This shader is used to create an effect of a sprite dissolving in a portal (when the protagonist visits a portal) in my game. The sprite should have the same size as the source image file.

Fragment_shader_for_sprite_in_portal_dissolving.fs

// Copyright © 2024 MGDS Studio. All rights reserved.

#ifdef GL_ES
#define LOWP lowp
precision mediump float;
#else
#define LOWP
#endif

const float VISIBLE_LINES_COUNT = 7.0;                          // Visible areas pieces
const float INVISIBLE_LINES_COUNT = VISIBLE_LINES_COUNT-1.0;    //Invisible areas between visible. They will be increased
const float TIME = 1.2;                                         //Max time for dissolvin
const float START_ALPHA = 0.0;                                  //Start alpha for dissolving
const float END_ALPHA = 0.75;                                   //End alpha for dissolving
const float END_HEIGHT = 1.0/(INVISIBLE_LINES_COUNT);           //End height for every area which will be increased at TIME-moment

varying vec2 v_texCoords;       //From 0.0 to 1.0! Only readable in fragment shader
uniform sampler2D u_texture;
uniform float time;             //Time from shader launch. Value must be set up from the game in seconds

void renderTransparent();

/** Remember! Coordinates from bottom!
*   Incoming data only time in seconds!
*   shaderProgram.setUniformf("time", deltaTime);
*/

void main(void) {
	float actualInvisibleHeight = (END_HEIGHT)*(time/TIME);	                     //(0 .. 1)
	float halfTestAreaHeight = actualInvisibleHeight/2.0;
	float top;                                                                  //top line of the  single area
	float bottom;                                                               //bottom line of the  single area
	bool areaInside = false;
	float actualY = END_HEIGHT/2.0;
	for (float i = 0.0; i < INVISIBLE_LINES_COUNT; i+=1.0){
		  float fI = float(i);                      //Convert to float
          top =     actualY+halfTestAreaHeight;     //Top line (Y-pos) of this invisible area
		  bottom =  actualY-halfTestAreaHeight;     //Top line (Y-pos) of this invisible area
		  if (v_texCoords.y>=bottom && v_texCoords.y <= top){
                areaInside = true;                  //Line in invisible area. Will not be rendered
                break;
          }
          actualY+=END_HEIGHT;
	}
	if (areaInside){
		gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);
	}
	else {
		renderTransparent();
	}
}

void renderTransparent(){
    float actualAlpha = 1.0-(START_ALPHA+(END_ALPHA-START_ALPHA)*(time/TIME));
    if (actualAlpha > 1.0) actualAlpha = 1.0;
    else if (actualAlpha <0.0) actualAlpha = 0.0;
    vec4 color = texture2D(u_texture, v_texCoords);
    float originalAlpha = color.a;
    if (originalAlpha<actualAlpha){
        actualAlpha = originalAlpha;
    }
    gl_FragColor = vec4(color.r, color.g, color.b, actualAlpha);
}