shader - how to get right coordinates inside a textured rectangle?

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:bust_in_silhouette: Asked By rnnr

I’m trying to use simple shader to draw square texture split to quarters - top right and bottom left red, the rest white.

I crated scene wit root CanvasLayer and TextureRect in it, and I associated a shader with the texture:

shader_type canvas_item;
render_mode unshaded;

// rectangle dimensions
uniform vec2 position;
uniform vec2 size;

void vertex() {
}

void fragment() {
	
	// relative coordinates (inside the rectangle)
	float x = FRAGCOORD.x-position.x;
	float y = FRAGCOORD.y-position.y;
	
	if 
	( 
	  //(x > size.x / 2.0) // whole right half   	  
	   (y < size.y / 2.0) // should be lower half
	)  	
	{
		COLOR = vec4(1.0, 0.0, 0.0, 1.0)
	}
	else
	{
		COLOR = vec4(1.0);
    }
}

The condition is reduced here to make the code short.

I set the position and size from GD script:

extends TextureRect

func _ready():
	self.material.set("shader_param/position", self.rect_position)
	self.material.set("shader_param/size", self.rect_size)

This way I should be able to move the textured rectangle around the scene and draw the desired “pattern”.

The problem is may code works for me only for x axis ( the condition with "whole right half " comment) – regardless of where the textured rectangle is, the right half is red as desired.

For y axis (commented “should be lower half”) it does not work - it seems to take y coordinate relatively to screen, not the textured rectangle, in other words the lower half is red only when the thing is in the center of the screen.

What am I doing wrong here ?