How to put a simple text watermark on an image?

[GildedHonour]

How do I create and put a text watermark on an image in a simple way?

I'll have to use composite. But how presicely?

Also, no need to write a result to a disk.

[jcupitt]

Hello @GildedHonour,

ruby-vips has a watermark example:

https://github.com/libvips/ruby-vips/blob/master/example/watermark.rb

You can run it like this:

$ ./watermark.rb ~/pics/IMG_2603.jpg x.jpg "Hello <i>World</i>"

To make eg.:

I'll make a Lua version tomorrow, though it should be easy to translate.

[jcupitt]

I added a lua version of that watermark example here:

https://github.com/libvips/lua-vips/blob/master/example/watermark.lua

There's an array example too, showing how to make vips images into arrays of pixel values.

[GildedHonour]

Thanks

[GildedHonour]

Apart from that, but related:

I'm writing an nginx module.

Maybe you know if there's a way to send an output image -- with watermark -- to nginx without having to save it to as a file first? That is, I'd send im as a binary stream to nginx. Perhaps, by transforming im to binary stream first somehow.

https://github.com/libvips/lua-vips/blob/master/example/watermark.lua#L32

[GildedHonour]

Namely, I'd need something similar to this:

    local f = io.open("some_image.jpg", "rb")
    local content = f:read("*all")
    f:close()

   -- 'content' now contains binary data  which can be sent to nginx

I need to transform im from https://github.com/libvips/lua-vips/blob/master/example/watermark.lua#L32 to the same content data type, without having to save im on a disk

[jcupitt]

Yes, you can do eg.:

local vips = require "vips"

local f = io.open(arg[1], "rb")
local content = f:read("*all")

local im = vips.Image.new_from_buffer(content, "", {access = "sequential"})

-- brighten 20%
im = (im * 1.2):cast("uchar")

-- print as mime jpg
local buffer = im:write_to_buffer(".jpg", {Q = 90})
print("Content-length: " .. #buffer)
print("Content-type: image/jpeg")
print("")
print(buffer)

See:

https://github.com/libvips/lua-vips#image--vipsimagenew_from_bufferstring--string_options-options

https://github.com/libvips/lua-vips#string--imagewrite_to_buffersuffix--options

[jcupitt]

Kleis has made an image proxy with openresty, luajit, lua-vips, nginx etc., it might be a useful reference:

https://github.com/weserv/images

[GildedHonour]

Thanks

[jcupitt]

I added that to the examples/ dir.

I added access = "sequential" too, it give a nice speedup and drop in memuse.

[GildedHonour]

Is there a way to put a watermark if an image is binary data comming from nginx as a response?

request --> nginx --> retrieve image file, send binary data back --> Lua script is on the way, put watermark on it --> response to a client

A Lua script would receive image as binary data, from nginx, before it reaches a client, then put watermark on it -- on the fly -- and send it to a client. A client would receive it as a normal image/file with watermark.

[GildedHonour]

local img = vips.Image.new_from_NNNN(binary_data_from_nginx_as_response_to_client)

And one difficulty might be is that nginx sends data in chunks. I've read about this. In what occassions presicely - I don't know yet, I haven't dug into that.

[jcupitt]

Sure, though you'll need to make sure you have the complete image before you set the watermark script going.

Something like:

1. Request
2. nginx
3. Retrieve image file, post binary data back
4. Wait for all parts of binary image to arrive
5. Run watermark script to make new binary image
6. Post watermarked binary to client

[GildedHonour]

Would I use "new_from_memory()" or "new_from_buffer()" in the step 4 to create an image?

And if in a separate case I needed to request an image from internet using an HTTP client and also put a watermark on an image on the fly, which function would I use?

[jcupitt]

The _memory functions work with images stored as simple arrays in memory, so a big ffi buffer with RGBRGBRGBRGB etc in. Theuy are handy if you want to send a simple array on to another image processing library without having to decode and recode.

The _buffer functions work with big strings containing formatted jpg, tiff etc. images. They are more useful for something like a web proxy.

So for 3 and 5, use the _buffer variants.