libGDX Framebuffer Alpha Issues - libgdx

Question

I'm having issues with the libGDX FrameBuffer and Alpha. Below are two images of the expected result and the actual result. Can someone please tell me what I am doing wrong and how can I correct it. Here is the code:
FrameBuffer buffer;
Sprite sprite;
SpriteBatch batch;
Texture texture1;
Texture texture2;
Texture texture3;
Sprite texture2Sprite;
#Override
public void create () {
batch = new SpriteBatch();
texture1 = new Texture("1.png");
texture2 = new Texture("2.png");
texture3 = new Texture("3.png");
texture2Sprite = new Sprite(texture2);
texture2Sprite.setAlpha(0.5f);
texture2Sprite.setPosition(100, 100);
buffer = new FrameBuffer(Pixmap.Format.RGBA8888, Gdx.graphics.getWidth(), Gdx.graphics.getHeight(), false);
sprite = new Sprite(buffer.getColorBufferTexture());
sprite.flip(false, true);
}
public void createFBO()
{
buffer.begin();
Gdx.gl.glClearColor(0f, 0f, 0f, 0f);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
batch.begin();
batch.draw(texture1, 0f, 0f);
texture2Sprite.draw(batch);
batch.end();
buffer.end();
}
#Override
public void render () {
createFBO();
Gdx.gl.glClearColor(0f, 0f, 1f, 1f);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
batch.begin();
batch.enableBlending();
batch.draw(texture3, 200, 200);
sprite.draw(batch);
batch.end();
}
Expected Result
Actual Result

Answer 1

I had pretty much the same issue with semi-transparent colours in the FrameBuffer, the problem and solution you can find on my badlogic forum topic here.
Basically, you need to draw to your FrameBuffer in a pre-multiplied alpha state. You do this by creating a custom fragment shader (my full code is on the topic). Then you set your blending mode to match (GL_ONE, GL_ONE_MINUS_SRC_ALPHA).
Then draw your sprite using the same blend function but default shader. You should end up with something like this;
public void createFBO(){
buffer.begin();
batch.begin();
Gdx.gl.glClearColor(0f, 0f, 0f, 0f);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
batch.setShader(pmaShaderProgram); //pre-multiplied alpha ShaderProgram
batch.setBlendFunction(GL20.GL_ONE, GL20.GL_ONE_MINUS_SRC_ALPHA);
batch.draw(texture1, 0f, 0f);
texture2Sprite.draw(batch);
batch.setShader(null); //default ShaderProgram
batch.setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA); //default blend mode
batch.end();
buffer.end();
}
#Override
public void render () {
createFBO();
batch.begin();
batch.enableBlending();
Gdx.gl.glClearColor(0f, 0f, 1f, 1f);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
batch.draw(texture3, 200, 200);
batch.setBlendFunction(GL20.GL_ONE, GL20.GL_ONE_MINUS_SRC_ALPHA);
sprite.draw(batch);
batch.setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);
batch.end();
}