Water vs Land on Earth

(older) waterworld.glsl

uniform sampler2D heightmap;
uniform sampler2D palette;
uniform sampler2D bordermap;
varying vec4 vertTexCoord;
uniform float progress;

void main() {
  
  //color
  vec4 heightColor = texture2D(heightmap, vertTexCoord.st);
  float elevation = (heightColor.r * 255. + heightColor.a) / 255.; //0..1?
  float zerolevel = 0.49659944;
  float correction = -0.0018;
  float f = elevation + correction - (progress-zerolevel)/2;
  vec4 color = texture2D(palette, vec2(f,0));

  //borders
  vec4 borderColor = texture2D(bordermap, vertTexCoord.st);
  color += borderColor * borderColor.a;

  gl_FragColor = color;
}

improved shader using map (and if statement...).glsl

uniform sampler2D heightmap;
uniform sampler2D heightmap2;
uniform sampler2D palette;

uniform sampler2D bordermap;
varying vec4 vertTexCoord;
uniform float progress;

float map(float value, float min1, float max1, float min2, float max2) {
  return min2 + (value - min1) * (max2 - min2) / (max1 - min1);
}

void main() {
  vec4 borderColor = texture2D(bordermap, vertTexCoord.st);
  vec4 heightColor = texture2D(heightmap, vertTexCoord.st);
  vec4 heightColor2 = texture2D(heightmap2, vertTexCoord.st);
  float elevation = (heightColor.r * 256. + heightColor2.r) / 256.; //0..1?

  float f = elevation<progress ?
    map(elevation,  0,progress,  0,.5) :
    map(elevation,  progress,1,  .5,1); [] 

  gl_FragColor = texture2D(palette, vec2(f,0)) + borderColor * borderColor.a;

}

water-vs-land-curve.pde

import viewport.*;
float lowest = -11000; //meters
float highest = 8848; //meters
float progress = .5589; //.5543 voor 4k plaatjes
PImage heightmap, heightmap2, earth_palette, water_palette, palette, bordermap;
PShader shader;
Viewport view;

void setup() {
  size(1024, 512, P2D);
  bordermap = loadImage("borders8k.png");
  heightmap = loadImage("8k-lo.png"); //zit verschil in niveau tussen 4k en 8k
  heightmap2 = loadImage("8k-hi.png");
  palette = loadImage("colors4k copy 9.png");
  view = new Viewport(this, 0, 0, 1024, 512, 1024, 512);
}

void draw() {
  view.mouseEnabled = keyPressed;

  background(0);
  try {
    shader = loadShader("waterworld.glsl");
    shader.set("palette", palette);
    shader.set("heightmap", heightmap);
    shader.set("heightmap2", heightmap2);
    shader.set("bordermap", bordermap);
    shader.set("progress", progress);
    shader(shader);
  } 
  catch (Exception e) {
    println(e);
  }
  view.begin();
  image(heightmap, 0, 0, width, height);
  view.end();

  resetShader();

  //loadPixels();
  //float waterArea = 0;
  //for (int i=0; i<width*height; i++) {
  //  if (blue(pixels[i])>100) waterArea++;
  //}
  //waterArea /= width*height;

  //text("fps: " + int(frameRate), 50, 35);
  text("progress: " + nf(progress*100, 0, 2) + "%", 50, 50);
  //text("area: " + nf(waterArea*100, 0, 2) + "%", 50, 65);

  //float zero = .502;
  //float meters = progress < zero ? map(progress, 0, zero, lowest, 0) : map(progress, zero, 1, 0, highest);

  //23% progress geen water
  //49.71% progress 0 niveau
  //70% progress alles onder water

  //text("meters: " + int(meters) + "m", 50, 80);
}

void mouseWheel(MouseEvent e) {
  if (!keyPressed) progress += e.getCount()*.00001;
}

void mouseMoved() {
  //progress = map(mouseX,0,width,0,1);
}

void keyPressed() {
  if (key=='[') progress=0;
  if (key==']') progress=1;
  if (key=='/') progress=.5543;
}