Debugging my simple shader test

Processing Coding Questions
3

Here’s the simpler version of your code working off of that blur example:

PImage img;
PShader shader;

void setup() {
  size(640, 480, P2D);
  img = loadImage(dataPath("image.jpg"));
  shader = loadShader("frag.glsl");
}

void draw() {
  background(127); // set background color to gray
  image(img, 0, 0, width, height);
  filter(shader);
}

with no vertex.glsl file. The frag.glsl file is:

#ifdef GL_ES
precision mediump float;
#endif

uniform sampler2D texture;

varying vec4 vertTexCoord;

void main() {
  vec4 col = texture2D(texture, vertTexCoord.st);
  gl_FragColor = vec4(col.r, 0.0, 0.0, col.a); // keep only the red channel
}