小中大平时图片保存应该注意的几点---tangdl2000的个人总结 [Re: tangdl2000]
Posted by: xatrixer
Posted on: 2005-12-27 23:58
Hi ALL:
It is a very thoughtful summary by “Tangdl2000” , though there are “principle” and “concepts” that need to be emphasized in order to fully understand “digital imaging” and its application in “digital publication”.
Now, above all, beyond anything, and the first, and the foremost, or before you even start to work on a digital device – Digital Camera (DC) or Scanner, REMEMBER that you want to be a “Digital Carpenter”, not a “Digital Iron Smith” when dealing with the digital file you acquired. In a plan Chinese, it is “要做木匠, 绝不能做铁匠”. This is an extension of a famous Chinese proverb --- “长木匠, 短铁匠”.
What does it mean? The "Principle of 长木匠, 短铁匠” in digital imaging has two components:
1. Acquired digital file has to be at its maximum capacity available in terms of physical dimension and (digital) information encompassed.
“Physical dimension” --- Suppose your DC can capture image at “1024v768”, “1280x1024” and “1600x1200”, based on the principle of “长木匠”, you should always capture the image at highest resolution (physical dimension) available, such as “1600x1200” in this assumption.
“Digital information (encompassed)” --- Suppose you scan a (B&W) gel film, the “color depth” of your scanner setting should be at 8-bit. If the original is a color picture, the setting can be 16-bit, 24-bit, or 32-bit. Of course, 32-bit should be the choice, though the differences between the 32 and 24, or even 16 may not be distinguishable by naked eyes.
After an image was acquired (scanned), the format (存储格式) in which an image to be saved should be “TIFF”. TIFF format is a LOSSLESS compression (or, non-lossy compression) compared to other format, such as JPEG --- meaning every “piece and bit” of information captured by the digital device will NOT lost during the “process of saving to disk”. In comparison, a maximum “data preserving rate” of JEPG is at 90% --- meaning at least 10% loss of captured information during the “saving (compression) process”.
2. The “work-flow”, or the sequential steps of working with digital file (here it is digital image file) is one-way, and ONE-way-ONLY. It is from “High to Low”, from “Raw to New”, and can NEVER be reversed.
Once an image was acquired with DC or scanner, named with identifiable name (ID), and saved in a TIFF format, make sure you are not going to work, change, manipulate, such as load up into Photoshop and then “adjusting and save”. This “VIRGIN” image is called “STOCK IMAGE”. You should ONLY work on the copy or copies of your STOCK IMAGE. Also, always utilize the “Duplicate Image” function in Photoshop and work on the copy of previous changed (manipulated) image so that you will always have what you have done previously.
Now, suppose you have trimmed down (crop) the size of the image from 1600x1200 to 800x600, then you thought it is might be too small to fit in the column of text. BE ADVISED, NEVER bring a smaller image (back) to a (previous) larger dimension. It is against the “Rule of the Game”, and it is “自欺欺人” . The exactly same "principle" applies to “Image format conversion” and “color depth conversion”
“Image format conversion” --- if you converted (saved) an image from TIFF to JEPG, a “lossy process”, you should not re-save the JPEG back to TIFF. Doing so is meaningless.
“Color depth conversion” --- if a 24 bit image was saved in a GIF format, which is an 8-bit, 256-color format, you should not re-save it, or bring back it to 24 bit anymore, even it can carry a JPG file extension.
Okay, All the above is the basic “principle” in acquiring and working with digital image.
More will be added later on in dealing with “Resolution (分辨率), DPI (dots per inch) , RGB and CMYK”, and image dimension, etc.
Also, should anyone have question related to “Digital Imaging” … feel free to post below.
Now let’s take a look at the “Resolution of a digital image”. Roughly, in referring a digital image, the resolution is defined as “the basic measurement of the amount of information on a display, presented as 640x480, or 1024x768, and so on”.
Does higher the resolution mean higher quality of a image? No, absolutely not! Without specifying other characters (parameters), the resolution of an image is only referring to its physical dimension --- the image size (not printed size).
The quality of a digital image saved on your PC is determined SOLELY by the “rate of compression”, or, in layman’s term, the format (存储格式) in which the image was saved as.
For example: a 640x480 image in TIFF has a better quality than the same size image in JPEG. Now, what about the same TIFF image and a 1280x1024 JPEG image? Again, the TIFF has better quality than the larger JPEG due to TIFF is a Lossless (compression) format when the image was saved.
So … please make no mistake about this concept that the QUALITY of an image on your computer disk is NOT determined by its resolution (physical dimension), IT IS DETERMINED by the way of how the image was saved --- compressed or non-compressed.
Now, what is the “DPI”? As it is has been known by anyone who has had a “contact” with digital image, “DPI” stands for “dot per inch”. But, who the hell made this “unit”, and what heck it is for?
“DPI” is a SPECIFICATION used for display devices, such as monitors, and output devices, such as printers, and single-pass scanners. At certain circumstances, people “messed-up” the concepts of resolution and DPI. It is very much understandable since they are two things closely related.
[ Note: Scanner’s DPI specification (old three-pass scanners) is not consistent with the DPI specs of printer and monitors. It is a long story, and it had involved manufacturers’ “scandals” which only hyped for selling scanners and was used to confuse the scanner buyers. ]
What is the relationship between “resolution and DPI” and how will it be reflected on the final output, such as on paper? Again, let’s make an assumption.
Suppose you like to stick with “instruction for author”, and try to acquire an image via a scanner with a printed dimension in 2-inch wide at 600-DPI in order to fit it in a single column on a double column page. What is the “resolution setting” on the scanner should be? Or, what is the minimum resolution you should scan? It is very simple. It should be (at least) 1200-DPI (600DPI x 2). One more … You have a 1600x1200 image and you would like to print it on a 300-DPI ink-jet printer. What is the maximum printed size of the image? It will be 5.3 x 4 inch (also as known as “5 x 4”).
Image’s DPI can be manipulated with image editing program, such as Photoshop, very easily. Now, do yourself a favor and try to answer the following questions:
Does higher DPI represent higher quality of the printed image on paper?
Does higher DPI image contain more “digital info” than a lower one?
Do you know what DPI a regular CTR monitor has?
Do you know why the publisher specify a DPI, such as 300 or 600 for color images and 1200 for monotone image?
What FORMAT an image should be achieved (saved)? Without a doubt, it will be TIFF. Or more precisely, the STOCK IMAGE in TIFF format. For those “Photoshop-touched” and “Ready-to-Send” images, it seems TIFF format is a more generic or popular format. How about the EPS format? EPS, stands for “Encapsulated PostScript”, is a standard file format for importing and exporting in or out between desktop publishing software packages made mainly by Adobe System. This format was initially introduced to MAC (苹果鸡), and subsequently to PC after WinTel PC dominated the personal computer market. So, EPS format is, at certain sense, only for the purpose of printing layout (排版), and has nothing to do with anything else we discussed above.
Of course, if it is REQUIRED by “Instruction to Authors”, preserve your image in EPS is the only way to make both you and publisher’s life easier.
One more note. There are many issues related to whether an image should be compressed or not. It even has “ventured” to “fields of Law and Order”… It is too complicated to elaborate here. Should anyone has interests in the matter, feel free to ask via PM or email.
