
Imagine a blob as…

A blob component works in a similar way. It has a certain radius, but it appears smaller than its radius, because we can only see the part of the component that has a strength greater than the threshold value.
Radius
blob { threshold Threshold sphere { <Center>, Radius, Strength } cylinder { <End1>, <End2>, Radius, Strength } }

We won't go into details here, but just take note of that the visible radius is never as big as the actual radius you specify.
Strength
blob { threshold Threshold sphere { <Center>, Radius, Strength } cylinder { <End1>, <End2>, Radius, Strength } }

As we can see in the image, the greater the strength, the more of the component is visible. If the strength is very high, the visible radius is almost as big as the actual radius. That's because most of the mountain is above the water level. If the strength is small, the visible radius is much smaller than the actual radius. That's because most of the mountain is below the water level.
Threshold
blob { threshold Threshold sphere { <Center>, Radius, Strength } cylinder { <End1>, <End2>, Radius, Strength } }

Similar with blob components. If we lower the threshold value, we can see more of the blob components. If we increase the threshold value, we can see less of the blob components. And if the threshold value is greater than the strength of a blob component, then the blob component becomes invisible.
Strength relative to threshold

It is important to note though, that the strength only affects one component at a time, while the threshold affects all the components in a blob. When you work with a blob with many components, it's a good idea to set the threshold value once and for all, and after that only modify the strength values of the individual components.
In the image: From the top image to bottom image the strength of all the components have been multiplied with 2, but the threshold has also been multiplied with 2, so the result remains the same.
Controlling the visible radius and the blobbiness

Different combinations of radius and strength can give the same visible radius, but with different degrees of "blobbiness" (see image). To be able to control the visible radius and the blobbiness precisely, you need to know some formulas and stuff, which are a bit complicated. To make it easier, I have made a macro:
#macro easyblob (_threshold,_visibleradius,_blobbiness) #local _strength = (1+1/_blobbiness)*_threshold; #local _actualradius = _visibleradius/sqrt(1-sqrt(_threshold/_strength)); _actualradius, _strength #end
For a sphere blob component, you use it as follows:
sphere { <Center>, easyblob (Threshold, Visible_radius, Blobbiness) }
For a cylinder blob component, you use it as follows:
cylinder { <End1>, <End2>, easyblob (Threshold, Visible_radius, Blobbiness) }The Threshold value should be the same as the threshold value of the blob. The Visible_radius is how big you want the visible radius to be. The Blobbiness is how much the blob component should blob together with other blob components. All three values must be positive.
The easyblob macro is not the solution for everything, but it is a nice tool when working with blobs.
Component interaction

In the image to the right we have three blob components with a strength of 1.5. What happens when we move them so they're all located at the same location? The strengths are added together!
Negative strengths

The image at the right shows three examples of sphere components with cylinder components subtracted from them. In all the examples the threshold is 0.5, and the strength of the sphere is 1.5. The difference between the threshold of 0.5 and the strength of 1.5 is 1.0. That means that a negative strength of more than 1.0 is required to cut a hole through the sphere.
In the first example the negative strength of the cylinder is 0.5. That isn't enough to cut a hole through the sphere. In the next example the negative strength is 1.0. That's on the edge between being a hole there or not. In the last example the negative strength of the cylinder is 1.5. That clearly is enough to cut a hole through the sphere.
Joining cylinders

However, at the area where the cylinders overlap, the strengths are added together, so we have a strength of 2. That causes an enlarged, bulbous region where the cylinders meet. If we want to get rid of that, we can just place a sphere with a strength of -1 at the place where the cylinders meet. That is far the easiest way to join cylinder components smoothly, and it works perfectly. You can see that in the lower image.
That's it for now…

Feedback about this tutorial is much appreciated!