GLES2.0+ framebuffer fade artifacts

Question

I'm writing a simple test program on android using GLES 2.0 or 3.0. (If the solution only works on one over the other, than that's what I'll go with.)

Problem:

• Basically I am running into strange pixel artifacts when fading colors.
• The problem can be "hidden" by making the cutoff threshold higher however the fade still has a patchy consistency and for my purpose this will not work.

Example:

• Example Pictures (Sorry, I cant post images directly yet)
• The Frame buffers resolution has been lowered to make the artifacts more visible.
• This issue occurs on all devices tested (Nexus 7 2012 and 2013, Galaxy Tab S 8.4, Galaxy S6 Edge, Galaxy Note 4)
• On the newer devices, the problem just seems to be small pink uniform dots, on the older devices it seems do be patchy as seen in the linked pictures.
• Running a blur pass on the final render is also out of the question as this needs to be power efficient and mobile friendly.

Layout:

• I create 2 frame buffers. (Using GLES20.GL_UNSIGNED_SHORT_5_6_5 format.)
• I set the first one as a render target.
• I draw a quad that is the size of the buffer to the framebuffer.
• The quads fragment shader is setup to render a black dot that moves in a circle.
• The quad can also take a texture and blend it with the dot.
• I then switch back to rendering to the screen.
• I render another quad using the previously set render target as the texture.
• This process is then repeated each frame switching the textures.

Render Code:

int i = 0;
@Override public void onDrawView() {
    setRenderTarget(mRenderTexture[i]);
    mTestShader.setTexture0(mRenderTexture[(i + 1) % 2].getTextureHandle());
    mTestShader.draw(mBlitQuad);
    setRenderTarget(null);
    mBlitShader.setTexture0(mRenderTexture[i].getTextureHandle());
    mBlitShader.draw(mBlitQuad);
    i = ++i % 2;
}

Shader Code:

precision highp float;

uniform float u_GlobalTime;
uniform vec3 u_Resolution;
uniform sampler2D u_Texture0;

varying vec2 v_TexCoord;

void main(void) {
    vec2 uv = v_TexCoord * 2.0 - 1.0;
    uv.x *= u_Resolution.z;
    vec2 pos = vec2(cos(u_GlobalTime), sin(u_GlobalTime)) * 0.5;
    vec4 circle = vec4(1.0 - smoothstep(0.09, 0.11, length(pos - uv)));
    vec4 px = max(texture2D(u_Texture0, v_TexCoord), circle) - 0.025;
    gl_FragColor = step(0.15, px) * px;
}

Any help on this issue?

EDIT:
After some googling on dither thanks to Jerem, I found GLES20.glDisable(GLES20.GL_DITHER). This helps alot with this problem and on newer devices it works perfect. On older devices I'm still running into artifacting as seen HERE.

EDIT2:
After some more testing, switching to a buffer format with even RGB colors seems to have done the trick GLES30.GL_UNSIGNED_SHORT_5_5_5_1. My guess is it had to do with how the GPU compresses the framebuffer as the green was more noticeable. (It would still be helpful to know the specific reason as to why it doesn't like the 5_6_5 format.)
Final result after both fixes applied: i.imgur.com/bQsG9YJ.png.

Answer 1

Its a multi-part solution, call GLES20.glDisable(GLES20.GL_DITHER), set render buffer format to GLES30.GL_UNSIGNED_SHORT_5_5_5_1, and as Jerem said, you also may be able to get even better results with alpha disabled, though experimentation with that on my device selection appeared to have no effect, possible on other devices it would.