确定BLOB是没有加载整个领域的形象？ [英] determine if blob is an image without loading entire field?
问题描述
有没有办法在一个数据库只读的几个字节出一个BLOB类型的字段（对于这个问题，没关系的DB品牌），并确定如果二进制内容是图像（假设它是一个：JPG，GIF，PNG）？我有一个Web应用程序存储在数据库中的文件，如果它是一个形象，我想显示缩略图，否则我要显示一个图标...但我没有保存在其他地方的斑点任何MIMETYPE信息（这不是我的设计）......因为它是一个Web应用程序，图像的加载需要从写出的＆lt发生在一个单独的调用（getImage.ashx）; IMG＆GT;标签..当我写出来的标签，我不希望有读取整个BLOB到库，确定它是否是一个图像，调整其大小......等等等等......我想可以看看前几个字节，并且知道我是否需要写在标签或没有。我不明白的图像文件的结构/格式不够好，知道是否有某种形式的前几个字节标准头的，我可以读说，如果它是一个JPG，GIF或PNG。
is there a way to read only a few bytes out of a BLOB type field in a database (for this question it doesn't matter the DB brand) and determine if the binary content is an image (assume it is one of: JPG, GIF, PNG)? I have a webapp that stores files in the database, and if it is an image, I want to show a thumbnail, otherwise I want to show an icon... but I don't have any MIMETYPE info stored anywhere else for the blob (it's not my design)... and because it is a webapp, the loading of the image needs to occur in a separate call (getImage.ashx) from writing out the <img> tag.. and when I'm writing out the tag I don't want to have to read the entire blob into a library, determine if it is an image, resize it... etc etc ... I'd like to be able to look at the first few bytes and know whether I need to write the tag or not. I don't understand image file structures/formats well enough to know if there is some sort of standard header in the first few bytes that I could read in to tell if it is a JPG, GIF, or PNG.
有意义吗？
如果你想帮助我明确（而不是通常回答是否是可以做到的），然后我使用.NET C＃和SQL 2005
if you want to help me specifically (instead of generally answering whether it can be done) then I'm using .NET C# and SQL 2005
谢谢！
推荐答案
PNG格式开始：89 50 4E 47 0D 0A 1A 0A ;看到 http://www.libpng.org/pub/png/规格/ 1.2 / PNG-Structure.html
PNGs start with: 89 50 4E 47 0D 0A 1A 0A ; see http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html
的GIF开始47 49 46 38 37 61（GIF87a）或47 49 46 38 39 61（参考GIF89a） ;看到 http://www.fileformat.info/format/gif/egff.htm
GIFs start with 47 49 46 38 37 61 (GIF87a) or 47 49 46 38 39 61 (GIF89a) ; see http://www.fileformat.info/format/gif/egff.htm
JPEG图片与FF D8 FF E0 xx月xx 4A开始46 49 46 00（编辑：添加标题的缺失部分; xx月xx是标题的长度，以字节为单位）;看到 http://www.obrador.com/essentialjpeg/headerinfo.htm
JPEGs start with FF D8 FF E0 xx xx 4A 46 49 46 00 ( adding missing portion of the header; xx xx is the header length, in bytes) ; see http://www.obrador.com/essentialjpeg/headerinfo.htm
参考： HTTP：//wangrui.word press.com / 2007/06/19 /文件签名表/
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