#ifdef LOG_DEPTH
varying float v_depthFromNearPlusOne;

#ifdef POLYGON_OFFSET
uniform vec2 u_polygonOffset;
#endif

#endif

/**
 * Writes the fragment depth to the logarithmic depth buffer.
 * <p>
 * Use this when the vertex shader does not call {@link czm_vertexlogDepth}, for example, when
 * ray-casting geometry using a full screen quad.
 * </p>
 * @name czm_writeLogDepth
 * @glslFunction
 *
 * @param {float} depth The depth coordinate, where 1.0 is on the near plane and
 *                      depth increases in eye-space units from there
 *
 * @example
 * czm_writeLogDepth((czm_projection * v_positionEyeCoordinates).w + 1.0);
 */
void czm_writeLogDepth(float depth)
{
#if defined(GL_EXT_frag_depth) && defined(LOG_DEPTH)
    // Discard the vertex if it's not between the near and far planes.
    // We allow a bit of epsilon on the near plane comparison because a 1.0
    // from the vertex shader (indicating the vertex should be _on_ the near
    // plane) will not necessarily come here as exactly 1.0.
    if (depth <= 0.9999999 || depth > czm_farDepthFromNearPlusOne) {
        discard;
    }

#ifdef POLYGON_OFFSET
    // Polygon offset: m * factor + r * units
    float factor = u_polygonOffset[0];
    float units = u_polygonOffset[1];

    // If we can't compute derivatives, just leave out the factor I guess?
#ifdef GL_OES_standard_derivatives
    // m = sqrt(dZdX^2 + dZdY^2);
    float x = dFdx(depth);
    float y = dFdy(depth);
    float m = sqrt(x * x + y * y);

    // Apply the factor before computing the log depth.
    depth += m * factor;
#endif

#endif

    gl_FragDepthEXT = log2(depth) * czm_oneOverLog2FarDepthFromNearPlusOne;

#ifdef POLYGON_OFFSET
    // Apply the units after the log depth.
    gl_FragDepthEXT += czm_epsilon7 * units;
#endif

#endif
}

/**
 * Writes the fragment depth to the logarithmic depth buffer.
 * <p>
 * Use this when the vertex shader calls {@link czm_vertexlogDepth}.
 * </p>
 *
 * @name czm_writeLogDepth
 * @glslFunction
 */
void czm_writeLogDepth() {
#ifdef LOG_DEPTH
    czm_writeLogDepth(v_depthFromNearPlusOne);
#endif
}