Writing Photoshop Color Swatch (aco) files using C#

The previous article in this mini-series described how to read files in Abode's Colour File format as used by applications such as Photoshop and other drawing programs.

In this second article, I'll describe how to write such files.

Getting started

I'm not going to go over the structure again, so if you haven't already done so, please read the previous article Reading Photoshop Color Swatch (aco) files using C# for full details on the file structure and how to read it.

Writing big-endian values

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

We can use the following two methods to write a short or a int respectively into a stream as a series of bytes. Of course, if you just want functions to convert these into bytes you could use similar code, just remove the bit-shift.

private void WriteInt16(Stream stream, short value)
{
  stream.WriteByte((byte)(value >> 8));
  stream.WriteByte((byte)(value >> 0));
}

private void WriteInt32(Stream stream, int value)
{
  stream.WriteByte((byte)((value & 0xFF000000) >> 24));
  stream.WriteByte((byte)((value & 0x00FF0000) >> 16));
  stream.WriteByte((byte)((value & 0x0000FF00) >> 8));
  stream.WriteByte((byte)((value & 0x000000FF) >> 0));
}

As with the equivalent read functions, the >> 0 shift is unnecessary but it does clarify the code.

We also need to store colour swatch names, so again we'll make use of the Encoding.BigEndianUnicode property to convert a string into a series of bytes to write out.

private void WriteString(Stream stream, string value)
{
  stream.Write(Encoding.BigEndianUnicode.GetBytes(value), 0, value.Length * 2);
}

Writing the file

When writing the file, I'm going to follow the specification's suggestion of writing a version 1 palette (for backwards compatibility), followed by a version 2 palette (for applications that support swatch names).

using (Stream stream = File.Create(fileName))
{
  this.WritePalette(stream, palette, FileVersion.Version1, ColorSpace.Rgb);
  this.WritePalette(stream, palette, FileVersion.Version2, ColorSpace.Rgb);
}

The core save routine follows. First, we write the version of format and then the number of colours in the palette.

private void WritePalette(Stream stream, ICollection<Color> palette, FileVersion version, ColorSpace colorSpace)
{
  int swatchIndex;

  this.WriteInt16(stream, (short)version);
  this.WriteInt16(stream, (short)palette.Count);

  swatchIndex = 0;

With that done, we loop through each colour, calculate the four values that comprise the colour data and then write that.

If it's a version 2 file, we also write the swatch name. As these basic examples are just using the Color class, there's no real flexibility in names, so we cheat - if it's a "named" colour, then we use the Color.Name property. Otherwise, we generate a Swatch <index> name.

  foreach (Color color in palette)
  {
    short value1;
    short value2;
    short value3;
    short value4;

    swatchIndex++;

    switch (colorSpace)
    {
      // Calculate color space values here!
      default:
        throw new InvalidOperationException("Color space not supported.");
    }

    this.WriteInt16(stream, (short)colorSpace);
    this.WriteInt16(stream, value1);
    this.WriteInt16(stream, value2);
    this.WriteInt16(stream, value3);
    this.WriteInt16(stream, value4);

    if (version == FileVersion.Version2)
    {
      string name;

      name = color.IsNamedColor ? color.Name : string.Format("Swatch {0}", swatchIndex);

      this.WriteInt32(stream, name.Length);
      this.WriteString(stream, name);
    }
  }
}

Converting colour spaces

As previously mentioned, the specification states that each colour is comprised of four values. Even if a particular colour space doesn't use all four (for example Grayscale just uses one, you still need to write the other values, typically as zero's.

Although it's a slight duplication, I'll include the description table for colour spaces to allow easy reference of the value types.

While supporting CMYK colours are beyond the scope of this article as they require colour profiles, and I haven't the foggiest on the Lab space, we can easily support RGB, HSL and Grayscale spaces.

RGB is the simplest as .NET colours are already in this format. The only thing we have to do is multiple each channel by 256 as the specification uses the range 0-65535 rather than the typical 0-255.

Notice value4 is simply initialized to zero as this space only needs 3 of the 4 values.

      case ColorSpace.Rgb:
        value1 = (short)(color.R * 256);
        value2 = (short)(color.G * 256);
        value3 = (short)(color.B * 256);
        value4 = 0;
        break;

We can also support HSL without too much trouble as the Color class already includes methods for extracting these values. Again, we need to do a little fiddling to change the numbers into the range used by the specification.

      case ColorSpace.Hsb:
        value1 = (short)(color.GetHue() * 182.04);
        value2 = (short)(color.GetSaturation() * 655.35);
        value3 = (short)(color.GetBrightness() * 655.35);
        value4 = 0;
        break;

The last format we can easily support is grayscale. If the source colour is already grey (i.e. the red, green and blue channels are all the same value), then we use that, otherwise we'll average the 3 channels and use that as the value.

      case ColorSpace.Grayscale:
        if (color.R == color.G && color.R == color.B)
        {
          // already grayscale
          value1 = (short)(color.R * 39.0625);
        }
        else
        {
          // color is not grayscale, convert
          value1 = (short)(((color.R + color.G + color.B) / 3.0) * 39.0625);
        }
        value2 = 0;
        value3 = 0;
        value4 = 0;
        break;

Demo Application

The usual sample application is available from the links at the end of this article. The sample generates a random 256 colour palette, then writes this to a temporary file using the specified colour space. It then reads it back in, and displays both palettes side by side for comparison.

Update History

• 2009-08-10 - First published
• 2020-11-21 - Updated formatting

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Files

• PhotoshopColorSwatchWriter.zip (16.86 KiB)