在Android上用OpenGL绘制圆

我成功地跟踪了 this guide 用OpenGL画一个三角形,现在我试图修改代码来画一个圆。教程没有教我怎么做,所以我试着使用 here ,在页面的“For your toolbox”部分,我采用的方法是在OpenGL中定义一个正方形,然后按照建议 here ,使用片段着色器在正方形内绘制圆。这是我的圆类代码:

package com.example.onwards

import android.content.res.Resources
import android.opengl.GLES30
import java.nio.ByteBuffer
import java.nio.ByteOrder
import java.nio.FloatBuffer

class Circle {
    private val vertexCoords = floatArrayOf(
        -0.5f, 0.5f, 0.0f, // top left
        -0.5f, -0.5f, 0.0f, // bottom left
        0.5f, -0.5f, 0.0f, // bottom right
        0.5f, 0.5f, 0.0f // top right
    )

    private val resolution = floatArrayOf(
        getScreenWidth().toFloat(),
        getScreenHeight().toFloat()
    )

    private val vertexShaderCode =
        """ |attribute vec4 vPosition;
            |void main() {
            |    gl_Position = vPosition;
            |}
            |""".trimMargin()

    private val fragmentShaderCode =
        """ |precision mediump float;
            |
            |uniform vec2 vResolution;
            |
            |float circle(in vec2 st, in float r) {
            |   vec2 dist = st - vec2(0.5);
            |   return 1. - step(r * r, dot(dist, dist));
            |}
            |
            |void main() {
            |   vec2 st = gl_FragCoord.xy / vResolution.xy;
            |   vec3 color = vec3(circle(st, 0.5));
            |   // TODO: Set alpha to 0 outside of circle
            |   gl_FragColor = vec4(color, 1.);
            |}
            |""".trimMargin()

    private val vertexBuffer: FloatBuffer =
        ByteBuffer.allocateDirect(vertexCoords.size * 4).run {
            order(ByteOrder.nativeOrder())
            asFloatBuffer().apply {
                put(vertexCoords)
                position(0)
            }
        }

    private val vertexCount: Int = vertexCoords.size / COORDS_PER_VERTEX
    private val vertexStride: Int = COORDS_PER_VERTEX * 4

    private var program: Int
    private var positionHandle: Int = 0

    init {
        val vertexShader: Int = loadShader(GLES30.GL_VERTEX_SHADER, vertexShaderCode)
        val fragmentShader: Int = loadShader(GLES30.GL_FRAGMENT_SHADER, fragmentShaderCode)

        program = GLES30.glCreateProgram().also {
            GLES30.glAttachShader(it, vertexShader)
            GLES30.glAttachShader(it, fragmentShader)
            GLES30.glLinkProgram(it)
        }
    }

    private fun loadShader(type: Int, shaderCode: String): Int {
        return GLES30.glCreateShader(type).also { shader ->
            GLES30.glShaderSource(shader, shaderCode)
            GLES30.glCompileShader(shader)
        }
    }

    private fun getScreenWidth(): Int {
        return Resources.getSystem().displayMetrics.widthPixels
    }

    private fun getScreenHeight(): Int {
        return Resources.getSystem().displayMetrics.heightPixels
    }

    fun draw() {
        GLES30.glUseProgram(program)

        positionHandle = GLES30.glGetAttribLocation(program, "vPosition").also {
            GLES30.glEnableVertexAttribArray(it)
            GLES30.glVertexAttribPointer(
                it,
                COORDS_PER_VERTEX,
                GLES30.GL_FLOAT,
                false,
                vertexStride,
                vertexBuffer
            )

            GLES30.glGetUniformLocation(program, "vResolution").also { resHandle ->
                GLES30.glUniform2fv(resHandle, 1, resolution, 0)
            }

            GLES30.glDrawArrays(GLES30.GL_TRIANGLES, 0, vertexCount)
            GLES30.glDisableVertexAttribArray(it)
        }
    }
}

但是,运行这个应用程序只会在屏幕中央产生一个白色三角形。它的顶点在左上角、左下角和右下角,所以OpenGL似乎只绘制了正方形所需的一个三角形。

我注意到 drawOrder 和 drawListBuffer 变量来自 this section 在本教程的下一部分中从未使用过;它们只专注于绘制三角形而不是正方形。也许这和它有关?