How to get webp image EXIF metadata in lua? - lua

Question

I can get this data with the following code. But it runs too slow:
local handle = io.popen("exiftool image.webp")
local result = handle:read("*a")
handle:close()
Is there a more elegant way to get the metadata?
UPD:
I use this software:
docker (20.10.7)
openresty/openresty:xenial (1.15.8.3)
luarocks (3.2.1)
LuaJIT (2.1.0-beta3)
Here is an example of a picture with a UserComment field: link
Exiftool sees this property:
$ exiftool -EXIF:UserComment Johnrogershousemay2020.webp
User Comment : {"foo":"bar"}

Answer 1

This method is less elegant, but it does not require to run external application :-)
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
Usage example:
local file_name = "path/to/Johnrogershousemay2020.webp"
print(get_webp_user_comment(file_name))

Answer 2

If all you need is 'UserComment', then pass that as a parameter during your popen call:
local handle = io.popen("exiftool -EXIF:UserComment image.webp")