Golang OpenGL元素缓冲区对象无法正确呈现

Question

I'm trying to use an element buffer object to render a simple rectangle in Golang using the go-gl bindings for OpenGL. The following is the main code:

package main

import (
    "gogame/shaders"
    "runtime"

    "github.com/go-gl/gl/v4.1-core/gl"
    "github.com/go-gl/glfw/v3.2/glfw"

    log "github.com/sirupsen/logrus"
)

var vertices = []float32{
    -0.5, -0.5, 0.0,
    -0.5, 0.5, 0.0,
    0.5, 0.5, 0.0,
    0.5, -0.5, 0.0,
}

var rectangle = []uint{
    0, 1, 2,
    2, 3, 0,
}

func init() {
    runtime.LockOSThread()
}

func main() {
    window := initGLFW()
    defer glfw.Terminate()

    program := initOpenGL()

    var vertexBuffer uint32
    var elementBuffer uint32
    var vertexArray uint32

    gl.GenBuffers(1, &vertexBuffer)
    gl.GenBuffers(1, &elementBuffer)
    gl.GenVertexArrays(1, &vertexArray)

    gl.BindVertexArray(vertexArray)

    gl.BindBuffer(gl.ARRAY_BUFFER, vertexBuffer)
    gl.BufferData(gl.ARRAY_BUFFER, 4*len(vertices), gl.Ptr(vertices), gl.STATIC_DRAW)

    gl.BindBuffer(gl.ELEMENT_ARRAY_BUFFER, elementBuffer)
    gl.BufferData(gl.ELEMENT_ARRAY_BUFFER, 4*len(rectangle), gl.Ptr(rectangle), gl.STATIC_DRAW)

    gl.VertexAttribPointer(0, 3, gl.FLOAT, false, 0, nil)
    gl.EnableVertexAttribArray(0)

    gl.BindBuffer(gl.ARRAY_BUFFER, 0)
    gl.BindVertexArray(0)

    if err := gl.GetError(); err != 0 {
        log.Error(err)
    }

    for !window.ShouldClose() {
        gl.ClearColor(0.5, 0.5, 0.5, 0.5)
        gl.Clear(gl.COLOR_BUFFER_BIT)
        gl.Clear(gl.DEPTH_BUFFER_BIT)
        gl.UseProgram(program)

        gl.BindVertexArray(vertexArray)
        gl.DrawElements(gl.TRIANGLES, 6, gl.UNSIGNED_INT, gl.PtrOffset(0))
        //gl.DrawArrays(gl.TRIANGLES, 0, 3)

        glfw.PollEvents()
        window.SwapBuffers()

        if err := gl.GetError(); err != 0 {
            log.Error(err)
        }
    }

    glfw.Terminate()
}

Theoretically, this should draw a rectangle. However, this is what I get when running it in Wireframe mode:

OpenGL Window

The code I left out is just to compile very basic shaders and initialize GLFW and OpenGL.

Answer 1

You are probably using a 64-bit operating system. At a 64-bit operations system the data type uint has a size of 64 bits. See Go language data types or A Tour of GO - Basic types.

The coordinates of the rectangle ar specified like this:

 1: -0.5, 0.5             2: 0.5, 0.5
             x-----------x
             |           |
             |           |
             |           |
             |           |
             x-----------x
0: -0.5, -0.5             3: 0.5, -0.5

The indices array is a array of 64 bit integer values:

var rectangle = []uint{
    0, 1, 2,
    2, 3, 0,
}

But it is treated as an array of 32 bit integers, when the geometry is draw (gl.UNSIGNED_INT):

gl.DrawElements(gl.TRIANGLES, 6, gl.UNSIGNED_INT, gl.PtrOffset(0))

This causes that each index of the array is splitted to 2 indices, which 32 bit each, where the 1st value is the index of the array and the 2nd is 0:

[0, 0,   1, 0,   2, 0,   2, 0,   3, 0   0, 0]

So the first 2 triangles (first 6 indices) are

0 - 0 - 1
0 - 2 - 0

In the image you can see this 2 triangles, which are narrowed down to 2 lines, because 2 points of each triangle are equal.

Use the data type uint32 to solve the issue:

var rectangle = []uint32{
    0, 1, 2,
    2, 3, 0,
}