2D Cast shadow

This shader applies a dynamic shadow effect to 2D canvas items. The shadow’s direction can be controlled using the look_right parameter. The shader works by stretching the x-coordinates of the vertices horizontally and then calculating a shadow effect in the fragment shader. This allows for flexible shadow positioning and can create a more realistic depth effect for 2D elements.

As this shader only casts the shadow (which is intentional), the correct positioning must be managed using the z-index to ensure proper layering and visual effects.

Why the need to orient the shadow? Because if you horizontally flip your sprite, you don’t want the shadow to disengage from the other shadows’ direction. By controlling the shadow orientation, you maintain a consistent and realistic shadow direction relative to other objects in your scene.

Shader code

shader_type canvas_item;
uniform bool look_right = true;

void vertex() {
	VERTEX.x = VERTEX.x * { -1.0, 1.0 }[int(look_right)];
		
	float xSkewRatio = step(UV.y, .5);
	vec2 skewTransition = vec2(abs(VERTEX.x) * xSkewRatio, 0.0);
	vec2 skewScale = vec2(VERTEX.x, (VERTEX.y + abs(VERTEX.y)) / 2.0);
	
	VERTEX = skewScale;

	VERTEX = VERTEX + skewTransition;
}

void fragment() {
	COLOR = vec4(vec3(.0), .5 * COLOR.a);
}

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