#ifdef LOG_DEPTH
// 1.0 at the near plane, increasing linearly from there.
varying float v_depthFromNearPlusOne;
#ifdef SHADOW_MAP
varying vec3 v_logPositionEC;
#endif
#endif

vec4 czm_updatePositionDepth(vec4 coords) {
#if defined(LOG_DEPTH)

#ifdef SHADOW_MAP
    vec3 logPositionEC = (czm_inverseProjection * coords).xyz;
    v_logPositionEC = logPositionEC;
#endif

    // With the very high far/near ratios used with the logarithmic depth
    // buffer, floating point rounding errors can cause linear depth values
    // to end up on the wrong side of the far plane, even for vertices that
    // are really nowhere near it. Since we always write a correct logarithmic
    // depth value in the fragment shader anyway, we just need to make sure
    // such errors don't cause the primitive to be clipped entirely before
    // we even get to the fragment shader.
    coords.z = clamp(coords.z / coords.w, -1.0, 1.0) * coords.w;
#endif

    return coords;
}

/**
 * Writes the logarithmic depth to gl_Position using the already computed gl_Position.
 *
 * @name czm_vertexLogDepth
 * @glslFunction
 */
void czm_vertexLogDepth()
{
#ifdef LOG_DEPTH
    v_depthFromNearPlusOne = (gl_Position.w - czm_currentFrustum.x) + 1.0;
    gl_Position = czm_updatePositionDepth(gl_Position);
#endif
}

/**
 * Writes the logarithmic depth to gl_Position using the provided clip coordinates.
 * <p>
 * An example use case for this function would be moving the vertex in window coordinates
 * before converting back to clip coordinates. Use the original vertex clip coordinates.
 * </p>
 * @name czm_vertexLogDepth
 * @glslFunction
 *
 * @param {vec4} clipCoords The vertex in clip coordinates.
 *
 * @example
 * czm_vertexLogDepth(czm_projection * vec4(positionEyeCoordinates, 1.0));
 */
void czm_vertexLogDepth(vec4 clipCoords)
{
#ifdef LOG_DEPTH
    v_depthFromNearPlusOne = (clipCoords.w - czm_currentFrustum.x) + 1.0;
    czm_updatePositionDepth(clipCoords);
#endif
}