readpixel multiple grames

IEnumerator TakeScreenshotAndSaveCoroutine(int width, int height, Camera screenshotCamera, string saveToFileName)
    {
        yield return new WaitForEndOfFrame();

        if (width <= 0 || height <= 0)
        {
            yield return null;
        }

        if (screenshotCamera == null) screenshotCamera = Camera.main;

        Texture2D screenshot = new Texture2D(width, height, TextureFormat.RGB24, false);
        RenderTexture renderTex = new RenderTexture(width, height, 24);
        screenshotCamera.targetTexture = renderTex;
        screenshotCamera.Render();
        RenderTexture.active = renderTex;

        RenderTexture current_tex;

        // Saving by one slice each frame
        screenshot.ReadPixels(new Rect(0, 0, width / 4, height), 0, 0);
        yield return null;

        for (int i = 1; i < 4; i++)
        {
            current_tex = RenderTexture.active;
            RenderTexture.active = renderTex;
            screenshot.ReadPixels(new Rect(width * ((float)i / 4), 0, width * ((float)i / 4), height), (int) (width * ((float)i / 4)), 0);
            RenderTexture.active = current_tex;
            screenshotCamera.targetTexture = null;

            yield return null;
        }

        screenshot.Apply(false);
        screenshotCamera.targetTexture = null;

        RenderTexture.active = null;
        Destroy(renderTex);

        yield return 0;

        if (screenshot != null && saveToFileName != null)
        {
            if (Application.platform == RuntimePlatform.OSXPlayer ||
            Application.platform == RuntimePlatform.WindowsPlayer &&
            Application.platform != RuntimePlatform.LinuxPlayer
            || Application.isEditor)
            {
                byte[] bytes;
                if (saveToFileName.ToLower().EndsWith(".jpg"))
                    bytes = screenshot.EncodeToJPG();
                else bytes = screenshot.EncodeToPNG();
                FileStream fs = new FileStream(saveToFileName, FileMode.OpenOrCreate);
                BinaryWriter w = new BinaryWriter(fs);
                w.Write(bytes);
                w.Close();
                fs.Close();
            }
        }

        yield return screenshot;
    }