Here, all particles whose age is over 20% of their Lifetime will change color.
Conditions can be used to specifically query the properties and states of individual particles and, for example, subject them to logical comparisons. Other Conditions allow the use of Fields or the evaluation of time if, for example, the behavior of particles should change at a certain point in the animation. The Conditions themselves do not change anything about the particles, but only sort out the particles, e.g., of a group, which fulfill the desired criteria. What should then happen with these particles is usually defined by Modifiers that are grouped directly under the Conditions. However, the same principle could also be used for Force objects under one Condition, thereby limiting them to a certain part of the particles.
Conditions also work if they are not assigned to a group, but they will then access all particles. This can sometimes be useful, but as a rule you will only apply Conditions to the particles of certain groups. The Multi Group is also available for this purpose, with which any number of Particle Groups can be combined, e.g., for processing by Conditions.
The following example gives you an idea of the basic use of Conditions. A simple Basic Emitter and its automatically generated Particle Group are used there. The Emitter emits green particles, which should now be changed at a certain point in the particle's life. For this, we will create a Condition and make it a Child of the Particle Group This will give it access to all particles of the Basic Emitter. In the Condition, we will now select the property of the particle that is to be used as a criterion for switching the color. We will use the Age Percentage Property. This is a percentage value that indicates the current Age of each particle in relation to its Lifetime. For this to work, however, you should limit the Lifetime of the Emitter. If the particles live indefinitely, their percentage Age will not give a meaningful value. We will use a Lifetime of 150 images on the Emitter.
Now we have to select a criteria in the Condition that is used to check the Particle property. These can, for example, be the common checks Greater than or Less than. Here we will select Greater than and a value of 20%. The Condition will therefore sort out all particles whose age is over 20% of the Lifetime
Here, all particles whose age is over 20% of their Lifetime will change color.
As already described, the Condition itself cannot be used to change any properties of the particles. The Modifiers, for example, are responsible for this. By subordinating a Modifier under the Condition, it can only change the selected particles. This can, for example, be a new force that acts on the particles or an optical change, such as the color change here. We will use a Color Mapper Modifier for this. A possible result is shown in the image above. Only the sorted out part of the particles will change color.
Conditions can also be combined to perform even more complex sorting of particles. Here, for example, not only the percentage Age but also the Y-position of the particles is included in the check.
This principle can be extended as required, as additional Conditions can be subordinated to a Condition in order to further refine the particle sorting. For example, we could query and compare another property, such as the Y-position of the particles. In our case, we will use the second Condition to check whether particles are greater than 50 cm on the Y-axis. Each Condition has an additional setting for the Combine Mode. This defines how the results of overriding Conditions are to be dealt with. The Combine Mode And then means, for example, that both Conditions must be applie. Under the second Condition, only those particles are found whose percentage age is over 20% and which have a height greater than 50 cm (see image above).
After changing the Combine Mode to Or, both Conditions can apply equally to cause a color change.
If the Combine Mode of the second Condition is changed to Or, we will get a different result (see image above). Now all particles that are either older than 20% of the Lifetime OR above 50 cm on the Y-axis will be recolored.
All previous sorting out of particles by Conditions was hard-edged, but randomness can also be introduced. The Chance value of each Condition can be used for this. For example, if we choose a small percentage value for Chance in the first Condition, the age of the particles will only be recorded over a longer period of time. The following image shows how this changes our result.
The reduction in Chance leads to a smooth transition in the particles sorted out by Age in the lower part of the particle stream.
Such randomness, which can be defined as a percentage via the Chance value, is very helpful. For example, consider a situation in which you generally want to randomly sort out 30% of the particles and assign them to a new group or recolor them. In this case, a structure as shown in the following image would be possible.
Here, the particles are divided into two new groups at a ratio of 30% to 70%.
As can be seen in the image above, a Basic Emitter also generates particles here and transfers them to the automatically generated Particle Group. The task now is to randomly sort out 30% of these particles and create corresponding new groups. We will start again with a new Condition and this time ask for the Age. This is the elapsed Lifetime of the particles, measured in frame. The query Greater Than or Equals with the value 0 would therefore actually capture all particles of the Emitter. However, by using a Chance of only 30%, only this 30% of the newly created particles in the animation image will be filtered out. A subordinate Switch Group Modifier, in which a new Particle Group is linked, ensures that this 30% of the particles can now be moved to a new group. For clarity, these particles have been colored red in the image above.
To ensure that the remaining 70% of the particles do not pass the 30% Condition again during the next processing cycle of the first, original Particle Group and thus 30% of the then available particles will be rejected again, we will also move these particles to a new group. To do this, we use a second one under the first Condition that uses the Inverse Combine mode. This means that this Condition will automatically sort out all particles that were not filtered out by the higher Conditions. This is very handy if you use multiple Conditions with low Chance values on top to get all remaining particles.
The second Condition also uses a Switch Group, which moves the remaining particles to a new group. In this setup, the original Particle Group only ever contains the new particles that have just been created at the Emitter for a short time and releases them almost immediately to the two new groups.
On the introductory page on using the particle system, you will find another example of how to use this randomness in Conditions.