Libgdx fbo background transparency

Question

I draw things on an FBO in libgdx. Than I just want to draw that fbo WITH TRANSPARENT background to my screen. But that is allways BLACK.

Is it possible to use a transparent background on the spriteBatch?

Tried a lot of things, but it looks like I cannot dop this simple task.

I created a custom shader and:

vec4 orig = texture2D(u_texture, tc);
if (orig.a==0.0) discard;

ths FBO create code:

fbo = new FrameBuffer(Format.RGB565, width, height, hasDepth);

than:

public void begin() {
    fbo.begin();

    Gdx.gl.glClearColor(1.0f, 0.0f, 0.0f, 0f);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT | GL20.GL_DEPTH_BUFFER_BIT);
}

public void render() {
    renderIt();
}

private void renderIt() {
    spriteBatch.begin();
    spriteBatch.setShader(shader);
    spriteBatch.draw(fboRegion, 0, 0, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
    spriteBatch.end();
}

public void end() {
    Gdx.gl.glDisable(GLES20.GL_BLEND);
    fbo.end();
}

It looks like the shader cannot determinate the FBO's alpha values. why? I just need to clear the alpha=0.0 pixels out.

So in my shader the background cut out:

if (orig.r==1.0) discard;

this is WORKING

if (orig.a==0.0) discard;

if (orig.a<0.2) discard;

if (orig.a<0.9) discard;

THIS IS NOT

my shader looks like this

void main() {

    vec4 sum = vec4(0.0);
    vec2 tc = v_texCoord0;
    //the amount to blur, i.e. how far off center to sample from
    //1.0 -> blur by one pixel
    //2.0 -> blur by two pixels, etc.
    float blur = radius/resolution;
    //the direction of our blur
    //(1.0, 0.0) -> x-axis blur
    //(0.0, 1.0) -> y-axis blur
    float hstep = dir.x;
    float vstep = dir.y;

    sum += texture2D(u_texture, vec2(tc.x - 4.0*blur*hstep, tc.y - 4.0*blur*vstep)) * 0.0162162162;
    sum += texture2D(u_texture, vec2(tc.x - 3.0*blur*hstep, tc.y - 3.0*blur*vstep)) * 0.0540540541;
    sum += texture2D(u_texture, vec2(tc.x - 2.0*blur*hstep, tc.y - 2.0*blur*vstep)) * 0.1216216216;
    sum += texture2D(u_texture, vec2(tc.x - 1.0*blur*hstep, tc.y - 1.0*blur*vstep)) * 0.1945945946;
    sum += texture2D(u_texture, vec2(tc.x, tc.y)) * 0.2270270270;
    sum += texture2D(u_texture, vec2(tc.x + 1.0*blur*hstep, tc.y + 1.0*blur*vstep)) * 0.1945945946;
    sum += texture2D(u_texture, vec2(tc.x + 2.0*blur*hstep, tc.y + 2.0*blur*vstep)) * 0.1216216216;
    sum += texture2D(u_texture, vec2(tc.x + 3.0*blur*hstep, tc.y + 3.0*blur*vstep)) * 0.0540540541;
    sum += texture2D(u_texture, vec2(tc.x + 4.0*blur*hstep, tc.y + 4.0*blur*vstep)) * 0.0162162162;

    vec4 all = v_color * vec4(sum.rgb, 1.0);
    if (all.a < 0.9) discard;
    else gl_FragColor = all;

}

Answer 1

LOL I was a fckin idiote.

    vec4 all = v_color * vec4(sum.rgb, 1.0);
    if (all.a < 0.9) discard;
    else gl_FragColor = all;

this is the BAD shader (its alpha is allways 1.0)

this is the worked one:

vec4 all = sum;
if (all.a  < 0.5) discard;
else
    gl_FragColor = all;