余弦波是渐变的，当ωt+φ的取值变化量很小时，Y的变化量也是很微小的。余弦波的这一特性保证了嵌入水印后相邻字符灰度值差别不大，人眼无法识别，从而保证了水印的不可见性和图像质量。-Cosine wave is gradual, and when ωt+ φ values variation is very small, Y amount of change is very small. Cosine waves This feature ensures that watermarked gray value of adjacent character is not very different, the human eye does not recognize, thus ensuring the invisibility of watermark and image quality.