Unity - Paint and blood splatters tests

The concept: trying to paint a level with blood or paint (nearly the same thing, only the color is different) in Unity 3D.

Link: http://dmayance.com/unity-paint-and-blood-splatters-tests/

First method: decals

Decals are simple sprites, repeated and drawn on top of other objects. They are usually used for visual enhancements of a scene more than for gameplay purposes.

Here’s an example of a decal for paint.

Using an existing script probably made by a Dexter fan (download and demo here) I was able to spawn decals in my level to stick paint everywhere.

The splash algorithm from the original script linked above is quite smart. From a point in space it raycasts in random direction and print a decal when it hits something.

I added some debug features to visualize it.

To have a proper display of decals, I used a community script called BlendedDecal. It removes all glitches due to “Z-fighting”.

The decals itself was:

• A quad
• The texture above with alpha as transparency…
• …texture that was on a BlendedDecal shader material

Nothing else. The fun part is when you add a rigidbody, you can have physics splash.

And here’s the script I used for painting. I put it online because the original script was in JavaScript so it may helps someone to have it done in C#.

	using UnityEngine;
	using System.Collections;
	using System.Collections.Generic;
	/// <summary>
	/// Generate paint decals
	/// </summary>
	public class PainterScript : MonoBehaviour
	{
	public static PainterScript Instance;
	/// <summary>
	/// A single paint decal to instantiate
	/// </summary>
	public Transform PaintPrefab;
	private int MinSplashs = 5;
	private int MaxSplashs = 15;
	private float SplashRange = 2f;
	private float MinScale = 0.25f;
	private float MaxScale = 2.5f;
	// DEBUG
	private bool mDrawDebug;
	private Vector3 mHitPoint;
	private List<Ray> mRaysDebug = new List<Ray>();
	void Awake()
	{
	if (Instance != null) Debug.LogError("More than one Painter has been instanciated in this scene!");
	Instance = this;
	if (PaintPrefab == null) Debug.LogError("Missing Paint decal prefab!");
	}
	void Update()
	{
	// Check for a click
	if (Input.GetMouseButtonDown(0))
	{
	// Raycast
	Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
	RaycastHit hit;
	if (Physics.Raycast(ray, out hit, Mathf.Infinity))
	{
	// Paint!
	// Step back a little for a better effect (that's what "normal * x" is for)
	Paint(hit.point + hit.normal * (SplashRange / 4f));
	}
	}
	}
	public void Paint(Vector3 location)
	{
	//DEBUG
	mHitPoint = location;
	mRaysDebug.Clear();
	mDrawDebug = true;
	int n = -1;
	int drops = Random.Range(MinSplashs, MaxSplashs);
	RaycastHit hit;
	// Generate multiple decals in once
	while (n <= drops)
	{
	n++;
	// Get a random direction (beween -n and n for each vector component)
	var fwd = transform.TransformDirection(Random.onUnitSphere * SplashRange);
	mRaysDebug.Add(new Ray(location, fwd));
	// Raycast around the position to splash everwhere we can
	if (Physics.Raycast(location, fwd, out hit, SplashRange))
	{
	// Create a splash if we found a surface
	var paintSplatter = GameObject.Instantiate(PaintPrefab,
	hit.point,
	// Rotation from the original sprite to the normal
	// Prefab are currently oriented to z+ so we use the opposite
	Quaternion.FromToRotation(Vector3.back, hit.normal)
	) as Transform;
	// Random scale
	var scaler = Random.Range(MinScale, MaxScale);
	paintSplatter.localScale = new Vector3(
	paintSplatter.localScale.x * scaler,
	paintSplatter.localScale.y * scaler,
	paintSplatter.localScale.z
	);
	// Random rotation effect
	var rater = Random.Range(0, 359);
	paintSplatter.transform.RotateAround(hit.point, hit.normal, rater);
	// TODO: What do we do here? We kill them after some sec?
	Destroy(paintSplatter.gameObject, 25);
	}
	}
	}
	void OnDrawGizmos()
	{
	// DEBUG
	if (mDrawDebug)
	{
	Gizmos.DrawSphere(mHitPoint, 0.2f);
	foreach (var r in mRaysDebug)
	{
	Gizmos.DrawRay(r);
	}
	}
	}
	}

view rawPainterScript.cs hosted with ❤ by GitHub

Advantages:

• Simple
• It manipulates GameObjects
• Nice splash algorithm

Drawbacks

• What do we do of those game object? Destroy it after x seconds? Let them accumulate until it crashes?
• As you can see of the screenshot above, your decals have to have a size that match where they are applied

The hard way: painting the texture

Unity allow to directly change the colour of a pixel on a texture.

You just have to use the SetPixel(s) method.

It’s a bit slow, but if you batch your modification you can optimize the paint.

Drawing some random pixels is easy, so I thought it could be nice to use the previous decal as a model, a shape.

I also added a random roulette for dcals so it’s always the same that is used. Here’s what I got:

or the painting version, the same with a color change

The wall behind is quite nice. But the painting on platforms clearly sucks. It’s all streched and small. I am not applying a ratio, so the texture is stretched but I’m doing as if it was not.

I mixed it with the previous splash algorithm so it’s raycasting everywhere to paint things.

Advantages:

• No objects are created
• Ingame permanent (but reseted at each launch)

Drawbacks

• Performance: cause a small 0.1sec freeze
• Paintable objects must have a texture AND a mesh collider (so you can get textureCoords for RaycastHit)
• Complexity
• You must perfectly understand texture. Not like me, trying to hack stuff.