I made a GLSL shader loader with ofelia. Most examples from http://glslsandbox.com work without modifications (like the examples that i included). While the examples from https://www.shadertoy.com do not work out of the box.
A lot of other shader examples do not work at all (perhaps because I did not modify them right?).
Or is it possibly because of the wrong GLSL version?
What I am searching for are mainly video effects, especially a video delay (something like that: ).
It would be great if there is already a shader like that which works with ofelia.
One question: Somehow Pure Data complains about a missing .vert file, but it is still working fine without. Can I make something like this:

void main()
{
    gl_TexCoord[0] = gl_TextureMatrix[0]*gl_MultiTexCoord0;
    gl_Position = gl_ModelViewProjectionMatrix*gl_Vertex;
}

as a .vert file dummy? At least it seems to work.

I set this GLSL variables in ofelia (because they fit to the sandbox examples):
uniform float time;
uniform vec2 mouse;
uniform vec2 resolution;

ofelia_GLSL_sandbox.zip

here is a much simpler example (that should add two images) that also does not work:

  // Fragment shader
  #version 120
  uniform sampler2D Texture0;
  uniform sampler2D Texture1;
  // --------------------
  varying vec2 TexCoord0;
  // --------------------
  void main()
  {
     vec4 texel0 = texture2D(Texture0, TexCoord0);
     vec4 texel1 = texture2D(Texture1, TexCoord0);
     gl_FragColor = clamp(texel0 + texel1, 0.0, 1.0);
  }

  // Vertex shader
  #version 120
  attribute vec4 InVertex;
  attribute vec2 InTexCoord0;
  uniform mat4 ProjectionModelviewMatrix;
  varying vec2 TexCoord0;
  // --------------------
  void main()
  {
    gl_Position = ProjectionModelviewMatrix * InVertex;
    TexCoord0 = InTexCoord0;
  }

https://www.khronos.org/opengl/wiki/Texture_Combiners

@Jona hmm... Does it not work if you change #version to 120?

@Cuinjune no, somehow not.

i define the textures like that:

shader:setUniformTexture("Texture0", videoPlayer:getTexture(), 0);
shader:setUniformTexture("Texture1", image:getTexture(), 1);

i tried to make a minimalistic example (that does not work):

ofelia_glsl_addImage.zip

if type(window) ~= "userdata" then;
window = ofWindow();
end;
;
local canvas = pdCanvas(this);
local clock = pdClock(this, "setup");
local shaderDir = canvas:getDir() .. "/data/";
local shader = ofShader();
local imageDir = canvas:getDir() .. "/data/";
local image1 = ofImage();
local image2 = ofImage();
;
function ofelia.new();
ofWindow.addListener("setup", this);
ofWindow.addListener("draw", this);
ofWindow.addListener("exit", this);
window:setPosition(30, 100);
window:setSize(800 + 40, 600 + 40);
window:create();
if ofWindow.exists then;
clock:delay(0);
end;
end;
;
function ofelia.free();
window:destroy();
ofWindow.removeListener("setup", this);
ofWindow.removeListener("draw", this);
ofWindow.removeListener("exit", this);
end;
;
function ofelia.setup();
ofSetWindowTitle("Add Image");
ofBackground(0, 0, 0, 255);
image1:load(imageDir .. "image1.jpg");
image2:load(imageDir .. "image2.jpg");
shader:load(shaderDir .. "addImage");
end;
;
function ofelia.draw();
shader:beginShader();
ofSetColor(255);
shader:setUniformTexture("Texture0", image1:getTexture(), 0);
shader:setUniformTexture("Texture1", image2:getTexture(), 1);
image1:draw(20, 20, 800, 600);
image2:draw(20, 20, 800, 600);
shader:endShader();
end;
;
function ofelia.exit();
image1:clear();
image2:clear();
shader:unload();
end;

@Jona It seems to work when I correct the "addImage.vert" file to below:

  // Vertex shader
  #version 120
  varying vec2 TexCoord0;
  // --------------------
  void main()
  {
    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
    TexCoord0 = gl_MultiTexCoord0.xy;
  }

wow, thats great! it works I was searching and trying for a long time

here are the blend examples from https://www.khronos.org/opengl/wiki/Texture_Combiners.
From the article above about the code:
"Warning: This article describes legacy OpenGL APIs that have been removed from core OpenGL 3.1 and above (they are only deprecated in OpenGL 3.0). It is recommended that you not use this functionality in your programs. Consider using the OpenGL Shading Language instead."
But I think that is only because it is openGL 2.1 code, which is the default for ofelia. And it works fine.
Only "textureBlendAlpha" shows no effect, because the alpha values of the example images is constantly 255.
ofelia_glsl_blend_2_images.zip
i still wonder why this is not working: http://editor.thebookofshaders.com/?log=180614121858

@Jona I think it's because your two images have RGB color space and don't have an alpha channel.

@Cuinjune Yes, I forgot to create an alpha channel. Now the alpha blending does work, but not the shader from the book of shaders.
I think that I need to create something like a buffer frames (fbo?) before using the shader.
This attempt sounds also quite interesting, but I do not know how to do that: https://en.wikibooks.org/wiki/OpenGL_Programming/Motion_Blur
Or this: https://forum.openframeworks.cc/t/video-frame-buffer-question/6183

It is hard for me to judge what is the best solution (for a motion blur / video delay)...

But what I would need is a Buffer for several frames.

Anyway, here is the blend patch with added alpha channels: ofelia_glsl_blend_2_images.zip
And here is a simple motion blur with fbo and without shaders: ofelia_videoeffect_motion_blur.pd