fix a bug + doc

batch.go

@@ -1,6 +1,7 @@
 package pixel
 
 import (
+ "fmt"
  "image/color"
  "math"
 
@@ -75,6 +76,7 @@ func (b *Batch) MakeTriangles(t Triangles) TargetTriangles {
 func (b *Batch) MakePicture(p Picture) TargetPicture {
  return &batchPicture{
  Picture: p,
+ b: b,
  }
 }
 
@@ -113,6 +115,8 @@ func (bt *batchTriangles) Draw() {
 
 type batchPicture struct {
  Picture
+
+ b *Batch
 }
 
 func (bp *batchPicture) Slice(r Rect) Picture {
@@ -122,5 +126,9 @@ func (bp *batchPicture) Slice(r Rect) Picture {
 }
 
 func (bp *batchPicture) Draw(t TargetTriangles) {
- t.(*batchTriangles).draw(bp)
+ bt := t.(*batchTriangles)
+ if bp.b != bt.b {
+ panic(fmt.Sprintf("%T.Draw: TargetTriangles generated by different Batch", bp))
+ }
+ bt.draw(bp)
 }

interface.go

@@ -19,7 +19,11 @@ type Target interface {
  // present) when making new TargetTriangles. This varies from Target to Target.
  MakeTriangles(Triangles) TargetTriangles
 
- //TODO: doc
+ // MakePicture generates a specialized copy of the provided Picture.
+ //
+ // When calling Draw method on the returned TargetPicture, the TargetPicture will be drawn
+ // onto the Target that generated it together with the TargetTriangles supplied to the Draw
+ // method.
  MakePicture(Picture) TargetPicture
 }
 
@@ -116,6 +120,9 @@ type Picture interface {
 type TargetPicture interface {
  Picture
 
+ // Draw draws the supplied TargetTriangles (which must be generated by the same Target as
+ // this TargetPicture) with this TargetPicture. The TargetTriangles should utilize the data
+ // from this TargetPicture in some way.
  Draw(TargetTriangles)
 }