overlapping Metal Point primitives and blending - metal

Question

I am rendering Points primitives that overlap partially. The fragment shader shades parts of each Point primitive square transparent (solid center circle). A point primitive that does not overlap any other point primitive, shades as expected (the transparent areas of the square show the background).
When such a point primitive overlaps another point primitive, the behavior is a unexpected. specifically: the transparent area does not show the opaque color of the unlerlying points primitive, but instead clears it and shows the background color.
In other words, point primitives with a higher vertex ID number clear the previsously shaded fragments of point primitives with a lower vertex ID number.
The points primitives are encoded like this:
command_encoder.drawPrimitives(.Point, vertexStart: 0, vertexCount: stroke.count)
And the vertex & fragment shaders looks like this:
vertex OutVertex vertex_func(constant InVertex* vertex_array [[buffer(0)]],
constant Uniforms& uniforms [[buffer(1)]],
uint vid [[vertex_id]]) {
OutVertex out;
InVertex in = vertex_array[vid];
// transform vertex into NDC space
out.position = uniforms.projection * float4(in.position.x, in.position.y, 0, 1);
out.pointSize = 60;
return out;
}
fragment float4 fragment_func(OutVertex vert [[stage_in]], float2 uv[[point_coord]]) {
float2 uvPos = uv;
uvPos.x -= 0.5f;
uvPos.y -= 0.5f;
uvPos *= 2.0f;
float dist = sqrt(uvPos.x*uvPos.x + uvPos.y*uvPos.y);
float circleAlpha = 1.0f-dist;
half4 color = half4(0.0f, 0.0f, 1.0f, 1.0f);
color *= circleAlpha;
return float4(color.r, color.g, color.b, circleAlpha);
}
The results looks like this:
I would like to find out how to prevent the clearing of overlapping transparent areas but maintain the transparency against the background.
Thank you.
Update 5/25/16:
I was able to turn on fixed function blending with these additions to my render pipeline descriptor:
rpld.colorAttachments[0].blendingEnabled = true
rpld.colorAttachments[0].rgbBlendOperation = .Add;
rpld.colorAttachments[0].alphaBlendOperation = .Add;
rpld.colorAttachments[0].sourceRGBBlendFactor = .One;
rpld.colorAttachments[0].sourceAlphaBlendFactor = .One;
rpld.colorAttachments[0].destinationRGBBlendFactor = .OneMinusSourceAlpha;
rpld.colorAttachments[0].destinationAlphaBlendFactor = .OneMinusSourceAlpha;
And now things look as expected: