有時，當使用我們的電腦時，會發生一些非常奇怪的事情，完全沒有意義……比如把一個簡單的影象複製到剪貼簿上，電腦就因此凍結了。影象就是影象，對嗎？今天的超級使用者帖子回答了一個困惑讀者的難題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

原始影象由維基媒體提供。

問題

超級使用者讀者Joban Dhillon想知道為什麼將影象複製到他電腦上的剪貼簿會使其凍結：

I was messing around with some height map images and found this one:

(http://upload.wikimedia.org/*********/comm***/1/15/Srtm_ramp2.world.21600×10800.jpg)

The image is 21,600*10,800 pixels in size. When I right click and select “Copy Image” in my browser (I am using Google Chrome), it slows down my computer until it freezes. After that I must restart. I am curious about why this happens. I presume it is the size of the image, although it is only about 6 MB when saved to my computer. I am also using Windows 8.1

為什麼一個簡單的影象在複製到剪貼簿後會凍結Joban的電腦？

答案

超級使用者貢獻者Mokubai為我們提供了答案：

“Copy Image” is copying the raw image data, rather than the image file itself, to your clipboard.

The raw image data will be 21,600 x 10,800 x 3 (24 bit image) = 699,840,000 bytes of data. That is approximately 700 MB of data your browser is trying to copy to the clipboard.

JPEG compresses the raw data using a lossy algorithm and can get pretty good compression. Hence the compressed file is only 6 MB.

The reason it makes your computer slow is that it is probably filling your memory up with at least the 700 MB of image data that your browser is using to show you the image, another 700 MB (along with whatever overhead the clipboard incurs) to store it on the clipboard, and a not insignificant amount of processing power to convert the image into a format that can be stored on the clipboard.

Chances are that if you have less than 4 GB of physical RAM, then those copies of the image data are forcing your computer to page memory out to the swap file in an attempt to fulfill both memory demands at the same time. This will cause programs and disk access to be sluggish as they use the disk and try to use the data that may have just been paged out.

In short: Do not use the clipboard for huge images unless you have a lot of memory and a bit of time to spare.

Like pretty graphs? This is what happens when I load that image in Google Chrome, then copy it to the clipboard on my machine with 12 GB of RAM:

It starts off at the lower point using 2.8 GB of RAM, loading the image punches it up to 3.6 GB (approximately the 700 MB), then copying it to the clipboard spikes way up there at 6.3 GB of RAM before settling back down at the 4.5-ish you would expect to see for a program and two copies of a rather large image.

That is a whopping 3.7 GB of image data being worked on at the peak, which is probably the initial image, a reserved quantity for the clipboard, and perhaps a couple of conversion buffers. That is enough to bring any machine with less than 8 GB of RAM to its knees.

Strangely, doing the same thing in Firefox just copies the image file rather than the image data (without the scary memory surge).

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