The Role of Animations in Hit Effects

This shall be my final post regarding the technical side of animation for a while! I try to write again but it might be more about the artistic side of animation.

Video games are all about entertainment. Game Developers always try to maximize this entertaining experience. One aspect is to let the players feel exactly what they did and receive a fair result of their selected action. This can be considered from different perspectives like game design, risk/reward or aesthetics. Players should receive a suitable feedback based on what they do. Hitting and attacking in action games follow this rule as well. When hitting or being hit, player should feel the impact.  Several techniques can be considered here to show the impact of the hits.
This article tries to address some of these techniques. The techniques are effectively used in Dead Mage’s latest released game named Epic of Kings. EOK is a hack and slash game designed for touch based devices. It’s released on iOS devices and will be released on Android very soon. Here you can see the trailer of the game:

In the rest of the article, I’m going to mention the techniques we used to show the hit impacts in Epic of Kings.

Controlling the Hit Impacts
Before reading this section I want to say that all the mentioned cases here are related to animations which can motivate player’s eyesight. It’s obvious that the audio have a huge impact on hit effects as well but this article is not going to talk about audio as I’m not a professional in the audio field.
So here are some of the animation techniques we used in EOK to control and improve the hit impacts:

1-Animations:
Surely the most important aspect to show the hit effects is the animations themselves. Animations should not be floaty. They have to be started from a damaged pose because the incoming attack has high kinetic energy and it makes the victim to accelerate in the direction of the attack. So the animation should show this. It should start very quick but ends slowly to show some effects of the attack like dizziness. Just note that the time of the hit animations in combat is very important. So the slower part of animations showing the dizziness should have a reasonable time length and it should have safe intervals providing good poses for blending to other animations (if it’s needed to be canceled to other animation). Here is one example of a light hit animation:

2- Uninterruptable Animations:
In hack and slash games, the enemies often has slower animations than the player. One reason, is because of the responsiveness. Responsiveness causes faster animations for player since playable characters are interacting directly with the player and they should respond well to the game input. Enemies are usually slower because player should have some time to see the enemy’s current action and he/she needs some time to make the correct decision. If the enemy animations are too fast he doesn’t have enough time to make the right decision. However the timing of enemy’s animations can be adjusted based on enemy type, attack type and the player progression in the game.
In many situations, this slower enemy animations can’t be cancelled by the player’s attacks. That means the enemy’s animation continues while player hitting him.  Although the player attacks, the enemy is not showing any reaction because his animation is not getting canceled. So here we can use additive animations to show some shakes on the enemy’s body. Here is a video showing additive animations’ role in this scenario:

And here is one additive animation in UE4 editor:

The shown additive animation is animated from reference pose so it can be added on different animation poses generally.

3- No Cross Fade Time (Transitional Blending):
To avoid floaty animations and showing the transferred kinetic energy for hits, the cross fade times should be equal to zero while transitioning to hit animations.

4- Specific Hit Animations:
This is an obvious point. If you have specific hit animations for different attacks, the feeling of the hits would be much better.
For example directional hit animations can help the feeling of hit impacts. Based on incoming attack’s direction, an animation showing the correct hit direction can be played.
One another example is the specific hit animations based on the current animation state. For example in an attack animation, when the time is between t1 and t2, if the character gets hit, animations other than normal hits are played.

5- Body IK as A Post Process:
In Epic of Kings, an IK chain is defined on characters’ spine. It acts as a post process on the poses of block and block hit animations. Post process here means that the original animation generates the base pose and the IK will add some custom poses on top of it so we can save the fidelity to the animations created by the artists.
By moving the end-effector in a reasonable range and blend the IK solution with FK, the spine always change position and creates non-repetitive poses which can improve the visual of the motion.

6- Camera Effects:
As mentioned in the first section, we want the player to feel the impact of the hits. Surely involving the eyesight of the player is very important as all mentioned cases above was about involving players’ eyesight via animation techniques. So camera movements can do a good job to transfer this feeling as well.
One common way is to use camera shakes. In EOK, plenty of different camera shakes with different properties were defined. Properties include frequency and amplitude for position, rotation, FOV. It also has fade in/out values to let the camera shake get added on top of the current camera movement. For example heavy attacks have more amplitude and frequency and light attacks have less frequency and amplitude or the beasts' footsteps have less frequency but more amplitude.
One other important aspect is about animating the camera FOV. In some cases, animating camera FOV on enemy attacks can make sense. Some years ago I watched a documentary movie about self-defense. It was showing that when the brain feels danger, the view of the eyes become more narrow letting them to just focus on the danger. We used this phenomena in EOK by reducing the FOV in some enemy attacks to let the player feel the danger more. Video here shows this in action:

Just note that I suggest to animate FOV just for the situations in which you’re fighting with one enemy which is also our case in epic of kings. For the situations in which you need to fight simultaneously with different characters, FOV should not be changed because player needs to focus on all the events and actions from the enemies around to do the appropriate reaction. Changing FOV in this kind of situations can distract the player a bit.

7- Hit Pauses
One other thing that you can find in many games like street fighter or god of war is hit pauses. Whenever an attack lands, time stops for a short period to show the impact of the attack. We added a slight hit pause in Epic of kings as well.

8- Physically Based Animation
Blending between physically based animation and keyframe animation has been used in many games so far. It can bring dynamic action scenes with non-repetitive animations in the game environment. One common way is to make the ragdoll to follow the animation while responding to external perturbations. With this, the ragdoll can have the overall shape of the animation and respond physically to external forces. This can be blended with the keyframe animation as well to create better and natural poses.
We developed a system on top on UE4 to demonstrate this feature. However we didn’t integrate it in the final game mostly because of the low time in the development and also because in games like Epic of Kings, action scenes are not that dynamic unlike a third person hack and slash or shooter game. So it was not a priority and we forget about integrating this feature into the game.
This video shows this feature in action:

In the video above, a random force applies to random physical bodies and the ragdoll tries to follow the animation while responding to the applied external force. Also it blends with the keyframe animation. If you want to know more about this kind of systems, I’ve written a post about blending between ragdoll and keyframe animation on my blog here.

9- Particle Effects:
There is no doubt that particles can do a great job in terms of aesthetics. Some kind of particles like sparks and blasts can help the hits to be felt better.

Conclusion
Some cases which can help to control and improve hit effects in action games were mentioned. These cases were effectively used in Epic of Kings game. Having all these cases can help the player to feel the action more and involve herself better in the game.

The Challenge of Having Responsiveness and Naturalness in Game Animation

Video games as software need to meet functional requirements and it's obvious that the most important functional requirement of a video game is to provide entertainment. Users want to have interesting moments while playing video games and there exists many factors which can bring this entertainment to the players.

One of the important factors is the animations within game. Animation is important because it can affect the game from different aspects. Beauty, controls, narration and driving the logic of the game are among them.

This post is trying to consider the animations in terms of responsiveness while trying to discuss some techniques to retain their naturalness as well.

Here I'm going to share some tips we used in the animations of the 3D action-platforming side-scroller game named "Shadow Blade: Reload". SB:R, PC version has been released 10th 2015 August via Steam and the console versions are on the way. So before going further, let's have a look at some parts of the gameplay here:

You may want to check the Steam page here too.

So here we can discuss the problem. First, consider a simple example in real world. You want to punch into a punching bag. You rotate your hip, torso and shoulder in order and consume energy to rotate and move your different limbs. You are feeling the momentum in your body limbs and muscles and then you are hearing the punch sound just after landing it into the bag. So you are sensing the momentum with your tactile sensation, hearing different voices and sounds related to your action and seeing the desired motion of your body. Everything is synchronized! You are feeling the whole process with your different senses. Everything is ordinary here and this is what our mind knows as something natural.

Now consider another example in a virtual world like a video game. This time you have a controller, you are pressing a button and you want to see a desired motion. This motion can be any animation like a jump or a punch. But this punch is different from the mentioned example in real world because the player is just moving his thumb on the controller and the virtual character should move his whole body in response to it. Each time player presses a button the character should do an appropriate move. If you receive a desired motion with good visual and sounds after pressing each button, we can say that you are going to be merged within the game because it's something almost like the example of the punching in real world. The synchronous response of the animations, controls and audios help the player feel himself more within the game. He uses his tactile sensation while interacting with controller, uses his eyesight to see the desired motion and his hearing sensation to hear the audios. Having all these synchronously at the right moment can bring both responsiveness and naturalness which is what we like to see in our games.

Now the problem is that when you want to have responsiveness you have to kill some naturalness in animations. In a game like Shadow Blade: Reload, the responsiveness is very important because any extra move can lead the player to fall of the edges or be killed by enemies. However we need good-looking animations as well. So here I'm going to list some tips we used to bring both responsiveness and naturalness into our playable character named Kuro:

1- Using Additive Animations: Additive animations can be used to show some asynchronous motions on top of the current animations. We used them in different situations to show the momentum over body while not interrupting player to show different animations. An example is the land animation. After player fall ends and he reaches the ground, he can continue running or attacking or throwing shurikens without any interruptions or land animations. So we are directly blending the fall with other animations like run. But blending directly between fall and run doesn't provide acceptable motion. So here we're just adding an additive land animation on top of the run or other animations to show the momentum over upper body. The additive animation just have visual purposes and the player can continue running or doing other actions without any interruption.


We also used some other additive animations there. For example a windmill additive animation on spine and hands. It's being played when the character stops and starts running consecutively. It can show momentum to hands and spine.

These additive animations are just being added on top the main animations and not interrupting them while the main animations like run and jump are already providing good responsiveness.


2- Specific Turn Animations: You see turn animations in many games. For instance, pressing the movement button in the opposite direction while running, makes the character slide and turn back. While this animation is very good for many games and brings good felling to the motions,  it is not suitable for an action-platforming game like SB:R because you are always moving back and forth on the platforms with low areas and such an extra movement can make you fall unintentionally and it also kills responsiveness. So for turning, we just rotate the character 180 degrees in one frame. But again, rotating the character 180 degrees in just one frame, is not providing a good-looking motion. So here we used two different turn animations. They are showing the character turning and are starting in a direction opposite to character's forward vector and end in a direction equal to character's forward vector. When we turn the character in just one frame, we play this animation and the animation can show the turn completely. It has the same speed of run animation so nothing is just going to be changed in terms of responsiveness and you will just see a turn animation which is showing momentum of a turn motion over the body and it can bring good visuals to the game.

One thing which has to be considered here is that the turn animation starts in a direction opposite to character's forward vector so for using this animation we turned off the transitional blending. because it can make jerky motions on root bone while blending.

To avoid frame mismatches and foot-skating, we used two different turn animations and played them based on the feet phases in run animation. You may check out the turn animation here:

3- Slower Enemies: While the main character is very agile, the enemies are not! Their animations have much more frames. This can help us to get the focus of players out from the main character in many situations . You might know that the human eye has a great ability to focus and zoom on different objects. So when you are looking at one enemy you can only see it clearly and not the others. Slower enemy animations with more frames help us to get the focus out from the player at many points.

As a side note, I want to say that I was watching a scientific show about human eyes a while ago and it showed that the women eyes has wider view than men and men has better focusing. You might want to check this research if you are interested about this topic.

4- Safe Blending Intervals to Cancel Animations: Assume a grappling animation. It can be started from idle pose and ended in idle pose again. The animation can do its job in its 50% of length. So the rest of its time is just for the character to get back to its idle pose safe and smoothly. At the most times, players don't want to see the animations until their ending point. They prefer to do other actions. In our game, players usually tend to cancel the attack and grappling animations after they kill enemies. They want to run, jump or dash and continue navigating. So for each animation which can be cancelled, we are setting a safe interval of blending which is used as the time to start cancelling current animations(s). This interval provides poses which can be blended well with run, jump, dash or other attacks. It provides less foot-skating, frame mismatches and good velocity blending during animation blending.


5- Continuous Animations: In SB:R, most of the animations are animated with respect to the animation(s) which is playing with higher probability before them.

For example we have run attacks for the player. When animating them, the animators have concatenated one loop of run before it and created the run attack just after that. With this, we can have a good speed blending between source and destination animations because the run attack animation has been created with respect to the original run animation. Also we can retain the speed and responsiveness of the previous animations into the current animation.

Another example here is the edge climb which is starting from the wall run animation.


6- Context Based Combat: In SB:R we have context based combat which is helping us using different animations based on the current state of the player (moving, standing,  jumping, distance and/or direction to enemies).

Attacking from each state, causing different animations to be selected which all are preserving almost the same speed and momentum of the player's current state (moving, standing, diving and so on).

For instance, we have run attacks, dash attacks, dive attacks, back stabs, Kusarigama grapples and many other animations. All are being started from their respective animations like run, jump, dash and stand and all trying to preserve the previous motion speed and responsiveness.


7- Physically Simulated Cloths as Secondary Motion: Although responsiveness can lower the rate of naturalness but adding some secondary motions like cloth simulations can help solving this issue. In SB:R we have a scarf for the main character Kuro which helps us showing more acceptable motions.


8- Tense Ragdolls and Lower Crossfade Time in Contacts: Removing cross fade transition times in hits and applying more force to the ragdolls can help more in receiving better hit effects.  However this is useful in many games not just in our case.

Conclusion

Responsiveness VS naturalness is always a huge challenge in video games and there are ways to achieve both. Most times you have to do trade-offs between both to achieve a decent result.

For those who are eager to find more about this topic, I can recommend this good paper from Motion in Games conference:

Aline Normoyle, Sophie Jorg, "Trade-offs between Responsiveness and Naturalness for Player Characters", 2014.

It shows interesting results about players' responses to animations with different amount of responsiveness and naturalness.