Accessing z-coordinate in world space in shader modifier SceneKit - ios

Question

Looking for
I’m having trouble accessing the z-coordinate of the rendered pixel in world space. In SceneKit, I am looking for a 3d plane whose rendered color is directly related to the z-coordinates of the rendered point.
Situation
I’m working with SpriteKit and I’m using a SK3DNode to embed a SceneKit scene inside my SpriteKit scene. For the SceneKit scene, I’m using a .dae Collada file exported from Blender. It contains a plane mesh and a light.
I’m applying shader modifiers to modify the geometry and the lighting model.
self.waterGeometry.shaderModifiers = #{
SCNShaderModifierEntryPointGeometry : self.geomModifier,
SCNShaderModifierEntryPointSurface : self.cellShadingModifier
};
The geometry modifier code (self.geomModifier):
// Waves Modifier
float Amplitude = 0.02;
float Frequency = 15.0;
vec2 nrm = _geometry.position.xz;
float len = length(nrm)+0.0001; // for robustness
nrm /= len;
float a = len + Amplitude*sin(Frequency * _geometry.position.z + u_time * 1.6);
_geometry.position.xz = nrm * a;
The geometry modifier applies a sine transformation to the _surface property to simulate waves. In the image below, the sketched sprites are SpriteKit sprites which have a higher zPosition and do not interfere with the SK3DNode. Notice the subtle waves (z displacement) as a result of the geometry modifier.
The next step, I want to output color to be computed based on the point’s z-coordinate in world space. This could be either _surface.diffuse or _output.color, that doesn't matter that much to (would imply a different point of insertion for the shader modifier but not an issue).
I have tried
The following code in the surface modifier (self.cellShadingModifier).
vec4 geometry = u_inverseViewTransform * vec4(_surface.position, 1.0);
if (geometry.y < 0.0) {
_surface.diffuse.rgb *= vec3(0.4);
}
_surface.position is in view space, and I hoped to transform it to world space by using u_inverseViewTransform. Apple docs says:
Geometric fields (such as position and normal) are expressed in view
space. You can use SceneKit’s uniforms (such as
u_inverseViewTransform) to operate in a different coordinate space,
[...]
As you can see it is flickering and does not appear to be based on the the geometry.position I just modified. I have tested this both in the simulator and on device (iPad Air). I believe I am making a simple error as I'm probably confusing _surface and _geometry properties.
Can anyone tell me where I can get the z-coordinate (world space) of the currently shaded point of the mesh, so I can use it in my rendering method?
Note
I have also tried to access _geometry inside the surface shader modifier, but I get the error Use of undeclared identifier '_geometry', which is strange because Apple documentation says:
You can use the structures defined by earlier entry points in later
entry points. For example, a snippet associated with the
SCNShaderModifierEntryPointFragment entry point can read from the
_surface structure defined by the SCNShaderModifierEntryPointSurface entry point.
Note 2
I could have the LightingModel shader calculate off of the generated sine wave (and avoid the search for the z-coordinate), but in the future I may be adding additional waves and using the z-coordinate would be more maintainable, not to mention elegant.

Answer 1

I've also been learning how to use the shader modifiers. I have a solution to this which works for me, using both the inverse model transform and the inverse view transform.
The following code will paint the right-hand side of the model at the centre of the scene with a red tint. You should be able to check the other position element (y, I think) to get the result you want.
vec4 orig = _surface.diffuse;
vec4 transformed_position = u_inverseModelTransform * u_inverseViewTransform * vec4(_surface.position, 1.0);
if (transformed_position.z < 0.0) {
_surface.diffuse = mix(vec4(1.0,0.0,0.0,1.0), orig, 0.5);
}