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.
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.
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.
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.
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).
The core save routine follows. First, we write the version of format and then the number of colours in the palette.
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
Swatch <index> name.
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
Although it's a slight duplication, I'll include the description table for colour spaces to allow easy reference of the value types.
|0||RGB. The first three values in the colour data are red, green, and blue. They are full unsigned 16-bit values as in Apple's RGBColordata structure. Pure red = 65535, 0, 0.|
|1||HSB. The first three values in the colour data are hue, saturation, and brightness. They are full unsigned 16-bit values as in Apple's HSVColordata structure. Pure red = 0,65535, 65535.|
|2||CMYK. The four values in the colour data are cyan, magenta, yellow, and black. They are full unsigned 16-bit values. For example, pure cyan = 0,65535,65535,65535.|
|7||Lab. The first three values in the colour data are lightness, a chrominance, and b chrominance. Lightness is a 16-bit value from 0...10000. Chrominance components are each 16-bit values from -12800...12700. Gray values are represented by chrominance components of 0. Pure white = 10000,0,0.|
|8||Grayscale. The first value in the colour data is the gray value, from 0...10000.|
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 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.
value4 is simply initialized to zero as this space only
needs 3 of the 4 values.
We can also support HSL without too much trouble as the
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.
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.
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.
- 2009-08-10 - First published
- 2020-11-21 - Updated formatting
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- Writing Photoshop Color Swatch (aco) files using C#
- Reading Photoshop Color Swatch (aco) files using C#