Add true streaming

This commit allows creating image sources from descriptor, file or memory
and image targets to descriptor, file or memory.

Custom classes are not implemented

Author: rolandlo

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to `lua-vips` will be documented in this file.
 
 # master
 
+- add `vips.Connection`, `vips.Source` and `vips.Target` for true streaming support [rolandlo]
+  
 # 1.1-11 - 2024-04-16
 
 - add standard Lua support [rolandlo]

README.md

@@ -547,6 +547,28 @@ print("written ", ffi.sizeof(mem), "bytes to", mem)
 
 An allocated char array pointer (GCd with a `ffi.gc` callback) and the length in bytes of the image data is directly returned from libvips (no intermediate FFI allocation).
 
+## True Streaming
+
+When processing images an image library would usually read an image from a file into memory, decode and process it and finally write the encoded result into a file. The processing can only start when the image is fully read into memory and the writing can only start when the processing step is completed.
+Libvips can process images directly from a pipe and write directly to a pipe, without the need to read the whole image to memory before being able to start and without the need to finish processing before being able to start writing. This is achieved using a technique called true streaming. In this context there are sources and targets and the processing step happens from source to target. Sources can be created from files, memory or descriptors (like stdin) and targets can be created to files, memory or descriptors (like stdout). Here is an example:
+
+```lua test.lua
+local vips = require "vips"
+local stdin, stdout = 0, 1
+local source = vips.Source.new_from_descriptor(stdin)
+local target = vips.Target.new_to_descriptor(stdout)
+local image = vips.Image.new_from_source(source, '', { access = 'sequential' })
+image = image:invert()
+image:write_to_target(target, '.jpg')
+```
+
+Running this script in a Unix terminal via 
+```term
+curl https://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/Cat03.jpg/600px-Cat03.jpg | lua test.lua > out.jpg
+```
+
+will feed a cat image from the internet into standard input, from which the Lua script reads and inverts it and writes it to standard output, where it is redirected to a file. This all happens simultaneously, so the processing and writing doesn't need to wait until the whole image is downloaded from the internet.
+
 ## Error handling
 
 Most `lua-vips` methods will call `error()` if they detect an error. Use

example/target.lua

@@ -0,0 +1,18 @@
+local vips = require "vips"
+
+if #arg ~= 2 then
+    error("Usage: lua target.lua ~/pics/k2.png .avif > x")
+end
+
+local infilename = arg[1]
+local fmt = arg[2]
+
+local descriptor = {
+    stdin = 0,
+    stdout = 1,
+    stderr = 2,
+}
+
+local image = vips.Image.new_from_file(infilename)
+local target = vips.Target.new_to_descriptor(descriptor.stdout)
+image:write_to_target(target, fmt)

lua-vips-1.1-11.rockspec

@@ -47,6 +47,9 @@ build = {
        ["vips.voperation"] = "src/vips/voperation.lua",
        ["vips.Image"] = "src/vips/Image.lua",
        ["vips.Image_methods"] = "src/vips/Image_methods.lua",
-       ["vips.Interpolate"] = "src/vips/Interpolate.lua"
+       ["vips.Interpolate"] = "src/vips/Interpolate.lua",
+       ["vips.Connection"] = "src/vips/Connection.lua",
+       ["vips.Source"] = "src/vips/Source.lua",
+       ["vips.Target"] = "src/vips/Target.lua",
    }
 }

spec/connection_spec.lua

@@ -0,0 +1,44 @@
+local vips = require "vips"
+local ffi = require "ffi"
+
+local JPEG_FILE = "./spec/images/Gugg_coloured.jpg"
+local TMP_FILE = ffi.os == "Windows" and os.getenv("TMP") .. "\\x.png" or "/tmp/x.png"
+
+describe("test connection", function()
+    setup(function()
+        -- vips.log.enable(true)
+    end)
+
+    describe("to file target", function()
+        local target
+
+        setup(function()
+            target = vips.Target.new_to_file(TMP_FILE)
+        end)
+
+        it("can create image from file source", function()
+            local source = vips.Source.new_from_file(JPEG_FILE)
+            local image = vips.Image.new_from_source(source, '', { access = 'sequential' })
+            image:write_to_target(target, '.png')
+
+            local image1 = vips.Image.new_from_file(JPEG_FILE, { access = 'sequential' })
+            local image2 = vips.Image.new_from_file(TMP_FILE, { access = 'sequential' })
+            assert.is_true((image1 - image2):abs():max() < 10)
+        end)
+
+        it("can create image from memory source", function()
+            local file = assert(io.open(JPEG_FILE, "rb"))
+            local content = file:read("*a")
+            file:close()
+            local mem = ffi.new("unsigned char[?]", #content)
+            ffi.copy(mem, content, #content)
+            local source = vips.Source.new_from_memory(mem)
+            local image = vips.Image.new_from_source(source, '', { access = 'sequential' })
+            image:write_to_target(target, '.png')
+
+            local image1 = vips.Image.new_from_file(JPEG_FILE, { access = 'sequential' })
+            local image2 = vips.Image.new_from_file(TMP_FILE, { access = 'sequential' })
+            assert.is_true((image1 - image2):abs():max() < 10)
+        end)
+    end)
+end)

src/vips.lua

@@ -23,6 +23,9 @@ local vips = {
     voperation = require "vips.voperation",
     Image = require "vips.Image_methods",
     Interpolate = require "vips.Interpolate",
+    Connection = require "vips.Connection",
+    Source = require "vips.Source",
+    Target = require "vips.Target",
 }
 
 function vips.leak_set(leak)

src/vips/Connection.lua

@@ -0,0 +1,55 @@
+-- abstract base Connection class
+
+local ffi = require "ffi"
+
+local vobject = require "vips.vobject"
+
+local vips_lib = ffi.load(ffi.os == "Windows" and "libvips-42.dll" or "vips")
+
+local Connection_method = {}
+
+local Connection = {
+    mt = {
+        __index = Connection_method,
+    }
+}
+
+function Connection.mt:__tostring()
+    return self:filename() or self:nick() or "(nil)"
+end
+
+Connection.new = function(vconnection)
+    local connection = {}
+    connection.vconnection = vobject.new(vconnection)
+    return setmetatable(connection, Connection.mt)
+end
+function Connection_method:vobject()
+    return ffi.cast(vobject.typeof, self.vconnection)
+end
+
+function Connection_method:filename()
+    -- Get the filename asscoiated with a connection. Return nil if there is no associated file.
+    local so = ffi.cast('VipsConnection *', self.vconnection)
+    local filename = vips_lib.vips_connection_filename(so)
+    if filename == ffi.NULL then
+        return nil
+    else
+        return ffi.string(filename)
+    end
+end
+
+function Connection_method:nick()
+    -- Make a human-readable name for a connection suitable for error messages.
+
+    local so = ffi.cast('VipsConnection *', self.vconnection)
+    local nick = vips_lib.vips_connection_nick(so)
+    if nick == ffi.NULL then
+        return nil
+    else
+        return ffi.string(nick)
+    end
+end
+
+return ffi.metatype("VipsConnection", {
+    __index = Connection
+})

src/vips/Image_methods.lua

@@ -218,6 +218,14 @@ function Image.new_from_image(base_image, value)
     return image
 end
 
+function Image.new_from_source(source, options, ...)
+    local name = vips_lib.vips_foreign_find_load_source(source.vconnection)
+    if name == ffi.NULL then
+        error("Unable to load from source")
+    end
+
+    return voperation.call(ffi.string(name), options, source.vconnection, unpack { ... })
+end
 -- overloads
 
 function Image.mt.__add(a, b)
@@ -413,6 +421,17 @@ function Image_method:write_to_memory_ptr()
     return ffi.gc(vips_memory, glib_lib.g_free), tonumber(psize[0])
 end
 
+function Image_method:write_to_target(target, format_string, ...)
+    collectgarbage("stop")
+    local options = to_string_copy(vips_lib.vips_filename_get_options(format_string))
+    local name = vips_lib.vips_foreign_find_save_target(format_string)
+    collectgarbage("restart")
+    if name == ffi.NULL then
+        error(verror.get())
+    end
+
+    return voperation.call(ffi.string(name), options, self, target.vconnection, unpack { ... })
+end
 -- get/set metadata
 
 function Image_method:get_typeof(name)

src/vips/Source.lua

@@ -0,0 +1,41 @@
+-- An output connection
+
+local ffi = require "ffi"
+
+local verror = require "vips.verror"
+local Connection = require "vips.Connection"
+
+local vips_lib = ffi.load(ffi.os == "Windows" and "libvips-42.dll" or "vips")
+
+local Source = {}
+
+Source.new_from_descriptor = function(descriptor)
+    local source = vips_lib.vips_source_new_from_descriptor(descriptor)
+    if source == ffi.NULL then
+        error("Can't create source from descriptor " .. descriptor .. "\n" .. verror.get())
+    end
+
+    return Connection.new(source)
+end
+
+Source.new_from_file = function(filename)
+    local source = vips_lib.vips_source_new_from_file(filename)
+    if source == ffi.NULL then
+        error("Can't create source from filename " .. filename .. "\n" .. verror.get())
+    end
+
+    return Connection.new(source)
+end
+
+Source.new_from_memory = function(data) -- data is an FFI memory array containing the image data
+    local source = vips_lib.vips_source_new_from_memory(data, ffi.sizeof(data))
+    if source == ffi.NULL then
+        error("Can't create input source from memory \n" .. verror.get())
+    end
+
+    return Connection.new(source)
+end
+
+return ffi.metatype("VipsSource", {
+    __index = Source
+})

src/vips/Target.lua

@@ -0,0 +1,46 @@
+-- An input connection
+
+local ffi = require "ffi"
+
+local Connection = require "vips.Connection"
+
+local vips_lib = ffi.load(ffi.os == "Windows" and "libvips-42.dll" or "vips")
+
+local Target = {}
+
+Target.new_to_descriptor = function(descriptor)
+    collectgarbage("stop")
+    local target = vips_lib.vips_target_new_to_descriptor(descriptor)
+    collectgarbage("restart")
+    if target == ffi.NULL then
+        error("can't create output target from descriptor " .. descriptor)
+    else
+        return Connection.new(target)
+    end
+end
+
+Target.new_to_file = function(filename)
+    collectgarbage("stop")
+    local target = vips_lib.vips_target_new_to_file(filename)
+    collectgarbage("restart")
+    if target == ffi.NULL then
+        error("can't create output target from filename " .. filename)
+    else
+        return Connection.new(target)
+    end
+end
+
+Target.new_to_memory = function()
+    collectgarbage("stop")
+    local target = vips_lib.vips_target_new_to_memory()
+    collectgarbage("restart")
+    if target == ffi.NULL then
+        error("can't create output target from memory")
+    else
+        return Connection.new(target)
+    end
+end
+
+return ffi.metatype("VipsTarget", {
+    __index = Target
+})