Readability and fairness
The player has to understand what killed them. Telegraphs, reaction time, contrast, and a hitbox that need not match the art
There is one test of fairness: dying, does the player blame themselves or the level? Everything on this page pushes the answer towards "themselves"
Telegraphing
A danger has to be visible before it becomes dangerous
A shape and a collider are two separate fields of one object. So a warning is simple: a beam with a shape and no collider glows and never touches you. Half a second later a second one appears beside it, this time with a collider
The reverse is equally legal: an invisible wall is an object with no shape and a real collider
An invisible danger is fair in exactly one case: the player already knows it is there, because the level showed it earlier
Reaction time
A person sees and reacts in roughly 0.2 to 0.25 seconds at best. At 60 level frames per second that is 12 to 15 frames
A danger appearing less than 12 frames before contact (10, for example) cannot be passed first time. It can only be learned. When that is acceptable - Difficulty and the curve › Memorisation
Contrast
What is dangerous must differ from the background more than decoration does. The most common failure is a dark projectile on a dark background, at the exact moment the background is also changing
Take both the background and the projectile colours from the theme. Then one theme edit keeps the contrast everywhere. Hand-typed colours keep it only until the first change
One danger, one look
Every kind of danger needs its own recognisable look, and its own sound if you use one. Once the player has learned that a thing hurts, everything that looks like it has to hurt too
Never contradict your own signals. If something that looks like decoration kills in one place, the player stops trusting everything that looks like decoration
Structure beats density
A screen full of motion reads as noise rather than as difficulty. A player does not distinguish 40 projectiles. They distinguish a wall with a hole, a wave, a spiral, a corridor
Eight objects with structure are harder and fairer than forty without
A level plays the same every time
A level has no conditions, counters or triggers reacting to the player. Everything is authored on the timeline. Randomness does not change either: the same spot produces the same numbers on any device
So a level cannot be made reactive. It also cannot become unfair by accident, only by design
What does change between runs is chosen by the player: lives, playback speed and checkpoints. More - Difficulty and the curve › What the player chooses
Next: Difficulty and the curve, Colour, themes and post-processing