Logic of depth picking based on fragment shader

Depth buffer as written by vertex shader (left) and with fragment shader (right). Note fragment shader changes depth buffer allowing subsequently drawn crosshairs to show a depth test intersection with the brain. If Metal can not write to the depth buffer, I need to restrict myself to OpenGL and WebGL2, both of which support this feature. vertexShader_fragrmentShader

This did end up working for me. I wish Apple could provide more explicit advice when things fail. For one of the worlds most valued companies, and one that prides itself on being user friendly, the app notarization process is frustrating for developers. Apple needs to invest in generating meaningful error messages when they change things. My software is all free, but I still had to sign the Paid Applications Agreement.