扩展变换抖动调制（spread transform dither modulation，STDM）是一种性能很好的水印算法，该算法中用于扩展变换的投影向量是影响其性能的一个重要因素。本文研究STDM中的投影向量，对随机投影向量、均匀投影向量、平行投影向量、从频率角度选择的投影向量和从HVS角度选择的投影向量进行了介绍，并对选择各种投影向量时STDM的鲁棒性进行了比较与分析，得到了不同噪声攻击下，使得STDM水印算法的鲁棒性达到最优的投影向量选择方法。此外，为了充分利用HVS的特性，介绍了结合HVS综合选择投影向量的方法，实验结果说明该方法可以进一步改善STDM算法的视觉效果和鲁棒性。

Spread Transform Dither Modulation(STDM) is a watermarking algorithm having good performance.In this algorithm,the Project-vector used for spread transform is an important factor that affects its performance.This paper studies on the project-vector of STDM.Random project vector,uniform project vector,parallel project vector,project vector chosen according frequency and project vector chosen according HVS are introduced,the robustness under different project vectors is compared and analyzed,the way of choosing project-vectors that makes the robustness of STDM best under different attacks is obtained.Further,in order to make full use of the HVS's characteristics,choosing project-vector combined with HVS is introduced,and the experiment results show that by using this method,the visual quality and robustness of the STDM algorithm can be improved further.