(PECL ps >= 1.1.0)
ps_setgray — Sets gray value
$psdoc, float $gray): boolSets the gray value for all following drawing operations.
psdocResource identifier of the postscript file as returned by ps_new().
grayThe value must be between 0 (white) and 1 (black).
成功时返回 true， 或者在失败时返回 false。