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