I want to retrieve data that has created by the compute shader but have no clue how it works. After researching a few forums and articles on how it should work and this is what I came up with.

Compute Shader Code

#version 430 core

layout  (local_size_x  =  100)  in;

layout(std430, binding=0) buffer Pos{
    float Position[];

void main(){
    Position[gl_GlobalInvocationID.x] = float(gl_GlobalInvocationID.x);

Program Code

public float[] useProgram(int x_size, int y_size, int z_size){
        this.group_x_size = (int)Math.ceil(x_size/100);
        this.group_y_size = (int)Math.ceil(y_size/100);
        this.group_z_size = (int)Math.ceil(z_size/100);
        float[] data = new float[x_size];
        FloatBuffer buffer = BufferUtils.createFloatBuffer(x_size);
        int ssbo = GL15.glGenBuffers();// creates empty VBO as an object and stores it in OpenGl memory
        GL15.glBindBuffer(GL43.GL_SHADER_STORAGE_BUFFER, ssbo);
        GL43.glBindBufferBase(GL43.GL_SHADER_STORAGE_BUFFER, 0, ssbo);
        GL15.glBindBuffer(GL43.GL_SHADER_STORAGE_BUFFER, 0);
        GL20.glUseProgram(computeProgram);//start the shader program
        GL43.glDispatchCompute(group_x_size, 1, 1);

        GL15.glBindBuffer(GL43.GL_SHADER_STORAGE_BUFFER, ssbo);
        String values = GL43.glMapBuffer(GL43.GL_SHADER_STORAGE_BUFFER, GL43.GL_WRITE_ONLY).asFloatBuffer().toString();
        return null;

However this code just simply will not work the way I want it to. When I go to print out the Values it comes up null. Is there something wrong with the code or is there a better way to do it. I've looked through many forums but most of them aren't written in java.

  • $\begingroup$ One thing is that when you map the buffer you're passing GL_WRITE_ONLY. But you want to retrieve the values in the buffer, so you probably want GL_READ_ONLY for that. $\endgroup$ Commented Jul 21, 2020 at 23:00
  • $\begingroup$ Also, I think you want GL_BUFFER_UPDATE_BARRIER_BIT instead of GL_SHADER_STORAGE_BARRIER_BIT. The barrier bits refer to operations after the barrier that need to wait for shader writes prior to the barrier. glMapBuffer falls under the BUFFER_UPDATE_BARRIER_BIT. $\endgroup$ Commented Jul 21, 2020 at 23:05
  • $\begingroup$ @NathanReed The program still somehow returns null whenever I run it. $\endgroup$
    – Ethan Ma
    Commented Jul 22, 2020 at 0:13
  • $\begingroup$ Next thing to try might be hooking up debug output to see if there are any errors/warnings that might shed light on what's going wrong. See also this question. $\endgroup$ Commented Jul 22, 2020 at 0:20
  • $\begingroup$ @NathanReed after I switched out GL_DYNAMIC_COPY code for GL_READ_ONLY instead of returning null the whole thing just crashed. $\endgroup$
    – Ethan Ma
    Commented Jul 22, 2020 at 0:24


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