如何在lua中获取webp图像EXIF元数据？

问题描述

我可以使用以下代码获取此数据。但是运行速度太慢：

local handle = io.popen("exiftool image.webp")
local result = handle:read("*a")
handle:close()

有没有更优雅的方式来获取元数据？

更新：

我使用这个软件：

• 码头工人 (20.10.7)
• openresty/openresty:xenial (1.15.8.3)
• luarocks (3.2.1)
• LuaJIT (2.1.0-beta3)

以下是带有 UserComment 字段的图片示例：link

Exiftool 看到这个属性：

$ exiftool -EXIF:UserComment Johnrogershousemay2020.webp  
User Comment                    : {"foo":"bar"}  

解决方法

这种方法不太优雅，但不需要运行外部应用程序:-)

function get_webp_user_comment(file_name)
   local file = io.open(file_name,"rb")
   local exif_offset,exif_found,is_big_endian,user_comment = 12

   local function read_string(offset,size)
      file:seek("set",offset)
      return file:read(size)
   end

   local function read_uint(offset,size)
      local n,s = 0,read_string(offset,size)
      for j = 1,size do
         n = n * 256 + s:byte(is_big_endian and j or size + 1 - j)
      end
      return n
   end

   local function read_uint32(disp)
      return read_uint(exif_offset + disp,4)
   end

   local function read_uint16(disp)
      return read_uint(exif_offset + disp,2)
   end

   local function search_for_tag(ifd_disp,tag)
      if ifd_disp ~= 0 then
         local entry_disp = ifd_disp + 2
         for j = 1,read_uint16(ifd_disp) do
            if read_uint16(entry_disp) == tag then
               return read_uint32(entry_disp + 8),read_uint32(entry_disp + 4)
            end
            entry_disp = entry_disp + 12
         end
         return search_for_tag(read_uint32(entry_disp),tag)
      end
   end

   if read_string(0,4) == "RIFF" and read_string(8,4) == "WEBP" then
      local max_offset = read_uint32(-8)
      while exif_offset < max_offset do
         local section_name = read_string(exif_offset,4)
         exif_offset = exif_offset + 8
         if section_name == "EXIF" then
            local endianness = read_string(exif_offset,2)
            is_big_endian = endianness == "MM"
            exif_found = is_big_endian or endianness == "II"
            if exif_found then
               break
            end
         end
         exif_offset = exif_offset + read_uint32(-4)
         exif_offset = exif_offset + exif_offset % 2
      end
   end
   if exif_found then
      local exif_ifd = search_for_tag(read_uint32(4),0x8769)
      if exif_ifd then
         local disp,count = search_for_tag(exif_ifd,0x9286)
         user_comment = read_string(exif_offset + disp + 8,count - 8)
      end
   end
   file:close()
   return user_comment
end

用法示例：

local file_name = "path/to/Johnrogershousemay2020.webp"
print(get_webp_user_comment(file_name))

,

如果您只需要“UserComment”，则在调用 popen 时将其作为参数传递：

local handle = io.popen("exiftool -EXIF:UserComment image.webp")