Render to texture not functioning - actionscript-3

Question

I'm trying to test Stage3D's setRenderToTexture function, but my test code is not providing any output. I was hoping someone could tell me why - I've tried practically everything to get this working. The code I've created is below:
import flash.events.Event;
import flash.display.*;
import flash.display3D.*;
import flash.display3D.textures.Texture;
import AGALMiniAssembler;
//using stage size in several locations for simplicity - needs to be square, 2^n
[SWF(width='512', height='512', backgroundColor='0x000000', frameRate='60')]
//vars
var context0:Context3D;
var vBuff:VertexBuffer3D;
var iBuff:IndexBuffer3D;
var tex0:Texture;
var tex1:Texture;
var vShader:AGALMiniAssembler = new AGALMiniAssembler();
var fShader:AGALMiniAssembler = new AGALMiniAssembler();
var program:Program3D;
//create a square
var square:Shape = new Shape();
square.graphics.beginFill(0xaa00ff,1);
square.graphics.drawRect(10, 10, stage.stageWidth - 20, stage.stageHeight - 20);
//draw square to bitmapdata and create one blank bitmapdata
var tex0data:BitmapData = new BitmapData(stage.stageWidth,stage.stageHeight,false,0);
tex0data.draw(square);
var tex1data:BitmapData = new BitmapData(stage.stageWidth,stage.stageHeight,false,0);
//initialize stage3d
stage.stage3Ds[0].addEventListener(Event.CONTEXT3D_CREATE, initStage3D);
stage.stage3Ds[0].requestContext3D();
function initStage3D(e:Event):void {
context0 = stage.stage3Ds[0].context3D;
context0.configureBackBuffer(stage.stageWidth, stage.stageHeight, 0);
//create buffers - simple quad
var vBuff_vec:Vector.<Number> = new <Number>[-1, -1, 0, 0, 1,
-1, 1, 0, 0, 0,
1, 1, 0, 1, 0,
1, -1, 0, 1, 1];
var iBuff_vec:Vector.<uint> = new <uint>[0, 1, 2, 0, 2, 3]
vBuff = context0.createVertexBuffer(4, 5);
vBuff.uploadFromVector(vBuff_vec, 0, 4);
iBuff = context0.createIndexBuffer(6);
iBuff.uploadFromVector(iBuff_vec, 0, 6);
//load bitmapdata into textures - square to tex0 and blank to tex1
tex0 = context0.createTexture(tex0data.width,tex0data.height,'bgra',false);
tex0.uploadFromBitmapData(tex0data);
tex1 = context0.createTexture(tex1data.width, tex1data.height,'bgra',true);
tex1.uploadFromBitmapData(tex1data);
//create and upload simple shader program
vShader.assemble('vertex', 'mov v0, va1 \n'+ //uv coords to v0
'mov op, va0'); //output xyz coords
fShader.assemble('fragment', 'tex oc, v0, fs0 <2d, linear, nomip>'); //output texture at fs0
program = context0.createProgram();
program.upload(vShader.agalcode, fShader.agalcode);
context0.setProgram(program);
//set buffers
context0.setVertexBufferAt(0, vBuff, 0, 'float3');
context0.setVertexBufferAt(1, vBuff, 3, 'float2');
//prep rendering
addEventListener(Event.ENTER_FRAME, render);
}
function render(e:Event):void {
context0.clear();
//render tex0 onto tex1
context0.setRenderToTexture(tex1);
context0.setTextureAt(0, tex0);
context0.drawTriangles(iBuff);
//render tex1 to back buffer and present
context0.setTextureAt(0, tex1);
context0.setRenderToBackBuffer();
context0.drawTriangles(iBuff);
context0.present();
}
EDIT: I have noticed that changing the color of the blank bitmap does change the color of the final output - it's being displayed, but the square isn't being rendered to it.

Answer 1

FINALLY figured this out! There is a subtle, but important, step that must be taken before a texture is viable as a render target - it must be cleared after the render target is switched. Revising the render function as follows causes the program to function as expected:
function render(e:Event):void {
//render tex0 onto tex1
context0.setRenderToTexture(tex1);
context0.clear();
context0.setTextureAt(0, tex0);
context0.drawTriangles(iBuff);
//render tex1 to back buffer and present
context0.setTextureAt(0, tex1);
context0.setRenderToBackBuffer();
context0.clear();
context0.drawTriangles(iBuff);
context0.present();
}