The buffer allocated is insufficient when encoding to Rgba16 UWP

Question

I am encoding a Canvas control which contains Textblocks as child using the following code but I get

The buffer allocated is insufficient

My code

using(InMemoryRandomAccessStream ras = new InMemoryRandomAccessStream())
{
    var displayInformation = DisplayInformation.GetForCurrentView();
    var renderTargetBitmap = new RenderTargetBitmap();
    await renderTargetBitmap.RenderAsync(Textify.CanvasControl);
    var width = renderTargetBitmap.PixelWidth;
    var height = renderTargetBitmap.PixelHeight;
    IBuffer textBuffer = await renderTargetBitmap.GetPixelsAsync();
    byte[] pixels = textBuffer.ToArray();

    //Encode text to PNG
    var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, ras);

    encoder.SetPixelData(BitmapPixelFormat.Rgba16,
                         BitmapAlphaMode.Premultiplied,
                         (uint)width, 
                         (uint)height,
                         displayInformation.LogicalDpi,
                         displayInformation.LogicalDpi,
                         pixels);

    await encoder.FlushAsync();
...

I know it's the byte[] pixels buffer that is no large enough as I want to encode to BitmapPixelFormat.Rgba16. I do not encounter the error when encoding to BitmapPixelFormat.Rgba8 or BitmapPixelFormat.Bgra8

I tried calculating the buffer using the following but it only produce empty white bitmap.

byte[] pixels = new byte[16 * width * height];

int index = 0;
for (int y = 0; y < height; ++y)
    for (int x = 0; x < width; ++x)
    {
        pixels[index++] = 255;  // B
        pixels[index++] = 255;  // G
        pixels[index++] = 255;  // R
        pixels[index++] = 255;  // A
    }

I want to use BitmapPixelFormat.Rgba16 because I want to improve the image quality. How to do this properly? Thanks.