image_filter module with ngx.location.capture problem

I'm trying to get image width and height information. I can do it directly by querying the proxied location with curl, but I can not do it from lua code with ngx.location.capture. it just returns the image without image_filter processing:

The config

load_module "modules/ngx_http_image_filter_module.so";
location /ttt/ {
    proxy_pass_request_headers off;
    proxy_pass https://scontent.xx.fbcdn.net/v/t1.0-1/c50.50.621.621/s50x50/601487_103890943140684_608897883_n.jpg?oh=04ba29ad280bf84ed80029ce8f384ff9&oe=59488366;
    image_filter size;
}

location /test3/ {
	content_by_lua_block {
		local url = "/ttt/"
		res = ngx.location.capture(url)
		ngx.say(res.status)
		ngx.say(res.body)
	}
}

Direct call: success

$ curl -i -X GET 'http://127.0.0.1:82/ttt/'
HTTP/1.1 200 OK
Server: openresty/1.11.2.2
Date: Sun, 07 May 2017 20:00:41 GMT
Content-Type: application/json
Content-Length: 59
Connection: keep-alive

{ "img" : { "width": 50, "height": 50, "type": "jpeg" } }

Indirect call: failure

$ curl -i -X GET 'http://127.0.0.1:82/test3/'
HTTP/1.1 200 OK
Server: openresty/1.11.2.2
Date: Sun, 07 May 2017 20:00:44 GMT
Content-Type: text/plain
Transfer-Encoding: chunked
Connection: keep-alive
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: POST, GET, OPTIONS
Access-Control-Allow-Headers: DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type

200
����JFIF


���Photoshop 3.08BIM
a lot of data there also ...

Can you please point me how to make it work from ngx.location.capture?

Start by comparing the /ttt/ location's direct image_filter response with the /test3/ content_by_lua_block call to ngx.location.capture. Trace how image_filter behaves for subrequests and define done as making the captured response expose the processed image metadata rather than the raw image body.