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# How to Read Data from a Compute Shader using a UBO?

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#### mattbick2003

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##### How to Read Data from a Compute Shader using a UBO?
« on: July 16, 2018, 06:14:49 »
I am trying to read a ton of floats from a compute shader into a buffer. I simply have no idea how to do this, and I have searched and tried to follow things such as tutorials and other forums. Sadly, I just don't understand how to read my data from my Compute Shader into a FloatBuffer.

`#version 430uniform vec3 startPos;uniform int xSize;uniform int ySize;uniform int zSize;layout(std430, binding = 1) buffer Vals { float vals[];};uniform Vals { uniform float vals[];};layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;vec3 random3(vec3 c) { float j = 4096.0*sin(dot(c,vec3(17.0, 59.4, 15.0))); vec3 r; r.z = fract(512.0*j); j *= .125; r.x = fract(512.0*j); j *= .125; r.y = fract(512.0*j); return r-0.5;}/* skew constants for 3d simplex functions */const float F3 =  0.3333333;const float G3 =  0.1666667;/* 3d simplex noise */float simplex3d(vec3 p) { /* 1. find current tetrahedron T and it's four vertices */ /* s, s+i1, s+i2, s+1.0 - absolute skewed (integer) coordinates of T vertices */ /* x, x1, x2, x3 - unskewed coordinates of p relative to each of T vertices*/ /* calculate s and x */ vec3 s = floor(p + dot(p, vec3(F3))); vec3 x = p - s + dot(s, vec3(G3)); /* calculate i1 and i2 */ vec3 e = step(vec3(0.0), x - x.yzx); vec3 i1 = e*(1.0 - e.zxy); vec3 i2 = 1.0 - e.zxy*(1.0 - e); /* x1, x2, x3 */ vec3 x1 = x - i1 + G3; vec3 x2 = x - i2 + 2.0*G3; vec3 x3 = x - 1.0 + 3.0*G3; /* 2. find four surflets and store them in d */ vec4 w, d; /* calculate surflet weights */ w.x = dot(x, x); w.y = dot(x1, x1); w.z = dot(x2, x2); w.w = dot(x3, x3); /* w fades from 0.6 at the center of the surflet to 0.0 at the margin */ w = max(0.6 - w, 0.0); /* calculate surflet components */ d.x = dot(random3(s), x); d.y = dot(random3(s + i1), x1); d.z = dot(random3(s + i2), x2); d.w = dot(random3(s + 1.0), x3); /* multiply d by w^4 */ w *= w; w *= w; d *= w; /* 3. return the sum of the four surflets */ return dot(d, vec4(52.0));}void main() { vals[gl_GlobalInvocationID.x + gl_GlobalInvocationID.y * xSize + gl_GlobalInvocationID.z * xSize * ySize] = simplex3d(gl_GlobalInvocationID);}`
`package shaders;import java.nio.ByteBuffer;import java.nio.FloatBuffer;import org.lwjgl.BufferUtils;import org.lwjgl.opengl.GL15;import org.lwjgl.opengl.GL20;import org.lwjgl.opengl.GL30;import org.lwjgl.opengl.GL31;import org.lwjgl.opengl.GL33;import org.lwjgl.opengl.GL42;import org.lwjgl.opengl.GL43;import math.Vector3f;public class SimplexNoiseComputeShader extends ComputeShaderProgram { private static final String shaderFile = "src/shaders/SimplexNoiseComputeShader.txt"; private int location_startPos; private int location_xSize; private int location_ySize; private int location_zSize; private boolean isDone = false; public FloatBuffer vals; private int xSize; private int ySize; private int zSize; private int totalSize; private int ubo; public SimplexNoiseComputeShader(Vector3f startPos, int xSize, int ySize, int zSize) { super(shaderFile); this.xSize = xSize; this.ySize = ySize; this.zSize = zSize; totalSize = xSize * ySize * zSize; // TODO Auto-generated constructor stub //Load the start position for our compute shader location_startPos = super.getUniformLocation("startPos"); location_xSize = super.getUniformLocation("xSize"); location_ySize = super.getUniformLocation("ySize"); location_zSize = super.getUniformLocation("zSize"); super.loadVector3f(location_startPos, startPos); super.loadInt(location_xSize, xSize); super.loadInt(location_ySize, ySize); super.loadInt(location_zSize, zSize); //start the compute shader! start(); run(); //ByteBuffer bb = GL30.glMapBufferRange(ssbo, 0, totalSize, GL15.GL_READ_ONLY); //vals = bb.asFloatBuffer(); //clean up the compute shader CleanUp(); isDone = true; } private void run() { GL43.glDispatchCompute(xSize, ySize, zSize); } }`