GameMaker Quirks
When we first started to use GameMaker, we thought it was an extremely weird engine. Now, we know that it's a lot better than we initially gave it credit for. But it still has some quirks.
Gamemaker Rectangle Drawing
Drawing rectangles in GameMaker is a little bit weird. In most graphics libraries and engines, a rectangle is defined by its x, y, width, and height. In GameMaker, however, a rectangle is defined by its top-left corner (x1, y1) and its bottom-right corner (x2, y2).
To find the width of the rectangle, you have to do (x2 - x1) + 1, and to find the height of the rectangle, you have to do (y2 - y1) + 1. It seems that GameMaker's (x2, y2) refer to the TOP-LEFT CORNER of the BOTTOM-RIGHT PIXEL of the rectangle. This means that the TRUE bottom-right point of the rectangle is at (x2 + 1, y2 + 1).
This actually does not happen by default in the latest version of GameMaker. In it, draw_rectangle always draws a rectangle with a width of x2-x1 and a height of y2-y1. This is because the "quirk" was removed in GameMaker 2024.14. To add this quirk back, check the "Legacy primitive drawing behaviour" option in GameMaker's Game Options.
GameMaker Object "Movement"
GameMaker's Event Order docs explain the following very well.
Let there be an object obj_test that is instantiated in the Step event of another object, obj_creator, using instance_create_depth or instance_create_layer. Inside of the instance creation function, obj_test's Create event is called, before returning to the Step event of obj_creator. During its Create event, the obj_test instance's direction is set to 45, and its speed is set to 10.
GameMaker runs the Step events for all objects that have been created, one after the other. So after running the Step event for obj_creator instances, it runs the Step events for other objects. Note that GameMaker does NOT run the Step event for obj_test, because it was created during a Step event on the current frame.
Once it is done with all these Step events, it moves all objects according to their hspeed and vspeed variables. Since the obj_test instance was created before this, it will also be moved. GameMaker will convert direction and speed to hspeed and vspeed. In this case, both hspeed and vspeed are equal to 10 * cos(45deg), which is about 7.07. So GameMaker moves the instance of obj_test about 7.07 pixels down, and about 7.07 pixels to the right.
The "move all instances" part of the game loop always happens right after the "run all Step events" part of the game loop. Since obj_test was created during a Step event, its first "movement" happens before its first Step event. Therefore, on obj_test's Nth Step event, it will have moved N times.
What if there was an object called obj_inroom that was instantiated by being inside a room (in the room editor)? When the room is entered, obj_inroom's Create event is called. Let's say that obj_inroom also sets its speed and direction variables in the Create event.
obj_inroom's first Step event is ran BEFORE its first movement, so on the Nth Step event, it will have moved N-1 times. It is very confusing that two similar objects could behave so differently, but remember that it is because obj_test was instantiated during a Step event, so it never ran its first Step before its first movement.
Things GameMaker does better than Godot
GameMaker's object management system is simple, but REALLY good at what it does. If you always have just one instance of an object, you can reference it with the object's name. This is what Deltarune's code does when referencing obj_mainchara. If you have multiple instances of an object, you can do something to ALL of them with a with statement. And you can use instance_number(object_name) to get the number of instances of an object. Meanwhile, Godot has a tree system, but that makes it a lot more complex to move around. You cannot easily just get a node of a specific type in Godot, or get the number of nodes of that type. You have to implement "object pooling" in order to do that.
(Every time I think of this, I feel shame. How could Godot do something so fundamental, worse than GameMaker? Surely there's something missing, right?)