OpenGLES使用glReadPixels保存并旋转处理一张Bitmap图片，记录一下

1.　必须GLThread线程里调用

fun saveFrame(filename: String, width: Int, height: Int) {
    val startTime = System.currentTimeMillis()
    //1.glReadPixels返回的是大端的RGBA Byte组数，我们使用小端Buffer接收得到ABGR Byte组数
    val buffer: ByteBuffer = ByteBuffer.allocateDirect(width * height * 4).order(ByteOrder.LITTLE_ENDIAN)
    GLES20.glReadPixels(0, 0, width, height, GLES20.GL_RGBA, GLES20.GL_UNSIGNED_BYTE, buffer)
    buffer.rewind()//reset position
    val pixelCount = width * height
    val colors = IntArray(pixelCount)
    buffer.asIntBuffer().get(colors)
    for (i in 0 until pixelCount) {
        val c = colors[i]   //2.每个int类型的c是接收到的ABGR，但bitmap需要ARGB格式，所以需要交换B和R的位置
        colors[i] = c and -0xff0100 or (c and 0x00ff0000 shr 16) or (c and 0x000000ff shl 16) //交换B和R，得到ARGB
    }
    //上下翻转
    for (y in 0 until height / 2) {
        for (x in 0 until width) {
            val temp: Int = colors[(height - y - 1) * width + x]
            colors[(height - y - 1) * width + x] = colors[y * width + x]
            colors[y * width + x] = temp
        }
    }
    //写入文件
    var fos: FileOutputStream? = null
    try {
        fos = FileOutputStream(filename)
        val bmp = Bitmap.createBitmap(colors, width, height, Bitmap.Config.ARGB_8888)
        bmp.compress(Bitmap.CompressFormat.PNG, 90, fos)
        bmp.recycle()
    } catch (ioe: IOException) {
        throw RuntimeException("Failed to write file $filename", ioe)
    } finally {
        try {
            fos?.close()
        } catch (ioe2: IOException) {
            throw RuntimeException("Failed to close file $filename", ioe2)
        }
    }
    val endTime = System.currentTimeMillis()
    Log.d(TAG, "Saved duration:" + (endTime - startTime) + "ms -> " + width + "x" + height + " frame as '" + filename + "'")
}

2.使用方法

glSurfaceView.queueEvent { //run on GLThread
    val filename = Environment.getExternalStorageDirectory().absolutePath + "/fbo_${System.currentTimeMillis()}.png"
    File(filename).createNewFile()
    GLHelper.saveFrame(filename, glSurfaceView.width, glSurfaceView.height)
    runOnUiThread {//run on UIThread
        Toast.makeText(this, "保存成功", Toast.LENGTH_SHORT).show()
    }
}