In processing why some uniform variables are not provided or some varying variables only appearing in fragment shader but they are used?

Question

I am currently studying the language Processing along with GLSL. According to GLSL, uniform variables should be given by the application while varying variables should be used in both vertex and fragment shader. However, look at the following example.

Application:

PImage label;
PShape can;
float angle;

PShader texShader;

void setup() {
  size(640, 360, P3D);  
  label = loadImage("lachoy.jpg");
  can = createCan(100, 200, 32, label);
  texShader = loadShader("texfrag.glsl", "texvert.glsl");
}

void draw() {    
  background(0);

  shader(texShader);  

  translate(width/2, height/2);
  rotateY(angle);  
  shape(can);  
  angle += 0.01;
}

PShape createCan(float r, float h, int detail, PImage tex) {
  textureMode(NORMAL);
  PShape sh = createShape();
  sh.beginShape(QUAD_STRIP);
  sh.noStroke();
  sh.texture(tex);
  for (int i = 0; i <= detail; i++) {
    float angle = TWO_PI / detail;
    float x = sin(i * angle);
    float z = cos(i * angle);
    float u = float(i) / detail;
    sh.normal(x, 0, z);
    sh.vertex(x * r, -h/2, z * r, u, 0);
    sh.vertex(x * r, +h/2, z * r, u, 1);    
  }
  sh.endShape(); 
  return sh;
}

texvert.glsl:

#define PROCESSING_TEXTURE_SHADER

uniform mat4 transform;
uniform mat4 texMatrix;

attribute vec4 vertex;
attribute vec4 color;
attribute vec2 texCoord;

varying vec4 vertColor;
varying vec4 vertTexCoord;

void main() {
  gl_Position = transform * vertex;

  vertColor = color;
  vertTexCoord = texMatrix * vec4(texCoord, 1.0, 1.0);
}

texfrag.glsl:

#ifdef GL_ES
precision mediump float;
precision mediump int;
#endif

uniform sampler2D texture;

varying vec4 vertColor;
varying vec4 vertTexCoord;

void main() {
  gl_FragColor = texture2D(texture, vertTexCoord.st) * vertColor;
}

It is a sample from the official website. As you can see, none of the uniform variables are initialized.

In another example.

#ifdef GL_ES
precision mediump float;
precision mediump int;
#endif

#define PROCESSING_TEXTURE_SHADER

uniform sampler2D texture;
uniform vec2 texOffset;

varying vec4 vertColor;
varying vec4 vertTexCoord;

void main(void) {
  // Grouping texcoord variables in order to make it work in the GMA 950. See post #13
  // in this thread:
  // http://www.idevgames.com/forums/thread-3467.html
  vec2 tc0 = vertTexCoord.st + vec2(-texOffset.s, -texOffset.t);
  vec2 tc1 = vertTexCoord.st + vec2(         0.0, -texOffset.t);
  vec2 tc2 = vertTexCoord.st + vec2(+texOffset.s, -texOffset.t);
  vec2 tc3 = vertTexCoord.st + vec2(-texOffset.s,          0.0);
  vec2 tc4 = vertTexCoord.st + vec2(         0.0,          0.0);
  vec2 tc5 = vertTexCoord.st + vec2(+texOffset.s,          0.0);
  vec2 tc6 = vertTexCoord.st + vec2(-texOffset.s, +texOffset.t);
  vec2 tc7 = vertTexCoord.st + vec2(         0.0, +texOffset.t);
  vec2 tc8 = vertTexCoord.st + vec2(+texOffset.s, +texOffset.t);

  vec4 col0 = texture2D(texture, tc0);
  vec4 col1 = texture2D(texture, tc1);
  vec4 col2 = texture2D(texture, tc2);
  vec4 col3 = texture2D(texture, tc3);
  vec4 col4 = texture2D(texture, tc4);
  vec4 col5 = texture2D(texture, tc5);
  vec4 col6 = texture2D(texture, tc6);
  vec4 col7 = texture2D(texture, tc7);
  vec4 col8 = texture2D(texture, tc8);

  vec4 sum = (1.0 * col0 + 2.0 * col1 + 1.0 * col2 +  
              2.0 * col3 + 4.0 * col4 + 2.0 * col5 +
              1.0 * col6 + 2.0 * col7 + 1.0 * col8) / 16.0;            
  gl_FragColor = vec4(sum.rgb, 1.0) * vertColor;  
}

It is a fragment shader, but no vertex shader is found in this example.

The application of this corresponding fragment shader is.

PShader blur;

void setup() {
  size(640, 360, P2D);
  blur = loadShader("blur.glsl"); 
  stroke(255, 0, 0);
  rectMode(CENTER);
}

void draw() {
  filter(blur);  
  rect(mouseX, mouseY, 150, 150); 
  ellipse(mouseX, mouseY, 100, 100);
}

I am confusing because these examples from processing disobey the tutorials of OpenGL. Why is that happening?

Answer 1

It seems like the tutorial back then was outdated. If I look now at https://processing.org/tutorials/pshader/ then the old #define statements are gone. Also things like vertex are renamed to position. Also the examples can be downloaded here: https://github.com/codeanticode/pshader-tutorials .

Al thought, for learning GLSL, I recommend using thebookofshaders.com .