# Why clipplanes are computed in eye space?

In ModelClippingPlanesStageFS, CesiumJS use clip function to compute the fragment will be display or not:

``````float clip(vec4 fragCoord, sampler2D clippingPlanes, mat4 clippingPlanesMatrix) {
vec4 position = czm_windowToEyeCoordinates(fragCoord);
vec3 clipNormal = vec3(0.0);
vec3 clipPosition = vec3(0.0);
float pixelWidth = czm_metersPerPixel(position);

#ifdef UNION_CLIPPING_REGIONS
float clipAmount; // For union planes, we want to get the min distance. So we set the initial value to the first plane distance in the loop below.
#else
float clipAmount = 0.0;
bool clipped = true;
#endif

for (int i = 0; i < CLIPPING_PLANES_LENGTH; ++i) {
vec4 clippingPlane = getClippingPlane(clippingPlanes, i, clippingPlanesMatrix);
clipNormal = clippingPlane.xyz;
clipPosition = -clippingPlane.w * clipNormal;
float amount = dot(clipNormal, (position.xyz - clipPosition)) / pixelWidth;

#ifdef UNION_CLIPPING_REGIONS
clipAmount = czm_branchFreeTernary(i == 0, amount, min(amount, clipAmount));
if (amount <= 0.0) {
}
#else
clipAmount = max(amount, clipAmount);
clipped = clipped && (amount <= 0.0);
#endif
}

#ifndef UNION_CLIPPING_REGIONS
if (clipped) {
}
#endif

return clipAmount;
}
``````

My question is why is the clipNormal and position that computes amout under eyeSpace:

``````vec4 clippingPlane = getClippingPlane(clippingPlanes, i, clippingPlanesMatrix);
clipNormal = clippingPlane.xyz;
clipPosition = -clippingPlane.w * clipNormal;
float amount = dot(clipNormal, (position.xyz - clipPosition)) / pixelWidth;
``````

Is there any reason for this?