Where is GLES30.glGetBufferSubData? how to read transform feedback data opengles 3.0

Question

So I am following one of the simplest examples I could find on transform feedback, here, and converting it to Android/Java, but Android seems to be missing the glGetBufferSubData function available in OpenGL ES 3.0.

I've tried searching the Android Source to see if this method is used when calling another method under the JNI, but I am not sure I am even looking in the correct place for that.

If this function doesn't exist how can I read the data out from the transform feedback? I've looked at glGetTransformfeedbackVarying, glGetBufferPointerv and glReadBuffer but none seem to do what I want.

Here is my code:

import com.harmonicprocesses.penelopefree.openGL.MyGLRenderer;
import android.opengl.GLES30;
import android.util.Log;

import java.nio.ByteBuffer;
import java.nio.FloatBuffer;

/**
* Created by izzy on 6/24/15.
*/
public class TransformFeedback {


// Vertex shader
private final String vertexShaderSrc =

        "in float inValue;\n" +
        "out float outValue;\n" +

        "void main() {\n" +
        "    outValue = sqrt(inValue);\n" +
        "}\n";

private final int mProgram;


public TransformFeedback{

    // Compile shader
    int vertexShader = MyGLRenderer.loadShader(GLES30.GL_VERTEX_SHADER,
            vertexShaderSrc);

    // Create program and specify transform feedback variables
    mProgram = GLES30.glCreateProgram();
    GLES30.glAttachShader(mProgram, vertexShader);

    //const GLchar* feedbackVaryings[] = { "outValue" };
    GLES30.glTransformFeedbackVaryings(mProgram, 1, {"outValue"}, GLES30.GL_INTERLEAVED_ATTRIBS);

    GLES30.glLinkProgram(mProgram);
    GLES30.glUseProgram(mProgram);

    // Create VAO
    int[] vao = new int[1];
    GLES30.glGenVertexArrays(1, vao, 0);
    GLES30.glBindVertexArray(vao[0]);

    // Create input VBO and vertex format
    int bufferLength = 5 * 4; //5 floats 4 bytes each
    ByteBuffer bb = ByteBuffer.allocateDirect(bufferLength);
    FloatBuffer data = bb.asFloatBuffer();
    float[] floatData = { 1.0f, 2.0f, 3.0f, 4.0f, 5.0f };
    data.put(floatData);
    data.position(0);


    int[] vbo = new int[1];
    GLES30.glGenBuffers(1, vbo, 0);
    GLES30.glBindBuffer(GLES30.GL_ARRAY_BUFFER, vbo[0]);
    GLES30.glBufferData(GLES30.GL_ARRAY_BUFFER, bufferLength, data, GLES30.GL_STATIC_DRAW);

    int inputAttrib = GLES30.glGetAttribLocation(mProgram, "inValue");
    GLES30.glEnableVertexAttribArray(inputAttrib);
    GLES30.glVertexAttribPointer(inputAttrib, 1, GLES30.GL_FLOAT, false, 0, 0);

    // Create transform feedback buffer
    int[] tbo = new int[1];
    GLES30.glGenBuffers(1, tbo, 0);
    GLES30.glBindBuffer(GLES30.GL_ARRAY_BUFFER, tbo[0]);
    GLES30.glBufferData(GLES30.GL_ARRAY_BUFFER, bufferLength, null, GLES30.GL_STATIC_READ);

    // Perform feedback transform
    GLES30.glEnable(GLES30.GL_RASTERIZER_DISCARD);

    GLES30.glBindBufferBase(GLES30.GL_TRANSFORM_FEEDBACK_BUFFER, 0, tbo[0]);

    GLES30.glBeginTransformFeedback(GLES30.GL_POINTS);
    GLES30.glDrawArrays(GLES30.GL_POINTS, 0, 5);
    GLES30.glEndTransformFeedback();

    GLES30.glDisable(GLES30.GL_RASTERIZER_DISCARD);

    GLES30.glFlush();

    // Fetch and print results
    float feedback[] = new float[5];
    GLES30.glGetBufferSubData(GLES30.GL_TRANSFORM_FEEDBACK_BUFFER, 0, bufferLength, feedback);

    Log.e("TransformFeedback", String.format("%f %f %f %f %f\n", feedback[0], feedback[1], feedback[2], feedback[3], feedback[4]));

}

My final implementation won't need to read the data out necessarily, just use it in the shader on the next pass(feedback). But this really is the first step.

Answer 1

glGetBufferSubData() is not available in ES 3.0, or ES 3.1 for that matter. So there is nothing missing in the Android Java bindings in this case.

The function to use for reading back buffer data in ES 3.0 is glMapBufferRange() . In the C/C++ bindings, this returns a pointer to CPU readable memory containing the buffer content. Don't forget to call glUnmapBuffer() when you're done reading.

In the Android Java bindings, the function returns an object of type Buffer. Since you can't directly read from a buffer, you'll have to cast it to a ByteBuffer first. This is not really documented, but my answer here explains why I believe this type cast is safe: Safe usage of glMapBufferRange() on Android/Java.