GLES20.glVertexAttribPointer/glDrawElements中的"offset"参数是什么,ptr/indices来自哪里？

// 生成顶点缓冲区和索引缓冲区
int[] buffers = new int[2];
GLES20.glGenBuffers(2, buffers, 0);

// 绑定顶点缓冲区并填充顶点数据
GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, buffers[0]);
GLES20.glBufferData(GLES20.GL_ARRAY_BUFFER, vertexData.capacity() * 4, vertexData, GLES20.GL_STATIC_DRAW);

// 绑定索引缓冲区并填充索引数据
GLES20.glBindBuffer(GLES20.GL_ELEMENT_ARRAY_BUFFER, buffers[1]);
GLES20.glBufferData(GLES20.GL_ELEMENT_ARRAY_BUFFER, indexData.capacity() * 2, indexData, GLES20.GL_STATIC_DRAW);

// 启用顶点属性数组
GLES20.glEnableVertexAttribArray(positionHandle);

// 指定顶点属性数据
GLES20.glVertexAttribPointer(positionHandle, 3, GLES20.GL_FLOAT, false, 12, 0);

// 绘制图形
GLES20.glDrawElements(GLES20.GL_TRIANGLES, indexData.capacity(), GLES20.GL_UNSIGNED_SHORT, 0);