Reading Photoshop Color Swatch (aco) files using C#

In a previous article I described how to read the colour map from a DeluxePaint LBM/BBM file. In the next pair of articles, I'm going to describe how to load and save colour swatch files used by Photoshop (those with the .aco extension).

Caveat Emptor

As usual, I'll start with a warning. I have a very limited set of sample files to test with, so it may be that there's an error in this code which means it can't handle all files. Certainly it can't handle all colour spaces (more on that later). However, I've tested it on a number of files download from the internet without problems.

Structure of a Photoshop colour swatch file

The structure of the aco file is straightforward, helped by Adobe themselves publishing the specification which is something to appreciate. This article was created using the October 2013 edition of this specification.

According to the specification, there's two versions of the format both of which are are fairly similar. The specification also implies that applications which support version 2 should write a version 1 palette first, which would admirably solve backwards compatibility problems. In practice this doesn't seem to be the case, as some of the files I tested only had version 2 palettes in them.

The structure is simple. There's a 2-byte version code, followed by 2-bytes describing the number of colours. Then, for each colour, there are 10 further bytes, 2 each describing the colour space and then four values to describe the colour. Version two palettes also then follow this with a four byte integer describing the length of the name, then the bytes which make up said name.

All the data in an aco file is stored in big-endian format and therefore needs to be reversed on Windows systems.

Most colour spaces only use three of the four available values, but regardless of how many are actually used, all must be specified.

Colour Spaces

I mentioned above that each colour has a description of what colour space it belongs to. The specification defines the following colour spaces:

To avoid complicating matters, this article will concentrate on RGB and Grayscale colour spaces, although I'll include the basics of HSV too for if you have a conversion class kicking around.

Reading short/int data types from bytes

As I mentioned above, the values in this file format are all big-endian. As Windows uses little-endian, we need to do some bit shifting when we read each byte comprising either a short (Int16) or an int (Int32), using the following helpers:

/// <summary>
/// Reads a 16bit unsigned integer in big-endian format.
/// </summary>
/// <param name="stream">The stream to read the data from.</param>
/// <returns>The unsigned 16bit integer cast to an <c>Int32</c>.</returns>
private int ReadInt16(Stream stream)
{
  return (stream.ReadByte() << 8) | (stream.ReadByte() << 0);
}

/// <summary>
/// Reads a 32bit unsigned integer in big-endian format.
/// </summary>
/// <param name="stream">The stream to read the data from.</param>
/// <returns>The unsigned 32bit integer cast to an <c>Int32</c>.</returns>
private int ReadInt32(Stream stream)
{
  return ((byte)stream.ReadByte() << 24) | ((byte)stream.ReadByte() << 16) | ((byte)stream.ReadByte() << 8) | ((byte)stream.ReadByte() << 0);
}

The << 0 bit-shift in the above methods is technically unnecessary and can be removed. However, I find it makes the intent of the code clearer.

Reading strings

For version 2 files, we need to read a string, which is comprised of two bytes per character. Fortunately for us, the .NET Framework includes a BigEndianUnicode (MSDN) class that we can use to convert a byte array to a string. As this class does the endian conversion for us, we don't need to do anything special when reading the bytes.

/// <summary>
/// Reads a unicode string of the specified length.
/// </summary>
/// <param name="stream">The stream to read the data from.</param>
/// <param name="length">The number of characters in the string.</param>
/// <returns>The string read from the stream.</returns>
private string ReadString(Stream stream, int length)
{
  byte[] buffer;

  buffer = new byte[length * 2];

  stream.Read(buffer, 0, buffer.Length);

  return Encoding.BigEndianUnicode.GetString(buffer);
}

Reading the file

With the preliminaries done with, lets read the file!

We start off by reading the file version so we know how to process the rest of the file, or at least the first part of it. If we don't have a version 1 or version 2 file, then we simply abort.

using (Stream stream = File.OpenRead(fileName))
{
  FileVersion version;

  // read the version, which occupies two bytes
  version = (FileVersion)this.ReadInt16(stream);

  if (version != FileVersion.Version1 && version != FileVersion.Version2)
    throw new InvalidDataException("Invalid version information.");

  colorPalette = this.ReadSwatches(stream, version);
  if (version == FileVersion.Version1)
  {
    version = (FileVersion)this.ReadInt16(stream);
    if (version == FileVersion.Version2)
      colorPalette = this.ReadSwatches(stream, version);
  }
}

In the above example, if a file has both versions, then I read them both (assuming the file contains version 1 followed by version 2). However, there's no point in doing this if you aren't going to do anything with the swatch name. For example, this demonstration program converts all the values into the standard .NET Color structure - which doesn't allow you to set the Name property. In this scenario, clearly it's a waste of time reading the version 2 data if you've just read the data from version 1. However, if you are storing the data in an object that supports the name, then it's probably a good idea to discard the previously read data and re-read the version 2 data.

Reading colour data

As the two documented file formats are almost identical, we can use the same code to handle reading the data, and then perform a little bit extra for the newer file format. The core of the code which reads the colour data looks like this.

// read the number of colors, which also occupies two bytes
colorCount = this.ReadInt16(stream);

for (int i = 0; i < colorCount; i++)
{
  ColorSpace colorSpace;
  int value1;
  int value2;
  int value3;
  int value4;

  // again, two bytes for the color space
  colorSpace = (ColorSpace)(this.ReadInt16(stream));

  // then the four values which comprise each color
  value1 = this.ReadInt16(stream);
  value2 = this.ReadInt16(stream);
  value3 = this.ReadInt16(stream);
  value4 = this.ReadInt16(stream);

  // and finally, the name of the swatch (version2 only)
  if (version == FileVersion.Version2)
  {
    int length;
    string name;

    length = ReadInt32(stream);
    name = this.ReadString(stream, length);
  }
}

Translating the colour spaces

Once we've read the colour space and the four values of the colour data, we need to process it.

The first space, RGB, is simple enough. The Adobe format is using the range 0-65535, so we just need to convert that to the standard 0-255 range:

switch (colorSpace)
{
  case ColorSpace.Rgb:
    int red;
    int green;
    int blue;

    red = value1 / 256; // 0-255
    green = value2 / 256; // 0-255
    blue = value3 / 256; // 0-255

    results.Add(Color.FromArgb(red, green, blue));
    break;

Next is HSL. How you process that depends on the class you are using, and the range of values it accepts.

case ColorSpace.Hsb:
  double hue;
  double saturation;
  double brightness;

  hue = value1 / 182.04; // 0-359
  saturation = value2 / 655.35; // 0-1
  brightness = value3 / 655.35; // 0-1

  results.Add(new HslColor(hue, saturation, brightness).ToRgbColor());
  break;

The last colour space we can easily support is gray scale.

case AdobePhotoshopColorSwatchColorSpace.Grayscale:

  int gray;

  // Grayscale.
  // The first value in the color data is the gray value, from 0...10000.
  gray = (int)(value1 / 39.0625);

  results.Add(Color.FromArgb(gray, gray, gray));
  break;

Files using the Lab or CMYK spaces will throw an exception as these are beyond the scope of this example.

  default:
    throw new InvalidDataException(string.Format("Color space '{0}' not supported.", colorSpace));
}

Although none of the sample files I tested mixed colour spaces, they were either all RGB, all Lab or all CMYK, the specification suggests that it's at least possible. In this case, throwing an exception might not be the right idea as it could be possible to load other colours. Therefore it may be a better idea to just ignore such errors to allow any valid data to be read.

Wrapping up

As with reading LBM colour maps, reading the Photoshop colour swatches was also quite an easy process.

You can download a fully working sample from the link below, and my next article will reverse the process to allow you to write your own aco files.

Downloads

• PhotoshopColorSwatchLoader.zip (24.13 KB)

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