gistfile1.txt

uniform mat4 u_projViewTrans;

attribute vec4 a_position;

void main()
{
  gl_Position = u_projViewTrans * a_position;
}

gistfile2.glsl

precision lowp float;

void main()
{
  gl_FragColor.rgb = vec3(0.0, 1.0, 0.0);
}

MyGdxGame.java

package io.github.mlatu.gravity;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.files.FileHandle;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.Mesh;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.VertexAttribute;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.graphics.g2d.TextureRegion;
import com.badlogic.gdx.graphics.glutils.ShaderProgram;
import com.badlogic.gdx.math.Matrix4;

public class MyGdxGame extends ApplicationAdapter {
	Texture texture;
    private ShaderProgram shaderProgram;
    private Mesh mesh;
    TextureRegion textureRegion;
    private SpriteBatch batch;
    private Sprite sprite;
    private OrthographicCamera cam = null;
    private OrthographicCamera cam2d = null;

    @Override
	public void create () {
        texture = new Texture(Gdx.files.internal("texture_default.jpg"));

        batch = new SpriteBatch();
        sprite = new Sprite(texture);

		FileHandle f = Gdx.files.internal("f_shader.glsl");
        FileHandle v = Gdx.files.internal("v_shader.glsl");

		shaderProgram = new ShaderProgram(v, f);
        //ShaderProgram.pedantic = false;

        mesh = new Mesh(true, 4, 6, VertexAttribute.Position());//, VertexAttribute.TexCoords(0));

        mesh.setVertices(new float[]
                      {-0.5f, -0.5f, 0,
                        0.5f, -0.5f, 0,
                        0.5f, 0.5f,  0,
                        -0.5f, 0.5f, 0, });
        mesh.setIndices(new short[]{0, 1, 2, 2, 3, 0});

    if(!shaderProgram.isCompiled()) {
        String log = shaderProgram.getLog();
        Gdx.app.log("Log", log);
    }
}

    @Override
    public void render () {
        Gdx.gl.glClearColor(0.3f, 0.3f, 0.3f, 1);
		Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        Matrix4 o = new Matrix4();
        o.setToOrtho2D(0f, 0f, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());

        shaderProgram.begin();
        shaderProgram.setUniformMatrix("u_projViewTrans", o);
        mesh.render(shaderProgram, GL20.GL_TRIANGLES);
        shaderProgram.end();
    }
}