少了这一行怎么这么慢？关于 MemoryImageSource.(50分)

片断如下：
MemoryImageSource memImage;
Image drawImage;
int scrWidth = 400;
int scrHeight = 300;
int pixelCount = 120000;
int[] pixels = new int[pixelCount];
// 为什么少了这一行会很慢很慢？
// for (int i=0;
i< pixelCount;
++i) pixels = 0xff000000;
//
offImage = new MemoryImageSource(scrWidth, scrHeight, pixels, 0, scrWidth);
memImage.setAnimated(true);
drawImage = createImage(offImage);