确定blob是否是一个没有加载整个字段的图像？ [英] determine if blob is an image without loading entire field?

问题描述

有一种方法只从数据库中的BLOB类型字段中读取几个字节（对于这个问题，DB品牌并不重要），并确定二进制内容是否是一个图像（假设它是一个：JPG，GIF，PNG）？我有一个webapp在数据库中存储文件，如果它是一个图像，我想显示一个缩略图，否则我想显示一个图标...但我没有任何其他地方存储任何MIMETYPE信息blob （它不是我的设计）...并且因为它是一个webapp，图像的加载需要发生在一个单独的调用（getImage.ashx）从写出< img>标签..当我写出标签时，我不想把整个blob读入库，确定它是一个图像，调整它的大小...等等...我想要能够查看前几个字节，知道我是否需要写标签。我不明白图像文件结构/格式足够好知道在头几个字节，我可以读，以告诉它是一个JPG，GIF或PNG。是否有某种标准头。

有意义吗？

如果你想帮我明确（而不是一般回答是否可以做）使用.NET C＃和SQL 2005

谢谢！

解决方案

PNG开始：89 50 4E 47 0D 0A 1A 0A
;请参阅 http://www.libpng.org/pub/png /spec/1.2/PNG-Structure.html

GIF以47 49 46 38 37 61（GIF87a）或47 49 46 38 39 61（GIF89a）开头）
;请参阅 http://www.fileformat.info/format/gif/egff.htm

JPEG以FF D8开头FF E0 xx xx 4A 46 49 46 00（EDIT：添加标题的缺失部分; xx xx是标题长度， ）;请参阅 http://www.obrador.com/essentialjpeg/headerinfo.htm p>

参考资料： http://wangrui.wordpress.com/2007/06/19/file-signatures-table/

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.

make sense?

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

Thanks!

解决方案

PNGs start with: 89 50 4E 47 0D 0A 1A 0A ; see http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html

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

JPEGs start with FF D8 FF E0 xx xx 4A 46 49 46 00 (EDIT: adding missing portion of the header; xx xx is the header length, in bytes) ; see http://www.obrador.com/essentialjpeg/headerinfo.htm

reference: http://wangrui.wordpress.com/2007/06/19/file-signatures-table/

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