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

gpu/path.go

@@ -38,7 +38,6 @@ type coverer struct {
 type stenciler struct {
 	ctx                Backend
 	defFBO             Framebuffer
-	indexBufQuads      int
 	prog               Program
 	iprog              Program
 	fbos               fboSet
@@ -70,6 +69,11 @@ var (
 	intersectAttribs = []string{"pos", "uv"}
 )
 
+const (
+	// Number of path quads per draw batch.
+	pathBatchSize = 10000
+)
+
 const (
 	attribPathCorner = 0
 	attribPathMaxY   = 1
@@ -128,6 +132,19 @@ func newStenciler(ctx Backend) *stenciler {
 	}
 	coverLoc := iprog.UniformFor("cover")
 	iprog.Uniform1i(coverLoc, 0)
+	// Allocate a suitably large index buffer for drawing paths.
+	indices := make([]uint16, pathBatchSize*6)
+	for i := 0; i < pathBatchSize; i++ {
+		i := uint16(i)
+		indices[i*6+0] = i*4 + 0
+		indices[i*6+1] = i*4 + 1
+		indices[i*6+2] = i*4 + 2
+		indices[i*6+3] = i*4 + 2
+		indices[i*6+4] = i*4 + 1
+		indices[i*6+5] = i*4 + 3
+	}
+	indexBuf := ctx.NewBuffer(BufferTypeIndices)
+	indexBuf.Upload(BufferUsageStaticDraw, gunsafe.BytesView(indices))
 	return &stenciler{
 		ctx:                ctx,
 		defFBO:             defFBO,
@@ -138,7 +155,7 @@ func newStenciler(ctx Backend) *stenciler {
 		uPathOffset:        prog.UniformFor("pathOffset"),
 		uIntersectUVScale:  iprog.UniformFor("uvScale"),
 		uIntersectUVOffset: iprog.UniformFor("uvOffset"),
-		indexBuf:           ctx.NewBuffer(BufferTypeIndices),
+		indexBuf:           indexBuf,
 	}
 }
 
@@ -274,24 +291,9 @@ func (s *stenciler) stencilPath(bounds image.Rectangle, offset f32.Point, uv ima
 	nquads := data.ncurves / 4
 	for start < nquads {
 		batch := nquads - start
-		if max := int(^uint16(0)) / 6; batch > max {
+		if max := pathBatchSize; batch > max {
 			batch = max
 		}
-		// Enlarge VBO if necessary.
-		if batch > s.indexBufQuads {
-			indices := make([]uint16, batch*6)
-			for i := 0; i < batch; i++ {
-				i := uint16(i)
-				indices[i*6+0] = i*4 + 0
-				indices[i*6+1] = i*4 + 1
-				indices[i*6+2] = i*4 + 2
-				indices[i*6+3] = i*4 + 2
-				indices[i*6+4] = i*4 + 1
-				indices[i*6+5] = i*4 + 3
-			}
-			s.indexBuf.Upload(BufferUsageStaticDraw, gunsafe.BytesView(indices))
-			s.indexBufQuads = batch
-		}
 		off := path.VertStride * start * 4
 		s.ctx.SetupVertexArray(attribPathCorner, 2, DataTypeShort, path.VertStride, off+int(unsafe.Offsetof((*(*path.Vertex)(nil)).CornerX)))
 		s.ctx.SetupVertexArray(attribPathMaxY, 1, DataTypeFloat, path.VertStride, off+int(unsafe.Offsetof((*(*path.Vertex)(nil)).MaxY)))