A raycaster is a project that renders a 3D scene by "casting rays" from a camera into a scene of objects. The ray's length and closest object hit is information used to then draw on the screen. While the term "ray casting" is broad and unspecifying of direction, in Scratch it is generally used to mean casting rays horizontally only.
Ray casting should not be mistaken with ray tracing, which renders with more physical accuracy, catering to reflection and refraction of light rays.
It should also not be mistaken with ray marching, which iteratively steps through a scene composed of SDFs.
Concept
Ray casting works by casting "rays" to measure the distance to the nearest wall, hence the term "ray caster". The program sends out rays starting from the player, moving forward until it hits an object, at which point it takes the distance it has traveled and colors the pixel based on the distance. The rays are sent in different directions. After all rays have been sent, the complete picture will be seen.
Sprite-Based vs List-Based
Two categories of raycasters can be defined for Scratch: sprite-based and list-based.
The sprite-based method uses the shape of a sprite costume for finding ray intersections. A separate sprite is moves around the 2D screen to simulate a ray. Intersections are found with the <touching ( v)?> block. Both sprites may be invisible with the ghost effect or be quickly shown then hidden so that the player doesn't see the detection take place.
The list-based method instead stores the scene in a list. Most often this is a 2D grid of tiles. To find intersections, mathematical formulae can be used. In the case of a grid of tiles, the DDA algorithm is used.[clarify]
Sprite-Based Raycaster
The necessary components of a sprite-based raycaster are:
- Map — a map of the walls in the scene.
- Sensor — measures distances from the player to each wall.
- Renderer — draws each wall based on the distance given by the sensor.
This tutorial will use four sprites to accomplish these three requirements. If stuck, here is a finished example.
Map
Make a map sprite with walls, preferably 480x360. Everywhere else should be transparent.
Then, add the following script to it:
when gf clicked go to x: (0) y: (0) show set [ghost v] effect to (100)
Person
Next, a person sprite that walks around the map will be made.
Make a person sprite that has a one pixel by one pixel costume. The costume should be centered on the pixel. This is important.
Before diving into the sprite's scripts, note that all (speed) should be replaced with how fast the player needs to move.
Put these scripts in the "person" sprite:
when gf clicked
go to x: (0) y: (10) // Or wherever you want the sprite in the map.
set [ghost v] effect to (100)
broadcast (sense v) and wait
forever
if <key (right arrow v) pressed?> then
turn cw (3) degrees // This can be adjusted to rotate faster, to fight lag.
else
if <key (left arrow v) pressed?> then
turn ccw (3) degrees // This can be adjusted to rotate faster, to fight lag.
end
end
if <key (up arrow v) pressed?> then
move (speed)::custom
else
if <key (down arrow v) pressed?> then
move ((-1) * (speed))::custom
end
end
broadcast (sense v) and wait
end
define move (speed) move (speed) steps if <touching (Map v)?> then // Wall sensing move ((-1) * (speed)) steps end
A map has now been created that can be walked around in
Distance sensing
This is where it starts to build up to 3D raycasting.
In the real world, the more distant an object is, the smaller it appears.
You will make a sensing script that measures the distance from the player to the walls around them. You need to do this for 96 different angles. All of this will happen when the "sense" broadcast is called.
Make a "distance sensing" sprite that has a one pixel by one pixel costume. The costume should be centered on the pixel.
Then, add this script:
when gf clicked
set [ghost v] effect to (100)
when I receive [sense v] // Used in "person"
sense
broadcast (draw v) and wait
define sense// Important: Make this custom block run without screen refresh.
. . .
delete all of [distances v]
set [distance v] to [0]
set [angle offset v] to [-48]
repeat (96)
set [distance v] to [0]
go to (person v)
point in direction (([direction v] of (person v)) + (angle offset))
repeat until <<touching (map v)?> or <(distance::variables) = [80]>>
move (1) steps
change [distance v] by (1)
end
add (distance::variables) to [distances v] // We hit a wall. Let's keep track of the distance.
change [angle offset v] by (1) // Let's test a new angle...
end
Drawing
Here is the information generated by "distance sensing" it taken and drawn. Remember, the more distant an object is, the smaller it is.
Make a "drawing" sprite with this script:
when I receive [draw v] draw define draw// Important: Make this custom block run without screen refresh. . . . go to x: (-237.5) y: (180) set pen size to (5) set pen color to [#7AF] // Pick the color that you want. erase all // Prepare the screen for drawing! pen up set [column v] to (1) repeat (length of [distances v]) set pen (brightness v) to ((50) + ((item (column) of [distances v]) * ((50) / (80)))) // Fade to white set y to ((-1200) / (item (column) of [distances v])) // Adjust -1200 to make walls larger or smaller. pen down set y to ((1200) / (item (column) of [distances v])) // Adjust 1200 to make walls larger or smaller. pen up change x by (5) change [column v] by (1) end
The project is now finished. The project will be slow when it is run in Scratch, unfortunately.
Jumping (optional)
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Although it is impossible to add true vertical movement to the project, it is possible to create the illusion of jumping. It works by drawing the walls at a lower height when the player is jumping. Walls closer to the player will move downwards more to simulate parallax. To add the option to jump, first add the script below to any sprite or background:
when gf clicked forever if <<key (space v) pressed?> and <(variable) = (1200)>> then set [variable v] to (900) broadcast (sense v) and wait set [variable v] to (750) broadcast (sense v) and wait set [variable v] to (675) broadcast (sense v) and wait set [variable v] to (650) broadcast (sense v) and wait set [variable v] to (650) broadcast (sense v) and wait set [variable v] to (675) broadcast (sense v) and wait set [variable v] to (750) broadcast (sense v) and wait set [variable v] to (900) broadcast (sense v) and wait set [variable v] to (1200) broadcast (sense v) and wait end end
Now, modify the script that draws the walls to the following:
define draw go to x: (-237.5) y: (180) set pen size to (5) set pen color to [#7AF] // Pick the color that you want. erase all pen up set [column v] to (1) repeat (length of [distances v]) set pen (brightness v) to ((50) + ((item (column) of [distances v]) * ((50) / (80)))) set y to (((variable) - (2400)) / (item (column) of [distances v])) //This is what creates the illusion of jumping. pen down set y to ((variable) / (item (column) of [distances v])) //This is what creates the illusion of jumping. pen up change x by (5) change [column v] by (1) end
Speed Optimization
There are several ways to reduce lag:
- Set the "person" sprite's rotation style to "don't rotate".
- In the "distance sensing" sprite, check for walls every 2 steps instead of 1.
- Instead of drawing 96 columns, draw fewer. 60 is a good number, assuming that in "distance sensing", the sprite is rotated 2 degrees instead of 1 and the pen size is 8.
- Replace the code in the forever loop in the "Player" sprite with this:
turn cw ((<key (right arrow v) pressed?> - <key (left arrow v) pressed?>) * (5)) degrees move ((<key (up arrow v) pressed?> - <key (down arrow v) pressed?>) * (speed)) :: custom broadcast (sense v) and wait
- Make the sensing both sense and draw
- Replace all
broadcast (sense v) and waitwithwait (0) secondsand make the sensing and drawing always sense and draw
The main speed bottleneck with a sprite-based raycaster is that the "distance sensing" sprite has to do lots of sensing.
List-Based Raycaster
A list-based raycaster relies on a map stored as a list and coordinates, such as that of the player and the ray, stored as variables. This method of raycasting is very virtual, all data is stored as numbers and there are no actual sprite costumes used. The only sprite required is a moving pen to draw the walls. If you want an example map, one is downloadable here.
This tutorial will teach you how to make a simple list-based raycaster. It uses custom blocks to make editing easier. Make sure to check off "run without screen refresh" box.
 Note: | Only try this method after you have completely understood the Sprite-Based Raycaster or are already familiar with arrays and raycasting. |
| Note: | Tutorial is not finished. |
Variables Required
The following variables are required for the tutorial:
(Actual Resolution) (Brightness :: variables) (Camera X) (Direction X) (Direction X Old) (Direction Y) (Distance X Delta) (Distance Y Delta) (Draw End) (Draw Start) (Height) (Line Height) (Map X) (Map XY) (Map Y) (Move Speed) (Perpendicular Wall Distance) (Plane X) (Plane X Old) (Plane Y) (Ray X Direction) (Ray X Position) (Ray Y Direction) (Ray Y Position) (Resolution) (Rotation Speed) (Side X Distance) (Side Y Distance) (side) (Step X) (Step Y) (Touching Wall) (Wall Found) (x) (X Direction) (X Position::variables) (Y Direction) (Y Position::variables)
Lists Required
The following lists are required for the tutorial:
(World Map::list)
Setting up the Variables
The code used to setup the variables is:
define Set up Variables set [X Position v] to [11] set [Y Position v] to [7] set [Direction X v] to [-1] set [Direction Y v] to [0] set [Plane X v] to [0] set [Plane Y v] to [0.66] set [Actual Resolution v] to [1] // If you aren't using resolution, you don't need this variable. set [Height v] to [300] set [Resolution v] to [12] // Resolution is not needed, but it is recommended.
Setting up the Lists
The (World Map::list)list should contain n items of n numbers of either 0 or 1, where n is the distance of each side of the grid that the list represents. Squares that should be filled in should be represented by a 1, and squares that should be empty should be represented as a 0. Here is a example 10x10 world map.
when flag clicked delete all of [world map v] add [1111111111] to [world map v] add [1000000001] to [world map v] add [1000110001] to [world map v] add [1000000001] to [world map v] add [1001001001] to [world map v] add [1001001001] to [world map v] add [1000000001] to [world map v] add [1000110001] to [world map v] add [1000000001] to [world map v] add [1111111111] to [world map v]
Pen Shade Replacement
This script is for replacing the set pen shade to () block:
define set pen shade to (shade) set pen (brightness v) to ((100)-(shade))
The Main Loop
This is the green flag script that starts all the other scripts.
when green flag clicked Set Up Variables::custom forever pen up // DadOfMrLog did some tests and found that setting pen size to 1 and using the pen up block reduces lag set pen size to (1) hide set [Actual Resolution v] to (((Resolution) - (16)) * (-1)) // If you want to have resolution, you need this script. Raycast::custom // This is the next script in the tutorial. end
The Raycasting Script
Next is the main raycasting script. This block controls most of the custom blocks. Be sure to make it a run without screen refresh block.
define Raycast
erase all
set [x v] to [-240] // "x" is the increment variable here.
Read Keys::custom // This is the next script in the tutorial.
repeat until <(x) > [240]>
pen up
set pen size to (1)
set pen shade to (0)::custom
set [Camera X v] to ((2) * ((x) / ((Actual Resolution) - (1)))
set [Ray X Position v] to (X Position::variables)
set [Ray Y Position v] to (Y Position::variables)
set [Ray X Direction v] to ((Direction X) + ((Plane X) * (Camera X)))
set [Ray Y Direction v] to ((Direction Y) + ((Plane Y) * (Camera X)))
set [Map X v] to ([floor v] of (Ray X Position))
set [Map Y v] to ([floor v] of (Ray Y Position))
Calculate Walls :: custom // Explained later on.
Draw Walls :: custom // Explained later on.
change [x v] by (Actual Resolution)
end
Controls
One of the custom blocks in the "raycast" script was the custom block, "Read Keys." Here, we'll focus on that script.
define Read Keys
if <<key (up arrow v) pressed?>and<not <(Touching Wall) = [1]>>> then
change [X Position v] by ((X Direction) * (Move Speed))
change [Y Position v] by ((Y Direction) * (Move Speed))
end
if <<key (down arrow v) pressed?>and<not <(Touching Wall) = [-1]>>> then
change [X Position v] by ((X Direction) * (Move Speed))
change [Y Position v] by ((Y Direction) * (Move Speed))
end
if <key (right arrow v) pressed?> then
set [Direction X Old v] to (Direction X)
set [Direction X v] to (((Direction X) * ([cos v] of ((Rotation Speed) * (-1)))) - ((Direction Y) * ([sin v] of ((Rotation Speed) * (-1)))))
set [Direction Y v] to (((Direction X Old) * ([sin v] of ((Rotation Speed) * (-1)))) + ((Direction Y) * ([cos v] of ((Rotation Speed) * (-1)))))
set [Plane X Old v] to (Plane X)
set [Plane X v] to (((Plane X) * ([cos v] of ((Rotation Speed) * (-1)))) - ((Plane Y) * ([sin v] of ((Rotation Speed) * (-1)))))
set [Plane Y v] to (((Plane X Old) * ([sin v] of ((Rotation Speed) * (-1)))) + ((Plane Y) * ([cos v] of ((Rotation Speed) * (-1)))))
end
if <key (left arrow v) pressed?> then
set [Direction X v] to (Direction X)
set [Direction X v] to (((Direction X Old) * ([cos v] of ((Rotation Speed) * (1)))) - ((Direction Y) * ([sin v] of ((Rotation Speed) * (1)))))
set [Direction Y v] to (((Direction X Old) * ([sin v] of ((Rotation Speed) * (1)))) + ((Direction Y) * ([cos v] of ((Rotation Speed) * (1)))))
set [Plane X Old v] to (Plane X)
set [Plane X v] to (((Plane X) * ([cos v] of ((Rotation Speed) * (1)))) - ((Plane Y) * ([sin v] of ((Rotation Speed) * (1)))))
set [Plane Y v] to (((Plane X Old) * ([sin v] of ((Rotation Speed) * (1)))) + ((Plane Y) * ([cos v] of ((Rotation Speed) * (1)))))
end
Calculating Walls
This section explains the block that calculates the walls. Unfortunately, wall touch detection is not included.
define Calculate Walls
set [Touching Wall v] to [0]
set [Distance X Delta v] to ([sqrt v] of ((1) + (((Ray Y Direction) * (Ray Y Direction)) / ((Ray X Direction) * (Ray X Direction)))))
set [Distance Y Delta v] to ([sqrt v] of ((1) + (((Ray X Direction) * (Ray X Direction)) / ((Ray Y Direction) * (Ray Y Direction)))))
set [Wall Found v] to [0] // Once the ray hits a wall, this variable is set to one, which is the same as the boolean value "true" in this case.
if <(Ray X Direction) < [0]> then
set [Step X v] to [-1]
set [Side X Distance v] to (((Ray X Position) - (Map X)) * (Distance X Delta))
else
set [Step X v] to [1]
set [Side X Distance v] to ((((Map X) + (1)) - (Ray X Position)) * (Distance X Delta))
end
if <(Ray Y Direction) < [0]> then
set [Step Y v] to [-1]
set [Side Y Distance v] to (((Ray Y Position) - (Map Y)) * (Distance Y Delta))
else
set [Step Y v] to [1]
set [Side Y Distance v] to ((((Map Y) + (1)) - (Ray Y Position)) * (Distance Y Delta))
end
repeat until <(Wall Found) = [1]>
if <(Side X Distance) < (Side Y Distance)> then
change [Side X Distance v] by (Distance X Delta)
change [Map X v] by (Step X)
set [side v] to [0] // The variable "side" is referring to which wall is being faced, a Y wall or an X wall. In this case, it is an X wall.
else
change [Side Y Distance v] by (Distance Y Delta)
change [Map Y v] by (Step Y)
set [side v] to [1] // In this case, the ray has hit a Y wall.
end
set [Map XY v] to (letter (Map Y) of (item (Map X) of [World Map v])) // Map XY is the item of the grid that the ray is in. Example: if X is eleven and Y is seven, then Map XY would be (letter 7 of (item 11 of World Map).
if <(Map XY) > [0]> then // If Map XY is more than zero, it has hit a wall.
set [Wall Found v] to [1]
end
end
if <(side) = [0]> then
set [Perpendicular Wall Distance v] to ([abs v] of ((((Map X) - (Ray X Position)) + (((1) - (Step X)) / (2))) / (Ray X Direction)))
else
set [Perpendicular Wall Distance v] to ([abs v] of ((((Map Y) - (Ray Y Position)) + (((1) - (Step Y)) / (2))) / (Ray Y Direction)))
end
set [Line Height v] to ([abs v] of ([floor v] of ((360)/(Perpendicular Wall Distance))
set [Draw Start v] to ((Line Height) / (-2)
set [Draw End v] to ((Line Height) / (2))
Drawing the Walls
The last part in this tutorial is the pen script that draws the walls. It is a very simple script.
define Draw Walls
set pen color to [#179fd7]
if <(side) = [1]> then
set [Brightness v] to (115) // This script makes the Y walls darker than the X walls for a nice effect.
else
set [Brightness v] to (150)
end
go to x: (x) y: (Draw Start)
set pen shade to ((Brightness :: variables) * (-0.2))::custom
set pen size to (Actual Resolution)
pen down
go to x: (x) y: ((2) * (Draw End))
pen up
set pen size to (1)
set pen shade to (0)::custom
Your list-based raycaster is now ready!
See Also
External Links
- Ray casting on Wikipedia
