I do not know the exact name of this visual effect (angular/clockwise fade-in/fade-out?), but it is one of the most common things that you can see in interface of RPG and RTS games, displaying progress of construction/upgrade/ability cooldown. So, to the point...

You can read how this is actually being implemented, or just scroll down to the bottom of post and grab the code and/or GMK file.

Implementation of such is actually pretty simple,

## 1. Angle

First, but still important thing to do, is to decide on a way that your code will calculate angle depending on progress. In this example, angle is being changed in a clock-like faschion (clockwise, starting and ending at top point), thus making formulae *angle = PI * 2 * progress - PI / 2*, where **progress** is a [0 ... 1] value, and **angle** is, well, output angle in radians, going clockwise from right-most point of a circle.

On image, cross in center indicates the zero (0, 0) point, while gray square is ((-1, -1) ... (+1, +1)) area, which final image must reside in.
Nothing complicated.

## 2. Make a vector

Second thing to do is to make a one-unit-long vector out of previously obtained angle. Here, "a vector" means a pair of values defining an offset. For this you will need sine/cosine functions or equivalent (even a lookup table would work, technically). In most cases this will remain as simple as *x = cos(angle); y = sin(angle);*. Image illustrates the resulting vector.

## 3. Normalize the vector

As you may or may not know, vector normalization is basically about modifying a vector to match given length while preserving direction. In this case, this is needed to make vector touch square boundary regardless of it's direction. "Touching" in this case means that either X or Y of vector must be -1 or +1. Figuring out, which of two is to be scaled, is also trivial - it's maximum of absolute values of two (*l = max(abs(x), abs(y))*). Afterwards, that is to be set to -1 or +1 accordingly, depending on it's original sign. The simplest (and valid) way of doing this is just to divide both of vector components by earlier found **l** (*x = x / l; y = y / l;*). After this operation, vector would look as illustrated on image.

## 4. Draw

Given the normalized vector, the final task remains - it has to be drawn as actual filled shape. Fortunately, GameMaker supports a "triangle fan" primitive type, which allows to define points in fan-like order (first point is center, every next two form a triangle), making it simpler. Other programming languages and frameworks tend to support a generic "path" shape, which would be used in just about the same way. The only minimal challenge here is to draw corners of rectangle. That is normally done by adding conditional checks against pre-calculated constants. In this case, corners happen at 1/8, 3/8, 5/8 and 7/8 of overall "progress", thus leading to values 0.125, 0.375, 0.625, 0.875. As illustrated on image, shape starts at center point, goes through the points that are "behind" the current progress, goes through previously calculated vector point, and ends back at the center.

That's all the logic behind this. If something is to be cleared up, comments are welcome, since I am considerably sleepy as of typing this.

You can download example here, or use the code below. Note that example has an advantage in form of second version of script, which is compatible with GameMaker: Lite.

/// draw_rectangle_cd(x1, y1, x2, y2, value) var v, x1, y1, x2, y2, xm, ym, vd, vx, vy, vl; v = argument4 if (v <= 0) return 0 // nothing to be drawn x1 = argument0; y1 = argument1; // top-left corner x2 = argument2; y2 = argument3; // bottom-right corner if (v >= 1) return draw_rectangle(x1, y1, x2, y2, false) // entirely filled xm = (x1 + x2) / 2; ym = (y1 + y2) / 2; // middle point draw_primitive_begin(pr_trianglefan) draw_vertex(xm, ym); draw_vertex(xm, y1) // draw corners: if (v >= 0.125) draw_vertex(x2, y1) if (v >= 0.375) draw_vertex(x2, y2) if (v >= 0.625) draw_vertex(x1, y2) if (v >= 0.875) draw_vertex(x1, y1) // calculate angle & vector from value: vd = pi * (v * 2 - 0.5) vx = cos(vd) vy = sin(vd) // normalize the vector, so it hits -1+1 at either side: vl = max(abs(vx), abs(vy)) if (vl < 1) { vx /= vl vy /= vl } draw_vertex(xm + vx * (x2 - x1) / 2, ym + vy * (y2 - y1) / 2) draw_primitive_end()

Great idea for a tutorial and great scripts too. Nice of you to take the time to create a Lite-compatible version.

Really neat example thanks, save us from having to do all that maths again.

Sharing is caring!

Thanks for sharing this code. I’ve used it on a personal prototyping project of mine and wish to thank you for allowing me to.

regards!

Thanks you very much 😀 Happy lucky 😀

Hey this is extremely neat. I noticed it is counter-clock-wise though. Is there an easy way to make this clock-wise?

Hey, sorry for the delayed response. The easiest way would be simply to swap x1 and x2 arguments when calling the script, causing it to draw a mirrored (and therefore CW) shape.

Is this applicable in Haxe/OpenFL ? Or should you use a shader for this?

You could do a path via display.Graphics. A shader would probably work too, but I think a path is a simpler approach (and ideally should convert into the few needed triangles automatically).

How can I make this to fit an alarm timer?

By passing value as

`1 - alarm[0] / (initial number of steps that you set alarm 0 to)`

.This is really useful, thank you! One thing though, what about if you wanted a rounded rectangle? I think you would need to look at when v is between certain values for the corners, and adjust the vector length, but am not sure if it would work. Any hints?

It would certainly be easier to adapt the function to draw corners of a textured polygon instead.

Clipping the resulted graphic would be another good option.

You certainly can make a custom normalization function to fit that shape (and iterate over near-corner values to triangulate them), but I do not believe that to be a good idea for most cases.