tagline
WEBP encoding & decoding

Required Library

libwebp

Note: After build library to "so" files, copy them to /usr/lib

local libwebp = require("resty.libwebp.webp")

A ffi binding for the libwebp with processing Webp image

API

---

• libwebp.decompress.[opt] set decode option, this option must be set before run load function
• libwebp.load_blob(blob) -> img: open a WEBP image from blob binary data for decoding
• libwebp.load_from_disk(<string>infile) -> img:open a WEBP image from file on disk for decoding
• img.compress.[opt]: set/read option for compress process (must set before run get_blob() for save() function)
• img:get_blob(): get WEBP image to binary string after compress
• img:save(): save WEBP image to disk (must set img.compress.outfile)

---

libwebp.decompress.[opt]

Set settings for decompress process. opt are some options as follow:

• bypass_filtering<boolean>: if true, skip the in-loop filtering
• no_fancy_upsampling<boolean>: if true, use faster pointwise upsampler
• scaled_width<int>: scale image to width pixel (keep aspect ratio)
• scaled_height<int>: scale image to height pixel (keep aspect ratio)
• use_threads<boolean>: if true, use multi-threaded decoding
• dithering_strength<int>: 0..100 range. = 0=off, 100=full
• alpha_dithering_strength<int>: 0..100 range. Alpha dithering strength
• format: RGB, RGBA, BRG, BRGA

---

libwebp.load_blob(blob) -> img

libwebp.load_from_disk(infile) -> img

Open a PNG image and read its header. blob is whole image binary string or infile is path to image file on disk

The return value is an image object which gives information about the file and can be used to load and decode the actual pixels. It has the fields:

• w, h: width and height of the image.

---

img.compress.[opt]

Set settings for compress process. opt are some options as follow:

• lossless<int> (default 0): Lossless encoding (0=lossy(default), 1=lossless).

• quality<float> (default 75): between 0 and 100. For lossy, 0 gives the smallest size and 100 the largest. For lossless, this parameter is the amount of effort put into the compression: 0 is the fastest but gives larger

• method<int> (default 4): quality/speed trade-off (0=fast, 6=slower-better)

• target_size<int> (default 0): if non-zero, set the desired target size in bytes. Takes precedence over the 'compression' parameter.

• target_PSNR<float> (default 0): if non-zero, specifies the minimal distortion to try to achieve. Takes precedence over target_size.

• segments<int> (default 4): maximum number of segments to use, in [1..4]

• sns_strength<int> (default 50): Spatial Noise Shaping. 0=off, 100=maximum.

• filter_strength<int> (default 60): range: [0 = off .. 100 = strongest]

• filter_sharpness<int> (default 0): range: [0 = off .. 7 = least sharp]

• filter_type<int> (default 1): filtering type: 0 = simple, 1 = strong (only used if filter_strength > 0 or autofilter > 0)

• autofilter<int> (default 0): Auto adjust filter's strength [0 = off, 1 = on]

• alpha_compression<int> (default 1): Algorithm for encoding the alpha plane (0 = none, 1 = compressed with WebP lossless). Default is 1.

• alpha_filtering<int> (default 1): Predictive filtering method for alpha plane. 0: none, 1: fast, 2: best. Default if 1.

• alpha_quality<int> (default 100): Between 0 (smallest size) and 100 (lossless). Default is 100.

• pass<int> (default 1): number of entropy-analysis passes (in [1..10]).

• show_compressed<int> (default 0): if true, export the compressed picture back. In-loop filtering is not applied.

• preprocessing<int> (default 0): preprocessing filter: 0=none, 1=segment-smooth, 2=pseudo-random dithering

• partitions<int> (default 0): log2(number of token partitions) in [0..3]. Default is set to 0 for easier progressive decoding.

• partition_limit<int> (default 0): quality degradation allowed to fit the 512k limit on prediction modes coding (0: no degradation, 100: maximum possible degradation).

• emulate_jpeg_size<boolean> (default false): If true, compression parameters will be remapped to better match the expected output size from JPEG compression. Generally, the output size will be similar but the degradation will be lower.

• thread_level<int> (default 1): If non-zero, try and use multi-threaded encoding.

• low_memory<int> (default 0): If set, reduce memory usage (but increase CPU use).

• near_lossless<int> (default 100): Near lossless encoding [0 = max loss .. 100 = off (default)].

• exact<int> (default 0): if non-zero, preserve the exact RGB values under transparent area. Otherwise, discard this invisible RGB information for better compression. The default value is 0.

• use_delta_palette<int> (default 0): reserved for future lossless feature

• use_sharp_yuv<int> (default 0): if needed, use sharp (and slow) RGB->YUV conversion

• outfile<string> (default nil): path to file on disk to save.

---

img:get_blob() -> return <string> binary data

Get image data in binary string after compress process

---

img:save()

Save image to disk base on img.compress.outfile setting. Must use before get_blob()

---

Sample Code

Nginx Configuration

server {
    listen 80;
    location = /favicon.ico {
        empty_gif;
    }
    location ~ /proxy(.*) {
        ## can use root or proxy_pass to get data from local or remote site
        # proxy_pass https://<origin>$1;
        root /dev/shm;
    }
    location / {
        content_by_lua_file resty-libwebp-sample.lua;
    }
}

---

resty-libwebp-sample.lua

local libwebp = require("resty.libwebp.webp") -- load library
-- set decompress options
libwebp.decompress.scaled_width = 1024
libwebp.decompress.colorspace = "RGBA"
-- get data direct from nginx
local res = ngx.location.capture('/proxy'..ngx.var.request_uri) -- get data from nginx location /proxy by subrequest 
local img = libwebp.load_blob(res.body) -- create object img
-- get data from disk
-- local img = libwebp.load_from_disk('/dev/shm/proxy/inputhd.webp') -- create object img
local outfile = '/dev/shm/proxy/inputhd_new.webp' -- declare outfile path
img.compress.outfile = outfile -- set outfile setting
img.compress.quality = 50 -- set quality
img.compress.method = 9 -- set speed to run
img:save() -- save file to disk
ngx.print(img:get_blob()) -- return image after compress to end user