vec4 czm_screenToEyeCoordinates(vec4 screenCoordinate)
{
    // Reconstruct NDC coordinates
    float x = 2.0 * screenCoordinate.x - 1.0;
    float y = 2.0 * screenCoordinate.y - 1.0;
    float z = (screenCoordinate.z - czm_viewportTransformation[3][2]) / czm_viewportTransformation[2][2];
    vec4 q = vec4(x, y, z, 1.0);

    // Reverse the perspective division to obtain clip coordinates.
    q /= screenCoordinate.w;

    // Reverse the projection transformation to obtain eye coordinates.
    if (!(czm_inverseProjection == mat4(0.0))) // IE and Edge sometimes do something weird with != between mat4s
    {
        q = czm_inverseProjection * q;
    }
    else
    {
        float top = czm_frustumPlanes.x;
        float bottom = czm_frustumPlanes.y;
        float left = czm_frustumPlanes.z;
        float right = czm_frustumPlanes.w;

        float near = czm_currentFrustum.x;
        float far = czm_currentFrustum.y;

        q.x = (q.x * (right - left) + left + right) * 0.5;
        q.y = (q.y * (top - bottom) + bottom + top) * 0.5;
        q.z = (q.z * (near - far) - near - far) * 0.5;
        q.w = 1.0;
    }

    return q;
}

/**
 * Transforms a position from window to eye coordinates.
 * The transform from window to normalized device coordinates is done using components
 * of (@link czm_viewport} and {@link czm_viewportTransformation} instead of calculating
 * the inverse of <code>czm_viewportTransformation</code>. The transformation from
 * normalized device coordinates to clip coordinates is done using <code>fragmentCoordinate.w</code>,
 * which is expected to be the scalar used in the perspective divide. The transformation
 * from clip to eye coordinates is done using {@link czm_inverseProjection}.
 *
 * @name czm_windowToEyeCoordinates
 * @glslFunction
 *
 * @param {vec4} fragmentCoordinate The position in window coordinates to transform.
 *
 * @returns {vec4} The transformed position in eye coordinates.
 *
 * @see czm_modelToWindowCoordinates
 * @see czm_eyeToWindowCoordinates
 * @see czm_inverseProjection
 * @see czm_viewport
 * @see czm_viewportTransformation
 *
 * @example
 * vec4 positionEC = czm_windowToEyeCoordinates(gl_FragCoord);
 */
vec4 czm_windowToEyeCoordinates(vec4 fragmentCoordinate)
{
    vec2 screenCoordXY = (fragmentCoordinate.xy - czm_viewport.xy) / czm_viewport.zw;
    return czm_screenToEyeCoordinates(vec4(screenCoordXY, fragmentCoordinate.zw));
}

vec4 czm_screenToEyeCoordinates(vec2 screenCoordinateXY, float depthOrLogDepth)
{
    // See reverseLogDepth.glsl. This is separate to re-use the pow.
#if defined(LOG_DEPTH) || defined(LOG_DEPTH_READ_ONLY)
    float near = czm_currentFrustum.x;
    float far = czm_currentFrustum.y;
    float log2Depth = depthOrLogDepth * czm_log2FarDepthFromNearPlusOne;
    float depthFromNear = pow(2.0, log2Depth) - 1.0;
    float depthFromCamera = depthFromNear + near;
    vec4 screenCoord = vec4(screenCoordinateXY, far * (1.0 - near / depthFromCamera) / (far - near), 1.0);
    vec4 eyeCoordinate = czm_screenToEyeCoordinates(screenCoord);
    eyeCoordinate.w = 1.0 / depthFromCamera; // Better precision
    return eyeCoordinate;
#else
    vec4 screenCoord = vec4(screenCoordinateXY, depthOrLogDepth, 1.0);
    vec4 eyeCoordinate = czm_screenToEyeCoordinates(screenCoord);
#endif
    return eyeCoordinate;
}

/**
 * Transforms a position given as window x/y and a depth or a log depth from window to eye coordinates.
 * This function produces more accurate results for window positions with log depth than
 * conventionally unpacking the log depth using czm_reverseLogDepth and using the standard version
 * of czm_windowToEyeCoordinates.
 *
 * @name czm_windowToEyeCoordinates
 * @glslFunction
 *
 * @param {vec2} fragmentCoordinateXY The XY position in window coordinates to transform.
 * @param {float} depthOrLogDepth A depth or log depth for the fragment.
 *
 * @see czm_modelToWindowCoordinates
 * @see czm_eyeToWindowCoordinates
 * @see czm_inverseProjection
 * @see czm_viewport
 * @see czm_viewportTransformation
 *
 * @returns {vec4} The transformed position in eye coordinates.
 */
vec4 czm_windowToEyeCoordinates(vec2 fragmentCoordinateXY, float depthOrLogDepth)
{
    vec2 screenCoordXY = (fragmentCoordinateXY.xy - czm_viewport.xy) / czm_viewport.zw;
    return czm_screenToEyeCoordinates(screenCoordXY, depthOrLogDepth);
}