When you paint with brushes, you can manually set their size or have it controlled based on various criteria:

• using a graphic pen from a tablet, you may change the brush size based on the pressure that you apply while painting
• it can be forced to simulate a tablet pen's stroke and change size at the tips: start and end is small, full size in the middle of the stroke. This is an option in the Curve tool.
  
• a post-processing envelope can modulate the size (and color/gradient)
  
• the scale can vary with the speed: moving faster or slower can be tied to increasing or decreasing the size of the brush image
• the size can also be made to change randomly
  

And then there is this one more:

• Z-scale Guide: The size may be affected by an underlying image, namely from the Swap image.  (which you could make a copy of the Main image, or parts of it, or it can be a totally different image - total freedom of expression!)
  

Let's explore this last idea. But first, let's recap some of the manual methods.

Graphic Tablet's Pen Pressure

If you own a graphic tablet, such as a Wacom Intuos, you know you can set the size to be affected by the pressure.

You can also re-calibrate the pen's pressure to map it to a finer range or, to the contrary, a rougher, less sensitive one.

Simulated Tablet Output with the Curve Tool

If you don't own a tablet, you can simulate its effect with the mouse, using the Curve tool in a special way. There are two ways in fact:

1. Individual single points: select the Curve tool (1), then select adding individual points (2), which is the default mode, and click in a few places on the image canvas to place the vew points, and see a curve established. After 4 points it turns from straight lines to smooth spline curved. When you're done entering the points for your curve, render it, using the second option: Stroke a curve. (3)

   

2. Multi-point Auto-stroke: In this mode, select the Auto mode to stroke the brush. (2). Then you use the mode for collectively adding multiple points when you don't click single points but instead draw a brush stroke, like you paint along the desired path. (3). This will auto-stroke the brush in a spline curve along the inserted positions. Each brush stroke automatically renders along the curve, and makes the tips thin down. In no time, you'll have a bunch of curves drawn out as if you had used a tablet's pen with pressure to change the size upon entry and exit of each brush stroke.
   

The Post Adjustment: Resizing Envelope

The Post Adjustment feature can be found in the View menu. When enabled, the size of the brush is affected by the envelope's amplitude from beginning to end of the brush stroke.

Here is an envelope that can simulate tablet pen pressure, starting small to reach full size in the middle and fading to small again in the end. However, it is not limited to just that. You can create many other envelopes, and use some of the presets.

Also, this can be used with many of the brushes, from internal (small image based) brushes, to the larger custom brushes, even animbrushes, and also most of the particle/foliage brushes.

Here's an example where we drew several brush strokes somewhat diagonally, lower left to upper right, and the size evolved from big (near lower left), to small (far upper right). A single brush stroke out of the wrist, using the mouse, no tablet needed.

Note that the Particles brushes also have an Age decrement feature which can create similar effects, but here we see the post processing envelope affect other types of brushes too, including bristle brushes.

Speed-Scaling

In the brush Settings panel, you'll find an option that affects the size based on the speed. You can make it get bigger with higher speed, or you can make it get smaller. The latter is useful when simulating ink on fibrous paper. WHen you slow down and take longer to move the pen, the ink has more time to penetrate the fibers of the paper, so we are simulating a blob. When you speed up, there's barely time for the ink to leave its mark. The line appears thinner.

Here is an example where we draw 3 similar lines, from left to right, at changing speeds: Starting slow, going faster near the center, and slowing down again to the end, well, best we could anyway. (If you thought it's hard to stop on a dime, try on a pixel.)

The top curve had speed scale left at default 0.

The middle curve had it set to slightly over 20. This is great for ballpoint pen and inking.

The bottom curve had it set to a similar but negative value. Now it starts small at slow speed, and gets bigger the faster you draw.

Random Scaling

Also in the Brush Settings panel, you'll find an option to set the scale randomly. Here are examples:

• the top row was with the large simple brush, the only thing changed is the random scale, and the step size to make the effect a little more evident. (if the step is very small, you may hardly see changes in sizes especially for other brushes with antialiasing or soft edges in their image.)
• The second line from the top had a much larger Step size to show more gaps and you get to really see the different random scale
• The 3rd line was with one of the large airbrushes
• Then 4th was with the Splatter, where the brush also uses random positioning. Still, you can tell that some splats are large and others are small. That's the random scaling
• try a few others. The last one was the flowery brush Organic effect group), you can see random scaling of the individual flowers, as well as random position, and random rotation. (The flowers don't all have the same orientation).
  

Z-scale Guide: The size may be affected by an underlying image

coming soon - but if you're curious:

- Enable Z-scale guide. Look for it in the Sidebar, at the very bottom, last group of items (Grids, which has visible grids and artist guides too)
- load an image into the swap buffer, such as a grid pattern or just a grey gradient from left to right.
- switch back to the main image, clear it.
- load a custom image-based brush (or make one from scratch)
- make sure the custom brush preview is enabled. ("eye" icon). You should see the brush preview move wth your mouse.
- move the mouse pointer across the blank image.It should change size based on what part of the swap image it is covering.
- The brightness of the swap image controls the scale of your brush.
- try that with various brushes, such as foliage or particle brushes too.


Ok, here goes.

Let's first start by selecting a brush, preferably one with a rather large image. Not super large necessarily, something like 100-200 pixels will do just fine. Use for example the Airbrush 200 brush. There are several ways to get to it. You could right-click the brush tool. Or left click to select the brush tool, then select the pulldown next to the Browse for media button.




Draw a simple brush stroke such as from left to right. No change in size.




Now switch to the Swap image. An easy way to jump is with the 'S' icon in the top-right icon collection. Another is from the Image menu, or keyboard shortcut 'j' for jump. (depends on keyboard focus)





You'll know if you're looking at the Swap image buffer when you see 'Swap' in the title bar next to the program's name (instead of Main). If you hit the toggle again, it will switch from Swap back to Main.




While still looking at the Swap image, load a linear gradient into it: Select the Linear gradient tool from the Gradients tool (below the Curve tool)




Draw the linear gradient across your Swap image, from left to right. SImply click-and-drag from somewhere near the left edge of the image to near the right edge.




Now that you have a horizontal gradient in the Swap image, switch back to the Main image.

Select the brush tool, and draw another brush stroke from left to right. Again, no difference in its size.  Just placing that gradient into the Swap image did not tell the system to use it as a way to control the size of the brush image.

But now comes the magic:

Look in the bottom of the sidebar. (the sidebar may be on the left or right side of teh screen - it's configurable.)

Look at the bottom, there's a category called Grids. It contains visible grids and artist guides.

It also shows the very last option:  Z scale guide

Enable Z scale guide.

Now draw a similar brush stroke from left to right, and observe how the size of the brush evolves, based on the linear gradient that's hidden underneath in the Swap image.



In case you didn't know where to enable the brush preview: use the eye icon in the context bar of the brush tool.




And that's the essence of using the Z scale guide.

So why do we call it the Z scale guide? Because in addition to the x and y position of the pixels in the image, you can think of the value of gradient from the Swap image as a depth or elevation, making it smaller when down low, and bigger when brighter.

Let's use other images in the Swap image, for fancier effects in Z-scaling.

For example: Switch back to the Swap mage, and render the brick pattern from Filter > Render > Brick texture...

And make sure that there is a high contrast between the two colors, i.e. almost black, and almost white, but don't let it go completely black in the grout parts, becasue we don't want it to cause the scaling all the way to zero size.




Switch back to the Main image, erase the image and draw any complex brush stroke. You'll see a wacky effect whereby the brush size is big when going over the bright parts of the Swap image, and small when going over the dark (grout) parts.




Here are other interesting patterns, and their effect on the brushes when Z scaling guide is enabled:


ex1 - linear horizontal gradient of vertical bars in several waves:




ex2 - A circular gradient of circles, and one brush stroke to paint this:




ex3 - blobs to confine clusters of populations - and flowers adhering to it, in just one brush stroke dabble all over the place

   


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