259 lines
9.4 KiB
Markdown
259 lines
9.4 KiB
Markdown
### Introduction
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This API is a thing layer over the libvips GObject API. it adds automatic
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reference counting, exceptions, operator overloads, and automatic constant
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expansion.
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You can drop down to the C API at any point, so all the C API docs also
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work for C++.
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### Example
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/* compile with:
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* g++ -g -Wall example.cc `pkg-config vips-cpp --cflags --libs`
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*/
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#include <vips/vips8>
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using namespace vips;
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int
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main (int argc, char **argv)
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{
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if (VIPS_INIT (argv[0]))
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vips_error_exit (NULL);
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if (argc != 3)
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vips_error_exit ("usage: %s input-file output-file", argv[0]);
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VImage in = VImage::new_from_file (argv[1],
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VImage::option ()->set ("access", VIPS_ACCESS_SEQUENTIAL));
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double avg = in.avg ();
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printf ("avg = %g\n", avg);
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printf ("width = %d\n", in.width ());
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in = VImage::new_from_file (argv[1],
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VImage::option ()->set ("access", VIPS_ACCESS_SEQUENTIAL));
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VImage out = in.embed (10, 10, 1000, 1000,
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VImage::option ()->
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set ("extend", "background")->
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set ("background", 128));
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out.write_to_file (argv[2]);
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vips_shutdown ();
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return 0;
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}
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Everything before `VImage in = VImage::new_from_file()` is exactly as the C
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API. `vips_error_exit()` just prints the arguments plus the libvips error
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log and exits with an error code.
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`VImage::new_from_file()` is the C++ equivalent of
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`vips_image_new_from_file()`. It works in the same way, the differences being:
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- VImage lifetime is managed automatically, like a smart pointer. You don't
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need to call `g_object_unref()`.
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- Instead of using varargs and a `NULL`-terminated option list, this
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function takes an optional `VOption` pointer. This gives a list of name /
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value pairs for optional arguments to the function.
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In this case we request unbuffered IO for the image, meaning, we expect
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to do a single top-to-bottom scan of the image and do not need it to be
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decompressed entirely. You can use the C enum name, as is done in this
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case, or use a string and have the string looked up. See below.
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The function will delete the `VOption` pointer for us when
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it's finished with it.
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- Instead of returning `NULL` on error, this constructor will raise a `VError`
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exception.
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There are a series of similar constructors which parallel the
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other constructors in the C API, see `VImage::new_from_memory()`,
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`VImage::new_from_buffer()`, and `VImage::new_matrix()`.
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The convenience function `VImage::new_from_image()` makes a constant image
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from an existing image. The image it returns will have the same size,
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interpretation, resolution and format as the image you call it on, but with
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every pixel having the constant value you specify. For example:
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new_image = image.new_from_image (12);
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Now `new_image` has the same size as `image`, but has one band, and every
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pixel has the value 12. You can pass a `std::vector<double>` as the
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argument to make a constant image with a different number of bands.
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There's also `VImage::new_memory()` and `VImage::new_temp_file()`, which when
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written to with `VImage::write()` will create whole images on memory or on disc.
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The next line finds the average pixel value, it's the equivalent of the
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`vips_avg()` function. The differences from the C API are:
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- `VImage::avg()` is a member function: the `this`
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parameter is the first (the only, in this case) input image.
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- The function returns the first output parameter, in this case the
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average pixel value. Other return values are via pointer arguments,
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as in the C API.
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- Like `VImage::new_from_file()`, function raises the `VError`
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exception on error.
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- Like `VImage::new_from_file()`, extra arguments are passed
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via an optional `VOption` parameter. There are none
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in this case, so the function brackets can be left empty.
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All other operations follow the same pattern, for example the C API call
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`vips_add(`):
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int vips_add (VipsImage *left, VipsImage *right, VipsImage **out, ...);
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appears in C++ as:
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VImage VImage::add (VImage right, VOption *options) const
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The next line uses `VImage::width()` to get the image width in pixels.
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There are similar functions paralleling `vips_image_get_format()` and
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friends. Use `VImage::set()` to set metadata fields, `VImage::get_int()` and
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c. to fetch metadata.
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Next we reload the image. The `VImage::avg()` will have scanned the image
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and reached the end of the file, we need to scan again for the next
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operation. If we'd selected random access mode (the default) in the
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original `VImage::new_from_file()`, we would not need to reload.
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The next line runs `vips_embed()` with two optional parameters. The first
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sets the value to an enum (here we use a string to set the value, it'll be
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looked up in the list of possible enum values, or you can use the symbols
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from the C API), the second sets the value to an `int`. The `"background"`
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parameter is actually a `VipsArrayDouble`: if you pass an `int` instead,
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it will be automatically converted to a one-element array for you. You can
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pass a `std::vector<double>` too: the utility function `VImage::to_vectorv()`
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is a convenient way to make one.
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Finally, `VImage::write_to_file()` will write the new image to the
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filesystem. You can add a `VOption` as a final parameter and set options for
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the writer if you wish. Again, the operation will throw a `VError` exception
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on error. The other writers from the C API are also present: you can write
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to a memory array, to a formatted image in memory, or to another image.
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The API docs have a [handy table of all vips
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operations](libvips/API/current/func-list.html), if you want to find out
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how to do something, try searching that.
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### Automatic constant expansion
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The C++ API will automatically turn constants into images in some cases.
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For example, you can join two images together bandwise (the
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bandwise join of two RGB images would be a six-band image) with:
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VImage rgb = ...;
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VImage six_band = rgb.bandjoin (rgb);
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You can also bandjoin a constant, for example:
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VImage rgb_with_alpha = rgb.bandjoin (255);
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Will add an extra band to an image, with every element in the new band having
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the value 255. This is quite a general feature. You can use a constant in
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most places where you can use an image and it will be converted. For example:
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VImage a = (a < 128).ifthenelse (128, a);
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Will set every band element of `a` less than 128 to 128.
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The C++ API includes the usual range of arithmetic operator overloads.
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You can mix constants, vectors and images freely.
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The API overloads `[]` to be `vips_extract_band()`. You can
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write:
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VImage xyz = VImage::xyz (256, 256) - VImage::to_vectorv (2, 128.0, 128.0);
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VImage mask = (xyz[0].pow (2) + xyz[1].pow (2)).pow (0.5) < 100;
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to make a circular mask, for example.
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The API overloads `()` to be `vips_getpoint()`. You can write:
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VImage xyz = VImage::xyz (256, 256) - VImage::to_vectorv (2, 128.0, 128.0);
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// this will have the value [0, 0]
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std::vector<double> point = xyz (128, 128);
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### Enum expansion
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libvips operations which implement several functions with a controlling
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enum, such as `vips_math()`, are expanded to a set of member functions
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named after the enum. For example, the C function:
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int vips_math (VipsImage *in, VipsImage **out, VipsOperationMath math, ...);
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where `VipsOperationMath` has the member `VIPS_OPERATION_MATH_SIN`, has a
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C convenience function `vips_sin()`:
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int vips_sin (VipsImage *in, VipsImage **out, ...);
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and a C++ member function `VImage::sin()`:
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VImage VImage::sin (VOption *options = 0) const
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### Image metadata
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libvips images can have a lot of metadata attached to them, giving things like
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ICC profiles, EXIF data, and so on. You can use the command-line program
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`vipsheader` with the `-a` flag to list all the fields.
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You can read metadata items with the member functions `get_int()`,
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`get_double()`, `get_string()` and `get_blob()`. Use `get_typeof()` to call
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`vips_image_get_typeof()` and read the type of an item. This will return 0
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for undefined fields.
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const char *VImage::get_string (const char *field);
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You can use the `set()` family of overloaded members to set metadata,
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for example:
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void VImage::set (const char *field, const char *value);
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You can use these functions to manipulate exif metadata, for example:
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VImage im = VImage::new_from_file ("x.jpg")
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int orientation = im.get_int (VIPS_META_ORIENTATION);
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im.set (VIPS_META_ORIENTATION, 2);
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im.write_to_file ("y.jpg");
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### Extending the C++ interface
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The C++ interface comes in two parts. First, `VImage8.h` defines a simple
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layer over `GObject` for automatic reference counting, then a generic way
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to call any vips8 operation with `VImage::call()`, then a few convenience
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functions, then a set of overloads.
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The member definition and declaration for each operation, for
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example `VImage::add()`, is generated by a small Python program called
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`gen-operators.py`. If you write a new libvips operator, you'll need to rerun
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this program to make it visible in the C++ interface.
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You can write the wrapper yourself, of course, they are very simple.
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The one for `VImage::add()` looks like this:
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VImage VImage::add (VImage right, VOption *options) const
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{
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VImage out;
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call("add",
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(options ? options : VImage::option())->
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set("out", &out)->
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set("left", *this)->
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set("right", right));
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return out;
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}
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Where `VImage::call()` is the generic call-a-vips8-operation function.
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