android - Additive blending without glClear

Question

I want to do additive blending on camera preview's surface texture binded to my opengl context.

I am getting weirdly rendered texture when i enable blending(20 noisy squares getting rendered), if I call GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT); preview is proper but i lose my additive blending as i have cleared the buffer!

With glClear call

Without glClear call

I have no clue whats the problem is, i am newbie to opengl-es, Any suggestions? If any piece of code is needed i can provide to better understand the issue.

putting relevant code only. Ask for any other part of code if necessary.

public void onSurfaceCreated(GL10 unused, EGLConfig config) {

//compile shader, link program.... etc omitted for brevity 

//enable additive blending
GLES20.glEnable(GLES20.GL_BLEND);
GLES20.glBlendFunc(GLES20.GL_ONE, GLES20.GL_ONE);
GLES20.glBlendEquation(GLES30.GL_MAX);
}

public void onDrawFrame(GL10 unused) {
// Do not want to do this, but if i do preview is normal but i lose my blending
//GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);    
surfaceTexture.updateTexImage();
    GLES20.glUseProgram(_onscreenShader); 

    int th = GLES20.glGetUniformLocation(_onscreenShader, "sTexture");
    GLES20.glActiveTexture(GLES20.GL_TEXTURE0);
    GLES20.glBindTexture(GLES11Ext.GL_TEXTURE_EXTERNAL_OES, _rawVideoTexture); 
    GLES20.glUniform1i(th, 0);
    GLES20.glVertexAttribPointer(_onscreenPositionAttribute, 2, GLES20.GL_FLOAT, false, 4 * 2, pVertex);
    GLES20.glVertexAttribPointer(_onscreenUVAttribute, 2, GLES20.GL_FLOAT, false, 4 * 2, pTexCoord);
    GLES20.glEnableVertexAttribArray(_onscreenPositionAttribute);
    GLES20.glEnableVertexAttribArray(_onscreenUVAttribute);
    GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, 0, 4); 
}

Answer 1

Most Android devices have a PowerVR, Mali, Adreno or Vivante GPU core which are all deferred, tiled-based renderers. This architecture requires the glClear operation at the start of each frame to tell the OpenGL ES driver when to clear internal triangle binning queues as well as the frame buffer. If you don't do the glClear, this caching design does not work properly and you will get weird results that will differ from one GPU type to another. So, you really must do the glClear.