There aren’t many bitmap tools that I look at and think “hmmm… I could probably crank that out in about half an hour.” But because of some previous work I had done, it turned out I wasn’t too far from the mark when it comes to the sponge tool.


In concept, the sponge tool is a simple tool. It either increases or decreases the saturation of an image. Saturation describes how intense a given color is. 0% saturation means the color is gray. In the implementation that I will show, the sponge tool will behave like a brush.

Thus, if I have a simple red square with about 50% saturation, the sponge tool in the saturation mode affects it like so:

Red square with saturation applied

The desaturation mode has the opposite effect on the same image:

Red square with desaturation applied

In addition to the saturate/desaturate mode, the sponge tool has a “flow” parameter, which determines how much to saturate or desaturate the image.

Implementation Overview

The sponge tool is simply a subclass of the FilterBrush class, which I created for the dodge and burn tutorial. In fact the only difference between this tutorial and that one is the new subclass Sponge. For that reason I’ll only be covering the Sponge class in this tutorial. For an explanation as to how the rest of the code works, please refer back to the dodge and burn tutorial.

As always, I’ve provided sample code for this article.



The sponge tool has two parameters which are initialized in the init method:

- (id) init
	self = [super init];
	if ( self != nil ) {		
		// Set the default values for our parameters
		mFlow = 0.5;
		mMode = kSpongeMode_Desaturate;
	return self;

The two parameters for the sponge tool are mFlow and mMode.

  • mFlow Flow determines how strong the saturation or desaturation effect is applied to the image. It ranges from 0.0 to 1.0, where 0.0 means the effect isn’t applied, and 1.0 is where the effect is at its strongest.

    Sponge in saturate mode, varying flow:

    mFlow Result
    0.25 Red square with 25% saturation applied
    0.5 Red square with 50% saturation applied
    1.0 Red square with 100% saturation applied

  • mMode Mode determines if the sponge saturates or desaturates the image. It has only two settings: saturate or desaturate. Saturate increases the color’s saturation, while desaturate decreases saturation.

    Sponge examples:

    mMode Result
    kSpongeMode_Saturate Red square with saturation applied
    kSpongeMode_Desaturate Red square with desaturation applied

In the examples I used a red rectangle with about a 50% saturation. That way, both saturation and desaturation effects will show up on it.

Creating the filter

If you recall from the dodge and burn tutorial, the only real responsibility of a FilterBrush subclass is to create a filter that will be applied to each brush stamp.

- (CIFilter *) filter
	// We need to create and configure our filter here. CIColorControls has controls
	//	for saturation which is what we care about.
	CIFilter * filter = [CIFilter filterWithName:@"CIColorControls"];
	[filter setDefaults];
	if ( mMode == kSpongeMode_Saturate )
		[filter setValue:[NSNumber numberWithFloat:1 + mFlow] forKey:@"inputSaturation"];
		[filter setValue:[NSNumber numberWithFloat:1 - mFlow] forKey:@"inputSaturation"];

	return filter;

Unlike the dodge and burn tools, I don’t use a custom filter. Instead, I use the system provided CIColorControls, which has a parameter for saturation. The saturation parameter has a range of 0 to 2, where 1 is the identity value. Thus, if I’m trying to saturate the image, I add 1.0 to mFlow, and if I’m trying to desaturate the image, I subtract mFlow from 1.0.

That’s it. The rest of the heavy lifting is done by the framework I created in the dodge and burn tutorial.


The main thing this tutorial demonstrates to me is that the time spent building a framework for filter brushes paid off. It should help in the future if I decide to tackle the blur and sharpen brushes as well.

Hopefully you found this post enlightening. Don’t forget to download the source code.