Just the Pixel Qi. In indirect light, the contrast ratio is much lower than, say, a Kindle, which is itself much lower than paper. So unless the sun is directly

Just the Pixel Qi. In indirect light, the contrast ratio is much lower than, say, a Kindle, which is itself much lower than paper. So unless the sun is directly on it, it's harder for sun-adjusted eyes to read than a Kindle, or than it is for indoors-adjusted eyes to read a normal backlit LCD.