Written By Steve Patterson

In the previous tutorial, we looked at how Photoshop's Color Dynamics options allow us to dynamically change and control various aspects of our brush's color as we paint. In this tutorial, we'll look at the sixth and final Brush Dynamics category in the Brushes panel, the one with the least descriptive and interesting name - Other Dynamics! Just as with Color Dynamics, the options found in Other Dynamics have nothing to do with the shape of our brush. Instead, they allow us to dynamically control the opacity and flow of our brush's color!

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What do "opacity" and "flow" mean, and how are they different from each other? Before we look at how to dynamically change them from within the Brushes panel, let's first see how we normally access these options so we can see what they do.

The Options Bar

Whenever we have the Brush Tool selected in Photoshop, the Options Bar along the top of the screen shows us various options that affect how the brush works. Two of these options are Opacity and Flow, and you'll find them side by side each other:

Opacity
Opacity controls the translucency of the brush color as we paint. When the Opacity value is set to 100% (the default value), the brush color is opaque, completely blocking anything below the area we're painting over from view. At 0% opacity, the brush color is transparent, allowing anything we paint over to show through (effectively making the brush color invisible). A value between 0% and 100% will make the brush color semi-transparent, with higher values making the color more opaque than lower values.

I'll paint a simple brush stroke using one of Photoshop's standard round brushes. I'll paint with black (by setting my Foreground color to black) and I'll increase the Spacing value to 50% in the Brush Tip Shape section of the Brushes panel so the individual brush tips will be easy to see, giving the stroke a "bumpy" look to it, with each "bump" being a new stamp of the brush tip. Here's my brush stroke with the Opacity value set to its default 100%:

Let's see what happens when I lower my brush opacity down to 25%:

This time, even though I'm still painting with black, the brush color appears as a much lighter gray:

The reason is that by lowering the opacity value, the white background of the document is now showing through the brush color. With a brush opacity of 25%, it means we're seeing only 25% of the brush color mixed in with 75% of the white background.

Here's the important part. Notice that even in the areas where the brush stroke looped back over itself, the opacity value didn't change. It remained at 25% throughout the entire length of the stroke, even in areas that were painted over twice. Also, even though the brush tips themselves are overlapping each other, it made no difference to the opacity level. That's the big difference between opacity and flow. Opacity controls the translucency of the entire brush stroke. Flow, on the other hand, controls the opacity level of each individual brush tip!

The only way I can affect the opacity of my initial brush stroke is by releasing my mouse button (or lifting my pen off the tablet) to end the first stroke, then painting a second, different brush stroke that passes over top of the first one. Here, I'll paint a second stroke, also at 25% opacity. The second stroke appears as the same light gray color, but in areas where the two strokes overlap each other, the opacity levels combine to create darker, more opaque sections:

Flow
I'm going to increase my Opacity value back up to 100% and this time, I'll lower the Flow value down to 25%:

Here's the same brush stroke again but with flow set to 25% instead of the opacity. This time, we're seeing something quite different. The stroke still starts out with the same light gray color, since we're still allowing the white background to show through, but the areas where the individual brush tips overlap each other are darker and more opaque, and the areas where the brush loops back over itself (where multiple brush tips are overlapping) are even darker:

Again, Flow controls the opacity level of each individual brush tip, unlike Opacity which controls the transparency of the stroke as a whole. With Flow, areas in the stroke where the brush tips overlap become more opaque than areas that don't overlap as the opacity levels of those areas combine. If I lower the spacing of my individual brush tips (I'll lower it to around 13% in the Brush Tip Shape section of the Brushes panel) and paint another stroke, we see much darker and more opaque results. Flow is still set to 25%, but because the brush tips are now closer together, they overlap more, and the more they overlap, the more opaque the brush stroke becomes:

If I leave my brush tip spacing the same and paint the same stroke again, this time with Opacity set to 25% instead of Flow (which I'll set back to its default 100%), we're back to seeing the same uniform transparency level throughout the entire stroke. The fact that the brush tips are close together and overlapping each other so much makes no difference with the Opacity option, since all it cares about is the transparency of the stroke as a whole:

Now that we've seen what the Opacity and Flow options are all about and how we normally set them in the Options Bar, let's see how we can dynamically control them from the Brushes panel!