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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);
    buffer.put(data);
    int ssbo = GL15.glGenBuffers();// creates empty VBO as an object and stores it in OpenGl memory
    GL15.glBindBuffer(GL43.GL_SHADER_STORAGE_BUFFER, ssbo);
    GL15.glBufferData(GL43.GL_SHADER_STORAGE_BUFFER,buffer,GL15.GL_DYNAMIC_COPY);
    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);
    GL43.glMemoryBarrier(GL43.GL_SHADER_STORAGE_BARRIER_BIT);

    GL15.glBindBuffer(GL43.GL_SHADER_STORAGE_BUFFER, ssbo);
    String values = GL43.glMapBuffer(GL43.GL_SHADER_STORAGE_BUFFER, GL43.GL_WRITE_ONLY).asFloatBuffer().toString();
    GL43.glUnmapBuffer(GL43.GL_SHADER_STORAGE_BUFFER);
    System.out.println(values);

    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.

Ethan Ma
  • 21
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  • 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. – Nathan Reed Jul 21 '20 at 23:00
  • 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. – Nathan Reed Jul 21 '20 at 23:05
  • @NathanReed The program still somehow returns null whenever I run it. – Ethan Ma Jul 22 '20 at 00:13
  • 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. – Nathan Reed Jul 22 '20 at 00:20
  • @NathanReed after I switched out GL_DYNAMIC_COPY code for GL_READ_ONLY instead of returning null the whole thing just crashed. – Ethan Ma Jul 22 '20 at 00:24
  • Not sure what you mean there, you don't need to change the GL_DYNAMIC_COPY (and GL_READ_ONLY is not a valid value for that anyway). But you do need to replace GL_WRITE_ONLY with GL_READ_ONLY in the glMapBuffer call. – Nathan Reed Jul 22 '20 at 23:26

0 Answers0