Question

Internally Flash obviously keeps a list of the primitives drawn using Graphics so I wondered if you have many such primitives in a Sprite, can you re-position/remove/alter individual items rather than clear and re-draw everything? Or is this deeper into the bowels of Flash than you’re allowed (or recommended) to go?

Answer 1

Unfortunately, it is impossible: Once the items are drawn, you can only modify the full shape, but not the drawing itself.

To give you more of an explanation, I googled about how Flash actually calculates display objects. Unfortunately, I couldn’t find anything specific.

But I found enough to make an educated guess: [EDIT]: I found a very interesting PDF on the Anatomy of a Flash. It explains the rendering tree and how graphics objects are treated internally.

I know for a fact that all shape tweens created in the IDE are compiled into shape sequences (each frame is stored as a separate image). And it makes sense to do it that way: Each new frame of the movie must be calculated, all vector images are added to a tree, each rendered as bitmaps, combined and drawn as one final bit plane, in order to be displayed. So it is only logical to do every possible shape calculation at compile time, rather than at runtime.

Then again, a bitmap would store 32 bits of color information for every single pixel, while vectors are stored in simple values, storing x and y coordinates, line style, fill style, etc. Some vectors can be grouped, so that for more complex shapes, line and fill styles only have to be stored once, and only coordinates are necessary for the rest. Also, primitive shapes like circles and rectangles require less information than objects combined from many individual points and lines.

[EDIT]: The above mentioned PDF says this:

Both AS3 and AS3 DisplayObjects are
converted to SObjects internally.
SObjects have a character associated.
Based on the character type it has
different drawing methods, but it all
resumes to drawing fills with
different source of colors.

It would take a very, very complex vector shape to require more single pieces of information than its bitmap representation, provided it is larger than a few pixels in width and height. Therefore, keeping simple shapes as vector representations consumes considerably less memory than storing full bitmaps – and so it is logical not to do shape rendering at compile time, as well (except for complicated shapes – then the “cacheAsBitmap” property comes into play).

Consider what I’ve said about vectors, line style and fill style, etc. – sounds quite a lot like the sequence of commands we have to write when drawing in ActionScript, right? I would assume these commands are simply converted 1:1 into exactly the kind of vector representations I was talking about. This would make the compiler faster, the binaries smaller, and the handling of both the IDE shapes and the AS shapes exactly the same.

[EDIT]: Turns out I was not quite right on that:

Edge & Colors

• LSObjects tree is traversed and a list of edges is created
  • Edges have colors associated
  • Strokes are converted to edges
  • Colors are sources of display data, eg. Bitmaps, Video, Solid fills,
    Gradients
• Rasterization
  • Edges are sorted and a color is calculated for each pixel – pixels are
    touched only once
• Presentation
  • After the main rasterizer is done painting, the memory buffer is
    copied to the screen

Now imagine all of those vectors were freely editable:
The sequence of commands would no longer be final! What if you were to add, move or erase one at runtime? For example: Having a rectangle inside of a filled rectangle subtracts the inner shape from the outer shape. What if you moved one of the corner points to the outside? The result would be a completely different shape! Or if you added one point? You could not store the shape as a rectangle any longer, requiring 5 point items to draw the same thing that once had been one rect item. In short: All the groupings and memory optimizations would no longer work. And it would also slow down runtime graphics considerably. That’s why it is only allowed to add new elements to the shape, but not to modify them once they are drawn. And why you have to clear and redraw your graphics, if you want existing shapes to change.

[EDIT]: You can always do complex stuff by doing the calculations yourself. I still believe it was a good decision not to integrate those into basic graphics functionality.