1.为什么要用到BASE64编码的图片信息
Base64是网络上最常见的用于传输8Bit字节代码的编码方式之一。Base64 主要不是加密，它主要的用途是把一些二进制数转成普通字符用于网络传输。由于一些二进制字符在传输协议中属于控制字符，不能直接传送需要转换一下。最常见的用途是作为电子邮件或WebService附件的传输编码.
2.base64编码定义
目前的internet e-mail标准--简单邮件传递协议(smtp)在rfc821中规定了两条重要但不难实现的限制：
1)邮件的内容必须全部为7-比特的美国ascii码。
2)每一行的长度不能超过1000的字符。
因此为了通过smtp用e-mail进行传送,内存的序列化对象必须转化为和以上相容的格式。
rfc1521提供了一个可行的方案。它定义了邮件的内容部分,使之能包涵多种形式的数据。这种标准就是目前众所周知的mime。
按照rfc1521编码过程为:输入是24个比特,输出是4个字节。24个比特输入组从左至右 由3个8比特的输入组形成。这24个比特被看成4个连续的6比特组,而每个6比特输入组被翻译为base64码表中的一个数字。依次反复不断进行，直到全部输入数据转换完成。
如果最后剩下两个输入数据，在编码结果后加1个“=”；如果最后剩下一个输入数据，编码结果后加2个“=”；如果没有剩下任何数据，就什么都不要加，这样才可以保证资料还原的正确性。
完整的base64定义可见 RFC1421和 RFC2045。编码后的数据比原始数据略长，为原来的4/3。在电子邮件中，根据RFC822规定，每76个字符，还需要加上一个回车换行。可以估算编码后数据长度大约为原长的135.1%。
3.如何进行base64编码
Base64 使用US-ASCII子集的65个字符, 每个字符用6位表示
对于文本串，编码过程如下。例如"men":
先转成US-ASCII值.
"m"十进制 109
"e"十进制 101
"n"十进制 110
二进制 :
m 01101101
e 01100101
n 01101110
三个8位连起来是24位
011011010110010101101110
然后分成4个6位
011011 010110 010101 101110
现在得到4个值，十进制为
27 22 21 46
对应的 Base64 字符是 : b W V u
编码总是基于3个字符，从而产生4个Base64字符。
如果只是2个字符的数据，使用特殊字符"="补齐Base64的4字。
如，编码"me"
01101101 01100101
0110110101100101
011011 010110 0101
111111 (与，补足6位)
011011 010110 010100
b W U
b W U = ("=" 补足4字符)
于是 "bWU=" 就是"me"的Base64值.
如果只是2个字符的数据,如编码 "m"
01101101
011011 01
111111
011011 010000
b Q = =
于是 "bQ==" 就是"m"的Base64值.
4.显示被存储为Base64编码字符串的图片的例子
1)使用data: URI直接在网页中嵌入.
data: URI定义于IETF标准的RFC 2397
data: URI的基本使用格式如下：
data:[<MIME-type>][;base64|charset=some_charset],<data>
mime-type是嵌入数据的mime类型，比如png图片就是image/png。
如果后面跟base64，说明后面的data是采用base64方式进行编码的
这种方法可以将任何文件嵌入到你的网页中，如HTM/HTML，主要代码：\'data:text/html;base64,+你的编码\'
下面给出MIME-Type 列表（文件类型参照）：
123 application/vnd.lotus-1-2-3
3gp video/3gpp
aab application/x-authoware-bin
aam application/x-authoware-map
aas application/x-authoware-seg
ai application/postscript
aif audio/x-aiff
aifc audio/x-aiff
aiff audio/x-aiff
als audio/X-Alpha5
amc application/x-mpeg
ani application/octet-stream
asc text/plain
asd application/astound
asf video/x-ms-asf
asn application/astound
asp application/x-asap
asx video/x-ms-asf
au audio/basic
avb application/octet-stream
avi video/x-msvideo
awb audio/amr-wb
bcpio application/x-bcpio
bin application/octet-stream
bld application/bld
bld2 application/bld2
bmp application/x-MS-bmp
bpk application/octet-stream
bz2 application/x-bzip2
cal image/x-cals
ccn application/x-cnc
cco application/x-cocoa
cdf application/x-netcdf
cgi magnus-internal/cgi
chat application/x-chat
class application/octet-stream
clp application/x-msclip
cmx application/x-cmx
co application/x-cult3d-object
cod image/cis-cod
cpio application/x-cpio
cpt application/mac-compactpro
crd application/x-mscardfile
csh application/x-csh
csm chemical/x-csml
csml chemical/x-csml
css text/css
cur application/octet-stream
dcm x-lml/x-evm
dcr application/x-director
dcx image/x-dcx
dhtml text/html
dir application/x-director
dll application/octet-stream
dmg application/octet-stream
dms application/octet-stream
doc application/msword
dot application/x-dot
dvi application/x-dvi
dwf drawing/x-dwf
dwg application/x-autocad
dxf application/x-autocad
dxr application/x-director
ebk application/x-expandedbook
emb chemical/x-embl-dl-nucleotide
embl chemical/x-embl-dl-nucleotide
eps application/postscript
eri image/x-eri
es audio/echospeech
esl audio/echospeech
etc application/x-earthtime
etx text/x-setext
evm x-lml/x-evm
evy application/x-envoy
exe application/octet-stream
fh4 image/x-freehand
fh5 image/x-freehand
fhc image/x-freehand
fif image/fif
fm application/x-maker
fpx image/x-fpx
fvi video/isivideo
gau chemical/x-gaussian-input
gca application/x-gca-compressed
gdb x-lml/x-gdb
gif image/gif
gps application/x-gps
gtar application/x-gtar
gz application/x-gzip
hdf application/x-hdf
hdm text/x-hdml
hdml text/x-hdml
hlp application/winhlp
hqx application/mac-binhex40
htm text/html
html text/html
hts text/html
ice x-conference/x-cooltalk
ico application/octet-stream
ief image/ief
ifm image/gif
ifs image/ifs
imy audio/melody
ins application/x-NET-Install
ips application/x-ipscript
ipx application/x-ipix
it audio/x-mod
itz audio/x-mod
ivr i-world/i-vrml
j2k image/j2k
jad text/vnd.sun.j2me.app-descriptor
jam application/x-jam
jar application/java-archive
jnlp application/x-java-jnlp-file
jpe image/jpeg
jpeg image/jpeg
jpg image/jpeg
jpz image/jpeg
js application/x-javascript
jwc application/jwc
kjx application/x-kjx
lak x-lml/x-lak
latex application/x-latex
lcc application/fastman
lcl application/x-digitalloca
lcr application/x-digitalloca
lgh application/lgh
lha application/octet-stream
lml x-lml/x-lml
lmlpack x-lml/x-lmlpack
lsf video/x-ms-asf
lsx video/x-ms-asf
lzh application/x-lzh
m13 application/x-msmediaview
m14 application/x-msmediaview
m15 audio/x-mod
m3u audio/x-mpegurl
m3url audio/x-mpegurl
ma1 audio/ma1
ma2 audio/ma2
ma3 audio/ma3
ma5 audio/ma5
man application/x-troff-man
map magnus-internal/imagemap
mbd application/mbedlet
mct application/x-mascot
mdb application/x-msaccess
mdz audio/x-mod
me application/x-troff-me
mel text/x-vmel
mi application/x-mif
mid audio/midi
midi audio/midi
mif application/x-mif
mil image/x-cals
mio audio/x-mio
mmf application/x-skt-lbs
mng video/x-mng
mny application/x-msmoney
moc application/x-mocha
mocha application/x-mocha
mod audio/x-mod
mof application/x-yumekara
mol chemical/x-mdl-molfile
mop chemical/x-mopac-input
mov video/quicktime
movie video/x-sgi-movie
mp2 audio/x-mpeg
mp3 audio/x-mpeg
mp4 video/mp4
mpc application/vnd.mpohun.certificate
mpe video/mpeg
mpeg video/mpeg
mpg video/mpeg
mpg4 video/mp4
mpga audio/mpeg
mpn application/vnd.mophun.application
mpp application/vnd.ms-project
mps application/x-mapserver
mrl text/x-mrml
mrm application/x-mrm
ms application/x-troff-ms
mts application/metastream
mtx application/metastream
mtz application/metastream
mzv application/metastream
nar application/zip
nbmp image/nbmp
nc application/x-netcdf
ndb x-lml/x-ndb
ndwn application/ndwn
nif application/x-nif
nmz application/x-scream
nokia-op-logo image/vnd.nok-oplogo-color
npx application/x-netfpx
nsnd audio/nsnd
nva application/x-neva1
oda application/oda
oom application/x-AtlasMate-Plugin
pac audio/x-pac
pae audio/x-epac
pan application/x-pan
pbm image/x-portable-bitmap
pcx image/x-pcx
pda image/x-pda
pdb chemical/x-pdb
pdf application/pdf
pfr application/font-tdpfr
pgm image/x-portable-graymap
pict image/x-pict
pm application/x-perl
pmd application/x-pmd
png image/png
pnm image/x-portable-anymap
pnz image/png
pot application/vnd.ms-powerpoint
ppm image/x-portable-pixmap
pps application/vnd.ms-powerpoint
ppt application/vnd.ms-powerpoint
pqf application/x-cprplayer
pqi application/cprplayer
prc application/x-prc
proxy application/x-ns-proxy-autoconfig
ps application/postscript
ptlk application/listenup
pub application/x-mspublisher
pvx video/x-pv-pvx
qcp audio/vnd.qcelp
qt video/quicktime
qti image/x-quicktime
qtif image/x-quicktime
r3t text/vnd.rn-realtext3d
ra audio/x-pn-realaudio
ram audio/x-pn-realaudio
rar application/x-rar-compressed
ras image/x-cmu-raster
rdf application/rdf+xml
rf image/vnd.rn-realflash
rgb image/x-rgb
rlf application/x-richlink
rm audio/x-pn-realaudio
rmf audio/x-rmf
rmm audio/x-pn-realaudio
rmvb audio/x-pn-realaudio
rnx application/vnd.rn-realplayer
roff application/x-troff
rp image/vnd.rn-realpix
rpm audio/x-pn-realaudio-plugin
rt text/vnd.rn-realtext
rte x-lml/x-gps
rtf application/rtf
rtg application/metastream
rtx text/richtext
rv video/vnd.rn-realvideo
rwc application/x-rogerwilco
s3m audio/x-mod
s3z audio/x-mod
sca application/x-supercard
scd application/x-msschedule
sdf application/e-score
sea application/x-stuffit
sgm text/x-sgml
sgml text/x-sgml
sh application/x-sh
shar application/x-shar
shtml magnus-internal/parsed-html
shw application/presentations
si6 image/si6
si7 image/vnd.stiwap.sis
si9 image/vnd.lgtwap.sis
sis application/vnd.symbian.install
sit application/x-stuffit
skd application/x-Koan
skm application/x-Koan
skp application/x-Koan
skt application/x-Koan
slc application/x-salsa
smd audio/x-smd
smi application/smil
smil application/smil
smp application/studiom
smz audio/x-smd
snd audio/basic
spc text/x-speech
spl application/futuresplash
spr application/x-sprite
sprite application/x-sprite
spt application/x-spt
src application/x-wais-source
stk application/hyperstudio
stm audio/x-mod
sv4cpio application/x-sv4cpio
sv4crc application/x-sv4crc
svf image/vnd
svg image/svg-xml
svh image/svh
svr x-world/x-svr
swf application/x-shockwave-flash
swfl application/x-shockwave-flash
t application/x-troff
tad application/octet-stream
talk text/x-speech
tar application/x-tar
taz application/x-tar
tbp application/x-timbuktu
tbt application/x-timbuktu
tcl application/x-tcl
tex application/x-tex
texi application/x-texinfo
texinfo application/x-texinfo
tgz application/x-tar
thm application/vnd.eri.thm
tif image/tiff
tiff image/tiff
tki application/x-tkined
tkined application/x-tkined
toc application/toc
toy image/toy
tr application/x-troff
trk x-lml/x-gps
trm application/x-msterminal
tsi audio/tsplayer
tsp application/dsptype
tsv text/tab-separated-values
tsv text/tab-separated-values
ttf application/octet-stream
ttz application/t-time
txt text/plain
ult audio/x-mod
ustar application/x-ustar
uu application/x-uuencode
uue application/x-uuencode
vcd application/x-cdlink
vcf text/x-vcard
vdo video/vdo
vib audio/vib
viv video/vivo
vivo video/vivo
vmd application/vocaltec-media-desc
vmf application/vocaltec-media-file
vmi application/x-dreamcast-vms-info
vms application/x-dreamcast-vms
vox audio/voxware
vqe audio/x-twinvq-plugin
vqf audio/x-twinvq
vql audio/x-twinvq
vre x-world/x-vream
vrml x-world/x-vrml
vrt x-world/x-vrt
vrw x-world/x-vream
vts workbook/formulaone
wav audio/x-wav
wax audio/x-ms-wax
wbmp image/vnd.wap.wbmp
web application/vnd.xara
wi image/wavelet
wis application/x-InstallShield
wm video/x-ms-wm
wma audio/x-ms-wma
wmd application/x-ms-wmd
wmf application/x-msmetafile
wml text/vnd.wap.wml
wmlc application/vnd.wap.wmlc
wmls text/vnd.wap.wmlscript
wmlsc application/vnd.wap.wmlscriptc
wmlscript text/vnd.wap.wmlscript
wmv audio/x-ms-wmv
wmx video/x-ms-wmx
wmz application/x-ms-wmz
wpng image/x-up-wpng
wpt x-lml/x-gps
wri application/x-mswrite
wrl x-world/x-vrml
wrz x-world/x-vrml
ws text/vnd.wap.wmlscript
wsc application/vnd.wap.wmlscriptc
wv video/wavelet
wvx video/x-ms-wvx
wxl application/x-wxl
x-gzip application/x-gzip
xar application/vnd.xara
xbm image/x-xbitmap
xdm application/x-xdma
xdma application/x-xdma
xdw application/vnd.fujixerox.docuworks
xht application/xhtml+xml
xhtm application/xhtml+xml
xhtml application/xhtml+xml
xla application/vnd.ms-excel
xlc application/vnd.ms-excel
xll application/x-excel
xlm application/vnd.ms-excel
xls application/vnd.ms-excel
xlt application/vnd.ms-excel
xlw application/vnd.ms-excel
xm audio/x-mod
xml text/xml
xmz audio/x-mod
xpi application/x-xpinstall
xpm image/x-xpixmap
xsit text/xml
xsl text/xml
xul text/xul
xwd image/x-xwindowdump
xyz chemical/x-pdb
yz1 application/x-yz1
z application/x-compress
zac application/x-zaurus-zac
zip application/zip
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源：http://hi.baidu.com/aidan_w/blog/item/ca9ad7bdb4b4260118d81f61.html
