对标准中传输语法的一段话理解不够，请多多指教

scarab

有如下一段话：(PS 3.5-2001 Page 46)
A.2 DICOM LITTLE ENDIAN TRANSFER SYNTAX (EXPLICIT VR)
......
-For the Value Representations OB and OW, the encoding shall meet the following specification
depending on the Data Element Tag:
--Data Element (7FE0,0010) Pixel Data
-where Bits Allocated (0028,0100) has a value greater than 8 shall have Value Representation OW and shall be encoded in Little Endian"
- where Bits Allocated (0028,0100) has a value less than or equal to 8 shall have the Value Representation OB or OW and shall be encoded in Little Endian.
......
针对"where Bits Allocated (0028,0100) has a value less than or equal to 8 shall have the Value Representation OB or OW and shall be encoded in Little Endian."
我的理解是：既然Bits Allocated<=8，那么Pixel Data的Value Representation应该为OB的，但是标准却说Pixel Data的VR是OB或者OW。
为什么可能会是OW呢？
多谢！

JB

The Rule of thumb is:
如果 pixel data 不是 encoded （即没有压缩）可以用 OW.

scarab

OK
让我想想。。
thx

scarab

JB :
我想了一下，我这样理解对不对：
除了Implicit Little endian transfer syntax之外，其余的transfer syntax都是Explicit，因此，我通过VR来判断是OB还是OW的来进行传输，而不用考虑是什么类型的图象也不用考虑该图象做过何种处理。
这样做法对吗？
谢谢

JB

可以。但是这个不很重要，从 Bits Allocated 你就应该知道到底一个 pixel 是 byte 还是 word。
题外话：何谓 “The Rule of thumb” 呀？有何来由啊？
我想考考你们英语老师的水平。我们那时主要还是靠自学。

Myth

to scarab
OB or OW 与 bit allocated 没有直接的关系，这就是说即使 bit allocated = 8，仍旧可以用OW，那么这代表什么呢，这表示 OW 仅仅是（7fe0,0010）的存放格式。
不知道我这样说清不清楚。
那么OB and OW在解图时的区别是什么呢，我认为当 transfer systax != 1.2.2 时，他们是一样的。

scarab

谢谢，哦，谢谢，呵呵，有机会我请客
>>何谓 “The Rule of thumb” 呀？有何来由啊？
这个，再让我想一想

ps:我也是自学，全是自己看标准，看程序
我没有接触过医学图象及图象处理，所以理解起来比较费劲

JB

那还是我来回答吧。
"The rule of thumb” 是一个常有语，意思就是“通用准则”。原来的意思却是相当不同，原意是“打老婆和孩子的棍子不能粗于拇子”。相当于当时的一条民间法律。
至于 OW 和 OB 嘛，OW 的主要意义是让你判断要不要做 byte swap.

tong

有意思。JB说的意思没错。
查了一下 Merriam-Webster.
Main Entry: rule of thumb
Date: 1692
1 : a method of procedure based on experience and common sense
2 : a general principle regarded as roughly correct but not intended to be scientifically accurate
另外一个有关出处的参考：http://phrases.shu.ac.uk/meanings/307000.html
Meaning  
    According to a rough and ready rule.  
Origin
    This has been said come from the belief that English law once allowed a man to beat his wife with a stick so long as it is was no thicker than his thumb. In 1782 Judge Buller made such a ruling in an English court. It has never been law though and there\'s no reason to connect the legal case with the phrase, which was in circulation well before 1782. Other explanations come from the numerous ways that thumbs have been used to estimate things - temperature of brews of beer, measurement using the estimated inch from the joint to the nail, etc.  

scarab

下面引用由JB在 2003/04/17 11:09am 发表的内容：
可以。但是这个不很重要，从 Bits Allocated 你就应该知道到底一个 pixel 是 byte 还是 word。

这就是我从标准中“where Bits Allocated (0028,0100) has a value less than or equal to 8 shall have the value Representation OB or OW and shall be encoded in Little Endian.”产生怀疑的原因。因为假如Bit allocated<=8的话，我认为他就应该是OB的，而标准中却说是OB或者OW的。

BTW：任何事情对我来说都很重要，我要把事情弄清楚才觉得坦然

scarab

另外，感谢myth，使我豁然开朗。
不过我确实糊涂了：（

Myth

难得糊涂