# SPDX-License-Identifier: BSD-3-Clause
# Copyright (c) Contributors to the OpenEXR Project.

The OpenEXRUtil Library
----------------------

The OpenEXRUtil library implements an in-memory image data structure, as
well as simple function calls for saving images in OpenEXR files, and for
constructing images from the contents of existing OpenEXR files.

The OpenEXR file format has a fairly large number of options for on-file
image storage, including arbitrary sets of channels, per-channel pixel
format selection, sub-sampled channels, multi-resolution images, deep
images, or storing images as tiles or scan lines.  While reading a simple
RGBA image does not require a lot of code, reading the contents of an
arbitrary OpenEXR file, and representing those contents in main memory
is not trivial.  The OpenEXRUtil library simplifies those tasks.

Image, Image Level, Image Channel
---------------------------------

An image (class Image) is a container for a set of image levels (class
ImageLevel), and an image level is a container for a set of image channels
(class ImageChannel).  An image channel contains an array of pixel values.

For example:

    image --+-- level 0,0 --+-- channel "R" --- pixel data
            |               |
            |               +-- channel "G" --- pixel data
            |               |
            |               +-- channel "B" --- pixel data
            |
            +-- level 1,1 --+-- channel "R" --- pixel data
            |               |
            |               +-- channel "G" --- pixel data
            |               |
            |               +-- channel "B" --- pixel data
            |
            +-- level 2,2 --+-- channel "R" --- pixel data
                            |
                            +-- channel "G" --- pixel data
                            |
                            +-- channel "B" --- pixel data

An image has a level mode (enum LevelMode), which can be ONE_LEVEL,
MIPMAP_LEVELS or RIPMAP_LEVELS.  A ONE_LEVEL image contains only a single
level, but a multi-resolution image, that is, one with level mode set to
MIPMAP_LEVELS or RIPMAP_LEVELS, contains multiple levels.  The levels are
analogous to the levels in an OpenEXR file, as described in the "Technical
Introduction to OpenEXR" document.

Levels are indexed by a pairs of level numbers.  Level (0,0) contains the
highest-resolution version of the image; level (lx,ly) contains an image
whose resolution is reduced in x and y by a factor of 2^lx and 2^ly
respectively.  The level has a data window that indicates the range of
x and y for which pixel data are stored in the level.

All levels in an image have the same set of image channels.

An image channel has a name (e.g. "R", "Z", or "xVelocity"), a type (HALF,
FLOAT or UINT) and x and y sampling rates.  A channel stores samples for
a pixel if the pixel is inside the data window of the level to which the
channel belongs, and the x and y coordinates of the pixel are divisible by
the x and y sampling rates of the channel.

An image can be either flat or deep.  In a flat image each channel in each
level stores at most one value per pixel.  In a deep image each channel in
each level stores an arbitrary number of values per pixel.  As an exception,
each level of a deep image has a sample count channel with a single value
per pixel; this value determines how many values each of the other channels
in the same level has at the same pixel location.

The Image, ImageLevel and ImageChannel classes are abstract base classes.
Two sets of classes, one for flat images and one for deep images, are
derived from the base classes.  The FlatImageChannel and DeepImageChannel
classes, derived from ImageChannel, are themselves base classes for the
templates TypedFlatImageChannel<T> and TypedDeepImageChannel<T>:

    Image -> FlatImage
          -> DeepImage

    ImageLevel -> FlatImageLevel
               -> DeepImageLevel


    ImageChannel -> FlatImageChannel -> TypedFlatImageChannel<T>
                 -> DeepImageChannel -> TypedDeepImageChannel<T>
                 -> SampleCountChannel

Channel objects of type TypedFlatImageChannel<T> and TypedDeepImageChannel<T> 
contain pixel values of type T, where T is either half, float or unsigned int.
For convenience, the following typedefs are provided:

    typedef TypedFlatImageChannel<half>         FlatHalfChannel;
    typedef TypedFlatImageChannel<float>        FlatFloatChannel;
    typedef TypedFlatImageChannel<unsigned int> FlatUIntChannel;

    typedef TypedDeepImageChannel<half>         DeepHalfChannel;
    typedef TypedDeepImageChannel<float>        DeepFloatChannel;
    typedef TypedDeepImageChannel<unsigned int> DeepUIntChannel;


File I/O
--------

An Image object can be saved in an OpenEXR file with a single function call:

    saveImage ("foo.exr", myImage);

The saveImage() function automatically creates a flat or a deep image file,
depending on the type of the image.  All channels and all image levels will
be saved in the file.

Optionally an OpenEXR Header object can be passed to the saveImage() function;
this allows application code save custom attributes in the file, and to control
how the file will be compressed:

    Header myHeader;
    myHeader.compression() = PIZ_COMPRESSION;
    myHeader.pixelAspectRatio() = 1.5;

    saveImage ("foo.exr", myHeader, myImage);

Loading an image from an OpenEXR file also requires only one function call,
either

    Image* myImage = loadImage ("foo.exr");

or

    Header myHeader;
    Image* myImage = loadImage ("foo.exr", myHeader);

The application owns the image that is returned by the loadImage() call.
It is the application's responsibility to delete the Image object.

The OpenEXRUtil library also provides versions of the saveImage() and
loadImage() functions that work only on flat images or only on deep images:

    saveFlatImage()
    saveFlatScanLineImage()
    saveFlatTiledImage()
    saveDeepImage()
    saveDeepScanLineImage()
    saveDeepTiledImage()

For details, refer to the ImfFlatImageIO.h and ImfDeepImageIO.h header files.


Manipulating Images in Memory
-----------------------------

Creating a mip-mapped flat image with two channels:

    FlatImage fimg (Box2i (V2i (0, 0), V2i (255, 255)),    // data window
                    MIPMAP_LEVELS);                        // level mode

    fimg.insertChannel ("R", HALF);
    fimg.insertChannel ("Z", FLOAT);

Creating a single-level deep image:

    DeepImage dimg (Box2i (V2i (0, 0), V2i (255, 255)),    // data window
                    ONE_LEVEL);                            // level mode

    dimg.insertChannel ("R", HALF);
    dimg.insertChannel ("Z", FLOAT);

Reading and writing pixels in level (2,2) of the mip-mapped flat image
(note: a mip-mapped image contains only levels where the x and y level
numbers are equal.  For convenience, mip-map levels can be addressed
using a single level number):

    FlatImageLevel &level = fimg.level (2);
    FlatHalfChannel &R = level.typedChannel<half> ("R);

    half r1 = R.at (20, 15);    // read pixel (20,15), with bounds checking
                                // (exception for access outside data window)

    half r2 = R (17, 4);        // read pixel (17,4) without bounds checking
                                // faster, but crashes for access outside
                                // data window

    R.at (20, 15) = 2 * r1;     // change pixel value, with and
    R (17, 4) = 2 * r2;         // without bounds checking

Reading and writing pixels in the single-level deep image:

    DeepImageLevel &level = dimg.level();
    DeepHalfChannel &R = level.typedChannel<half> ("R);

    // with bounds checking

    unsigned int n1 = R.sampleCounts().at (20, 15);
    half r1;

    if (n1 > 0)
        r1 = R.at(20, 15)[n1 - 1];  // read the last sample in pixel (20,15)

    // without bounds checking

    unsigned int n2 = R.sampleCounts()(20, 15);
    half r2;

    if (n > 0)
        r2 = R(17, 4)[n2 - 1];     // read the last sample in pixel (17,4)

    // change the value of an existing sample

    if (n1 > 0)
        R(20,15)[n1 - 1] = r1 * 2;

    // append a new sample to a pixel and set the sample to 3.0

    R.sampleCounts().set (20, 15, n1 + 1);
    R.(20, 15)[n1] = 3.0;

In addition to functions for reading and writing individual pixels, there
are functions for accessing a whole row of pixels with a single function
call.  For details see the ImfFlatImageChannel.h, ImfDeepImageChannel.h
and ImfSampleCountChannel.h header files in the OpenEXR library:

    T*                   TypedFlatImageChannel<T>::row (int r);
    const T*             TypedFlatImageChannel<T>::row (int r) const;

    T * const *          TypedDeepImageChannel<T>::row (int r);
    const T * const *    TypedDeepImageChannel<T>::row (int r) const;
    const unsigned int * SampleCountChannel::row (int r) const;

To change the number of samples in all pixels in one row of a deep image
level, use:

    void SampleCountChannel::set (int r, unsigned int newNumSamples[]);

Use an Edit object to temporarily make all sample counts in a deep image
level editable:

    class SampleCountChannel::Edit;

Miscellaneous Functions:
------------------------

Change the data window and the level mode of an image (pixel data are not
preserved across the call):

    void Image::resize (const Box2i &dataWindow, LevelMode levelMode);

Shift the data window in x and y; shift the pixels along with the data window:

    void Image::shiftPixels (int dx, int dy);

Erase a channel, rename a channel, rename multiple channels at the same time:

    void Image::eraseChannel (const string &name);
    void Image::renameChannel (const string &oldName, const string &newName);
    void Image::renameChannels (const RenamingMap &oldToNewNames);

Missing Functionality:
----------------------

At this point, the OpenEXRUtil library cannot read or write multi-part
files.  A future version of the library should probably define a new class
MultiPartImage that contains a set of regular images.  The library should
also define corresponding loadMultiPartImage() and saveMultiPartImage()
functions.

Sample Code
-----------

See the exrsave, exrmakescanlines, exrclip utilities.


