background

when build web applications, we always want to create many thumnails. not using the origin images.

solution

using the imagemagic’s “convert” command to create thumbnail

convert imput -thumbnail 80x80 -gravity center -extent 80x80 output

we can also using this command to get input image from stdin, and output thumbnail to stdout. so we can using LUA function “io.popen” to execute the convert command, and output the thumbnail to the response.

local fp = assert(io.popen("convert " .. ngx.var.image ..
  " -thumbnail 80x80 -gravity center -extent 80x80 - 2>/dev/null"), 
  "convert error, can not open pipe.")
local content = fp:read("*all")
fp:close()
ngx.print(content)

my image data was stored into mysql, but the function “io.popen” can not open one pipe, to read and write data. so we need another method to get the image data from mysql. mysql -e option can exec the SQL command. mysql –raw option, just write the resultset to output. mysql –skip-column-names option, do not display the column names. so using this options, can just write the blob field to stdout. and then pipe it to convert command.

local fp = assert(io.popen("mysql -h$OPENSHIFT_MYSQL_DB_HOST -P$OPENSHIFT_MYSQL_DB_PORT "..
  "-u$OPENSHIFT_MYSQL_DB_USERNAME -p$OPENSHIFT_MYSQL_DB_PASSWORD -D api "..
   "--disable-pager -e \'".. ngx.var.sql .."\' --raw --skip-column-names | convert - " ..
   " -thumbnail 80x80 -gravity center -extent 80x80 - 2>/dev/null"), 
   "convert error, can not open pipe.")
local content = fp:read("*all")
fp:close()
ngx.print(content)

perfomance of this solution was not good, so we need cache for this request, “https://github.com/openresty/srcache-nginx-module” is a good choice.