问题
this question is similar to something I asked here Android OpenGL ES 2: Introduction to VBOs however I tried multiple aproaches since then and I still haven't succeeded, so I think posting another question where I offer aditional details would be a better aproach.
I am new to OpenGL ES 2 on Android (I have never worked with another OpenGL, I just need to draw something for an app I am developing for Android) and I would very much like to understand how to use VBOs. I tried to modify this OpenGL ES 2 for Android tutorial to use VBOs when drawing the triangle. I tried to use this step by step guide and this tutorial but I still don't understand everything, I am rather new to all of these things. My app currently crashes on start. Here's what I have:
public class Triangle {
private final String vertexShaderCode =
// This matrix member variable provides a hook to manipulate
// the coordinates of the objects that use this vertex shader
"uniform mat4 uMVPMatrix;" +
"attribute vec4 vPosition;" +
"void main() {" +
// the matrix must be included as a modifier of gl_Position
// Note that the uMVPMatrix factor *must be first* in order
// for the matrix multiplication product to be correct.
" gl_Position = uMVPMatrix * vPosition;" +
"}";
private final String fragmentShaderCode =
"precision mediump float;" +
"uniform vec4 vColor;" +
"void main() {" +
" gl_FragColor = vColor;" +
"}";
private final FloatBuffer vertexBuffer;
private final int mProgram;
private int mPositionHandle;
private int mColorHandle;
private int mMVPMatrixHandle;
private final int buffer[] = new int[1];
// number of coordinates per vertex in this array
static final int COORDS_PER_VERTEX = 3;
static float triangleCoords[] = {
// in counterclockwise order:
0.0f, 0.622008459f, 0.0f, // top
-0.5f, -0.311004243f, 0.0f, // bottom left
0.5f, -0.311004243f, 0.0f // bottom right
};
private final int vertexCount = triangleCoords.length / COORDS_PER_VERTEX;
private final int vertexStride = COORDS_PER_VERTEX * 4; // 4 bytes per vertex
float color[] = { 0.63671875f, 0.76953125f, 0.22265625f, 0.0f };
/**
* Sets up the drawing object data for use in an OpenGL ES context.
*/
public Triangle() {
// initialize vertex byte buffer for shape coordinates
ByteBuffer bb = ByteBuffer.allocateDirect(
// (number of coordinate values * 4 bytes per float)
triangleCoords.length * 4);
// use the device hardware's native byte order
bb.order(ByteOrder.nativeOrder());
// create a floating point buffer from the ByteBuffer
vertexBuffer = bb.asFloatBuffer();
// add the coordinates to the FloatBuffer
vertexBuffer.put(triangleCoords);
// set the buffer to read the first coordinate
vertexBuffer.position(0);
// First, generate as many buffers as we need.
// This will give us the OpenGL handles for these buffers.
GLES20.glGenBuffers(1, buffer, 0);
// prepare shaders and OpenGL program
int vertexShader = MyGLRenderer.loadShader(
GLES20.GL_VERTEX_SHADER, vertexShaderCode);
int fragmentShader = MyGLRenderer.loadShader(
GLES20.GL_FRAGMENT_SHADER, fragmentShaderCode);
mProgram = GLES20.glCreateProgram(); // create empty OpenGL Program
GLES20.glAttachShader(mProgram, vertexShader); // add the vertex shader to program
GLES20.glAttachShader(mProgram, fragmentShader); // add the fragment shader to program
GLES20.glLinkProgram(mProgram); // create OpenGL program executables
}
/**
* Encapsulates the OpenGL ES instructions for drawing this shape.
*
* @param mvpMatrix - The Model View Project matrix in which to draw
* this shape.
*/
public void draw(float[] mvpMatrix) {
// get handle to fragment shader's vColor member
mColorHandle = GLES20.glGetUniformLocation(mProgram, "vColor");
// Set color for drawing the triangle
GLES20.glUniform4fv(mColorHandle, 1, color, 0);
// get handle to shape's transformation matrix
mMVPMatrixHandle = GLES20.glGetUniformLocation(mProgram, "uMVPMatrix");
MyGLRenderer.checkGlError("glGetUniformLocation");
// get handle to vertex shader's vPosition member
mPositionHandle = GLES20.glGetAttribLocation(mProgram, "vPosition");
//these I don't fully understand
GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, buffer[0]);
GLES20.glBufferData(GLES20.GL_ARRAY_BUFFER,vertexBuffer.capacity() * 4,vertexBuffer,GLES20.GL_STATIC_DRAW);
// Add program to OpenGL environment
GLES20.glUseProgram(mProgram);
// Enable a handle to the triangle vertices
GLES20.glEnableVertexAttribArray(mPositionHandle);
// Prepare the triangle coordinate data
GLES20.glVertexAttribPointer(
mPositionHandle, COORDS_PER_VERTEX,
GLES20.GL_FLOAT, false,
vertexStride, 0);
// Apply the projection and view transformation
GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mvpMatrix, 0);
MyGLRenderer.checkGlError("glUniformMatrix4fv");
// Draw the triangle
GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount);
// //is this still necesary? or do i have to use glDeleteBuffers?
// GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, 0);
}
}
when I put 0 instead of vertexBuffer inside glVertexAttribPointer() I get an error saying I don't provide the necesarry parameter: expected parameter: ptr: java.nio.Buffer; actual arguments: 0(int)
回答1:
The transition to the VBO can be a bit strange due to the data pointer usage.
From a quick inspection your main issue is in
GLES20.glVertexAttribPointer(
mPositionHandle, COORDS_PER_VERTEX,
GLES20.GL_FLOAT, false,
vertexStride, vertexBuffer);
as this should be
GLES20.glVertexAttribPointer(
mPositionHandle, COORDS_PER_VERTEX,
GLES20.GL_FLOAT, false,
vertexStride, 0);
So about this buffers: A VBO is a custom buffer on the GPU generally used and optimised to store the vertex data directly to the GPU. The performance you gain by doing so is that the vertex data do not need to be copied to the GPU on every draw call.
These buffers are still custom and on generating them all you need to set is their size. I see you are using factor *4 on the vertex count assuming a float value has a size of 4 bytes, this is not the best idea since that might not always be true. If possible always try to use some form of "sizeOf". Anyway your buffer is created correctly and data are sent to it.
After the data are sent to the VBO you should keep them there until you need them. That means you generally create a single VBO per unique object (a square for instance) and then just hold its ID. Whenever you wish to draw it you just simply bind the buffer and draw as you did. In other words the buffer should never be created in the draw method. What you did there is a memory leak as well since you are responsible for releasing the buffer by calling delete once the buffer is no longer needed.
So about your pointer issue on glVertexAttribPointer: There are 2 ways to use this method. Without the VBO the last parameter is the pointer to the data on your CPU. With the VBO you need to set that as a relative pointer inside the VBO. That means when VBO is bound the beginning of the buffer would be NULL (0), you might even need to typecast that value. For other positions in the buffer you need to manually calculate them.
And the code you posted specifically:
// First, generate as many buffers as we need.
// This will give us the OpenGL handles for these buffers.
final int buffer[] = new int[1];
GLES20.glGenBuffers(1, buffer, 0);
//these I don't fully understand
GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, buffer[0]);
GLES20.glBufferData(GLES20.GL_ARRAY_BUFFER,vertexBuffer.capacity() * 4,vertexBuffer,GLES20.GL_STATIC_DRAW);
This all goes into some load time and have a reference to the buffer[] beside that you should add GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, 0); what this call does is unbind the buffer. This is not something you NEED to do but it is best that you do so you have no confusion what buffer is bound if any.
Before the glVertexAttribPointer is called you need to bind your buffer. Then set the last parameter as described above.
After you are done using this buffer (done drawing) you should (again not necessary) unbind the buffer.
来源：https://stackoverflow.com/questions/25053786/opengl-es-2-on-android-how-to-use-vbos