user1677: I have some 1080p content and a 1080p display.
Currently, if I play 720 content, it's not as sharp on the display, obviously, as it is a lesser resolution.
Will this be the same issue if I relay my 1080p content on to a 4K display?
Or will I still get the same crispness of the 1080p content as it is on the 1080p display?

user1038: Obviously, it depends a lot on the quality of the displays. A high-quality 720p display may actually give better result than a bad 1080p one, and likewise a bad 4k display isn't that much use!
But assuming the quality is "the same" (though I'm not sure there is a good way to compare this objectively), 1080p footage will look at least as good on 4k display as it does on the 1080p one. In doubt, it can just upsample the footage, that's a lossless operation (in theory) and may in fact already improve the picture, since it's doesn't have the 1080p's pixel grainyness. Likely, it will also do some clever sharpening, which actually gives a "crisper" result, though that may well seem unnatural.
Remember that high-definition TV sets were on the market well before much HD material was actually widely obtainable, and even then it was a substantial quality improvement over the old standards.

user2836: It depends on which resolution of 4k your display supports.  Displays have what is known as a native resolution.  Even when you increase the number of pixels significantly, if it isn't an even multiple of both horizontal and vertical resolution as well as frame rate, then the output video must be interpolated to create pixels that fill in the gaps.  This results in distortion and softening of the image as well as potentially producing artifact from trying to guess at what the pixels in between should be.
If the screen is an exact multiple, then it can simply use 4 pixels for each of the previous pixels and there are no impacts on sharpness.
That said, as leftaroundabout mentioned, there are also far more details than simple sharpness that "crisp" could refer to.  Contrast ratio and overall brightness also contribute to how crisp an image looks and that varies based on the quality of the TV itself, independent of resolution.

1677: <p>I have some 1080p content and a 1080p display.<br>
Currently, if I play 720 content, it's not as sharp on the display, obviously, as it is a lesser resolution.</p>

<p>Will this be the same issue if I relay my 1080p content on to a 4K display?<br>
Or will I still get the same crispness of the 1080p content as it is on the 1080p display?</p>
 1038: <p>Obviously, it depends a lot on the quality of the displays. A high-quality 720p display may actually give <em>better</em> result than a bad 1080p one, and likewise a bad 4k display isn't that much use!</p>

<p>But assuming the quality is "the same" (though I'm not sure there is a good way to compare this objectively), 1080p footage will look at least as good on 4k display as it does on the 1080p one. In doubt, it can just upsample the footage, that's a lossless operation (<a href="http://en.wikipedia.org/wiki/Shannon-Nyquist_sampling_theorem" rel="nofollow">in theory</a>) and may in fact already improve the picture, since it's doesn't have the 1080p's pixel grainyness. Likely, it will also do some clever sharpening, which actually gives a "crisper" result, though that may well seem unnatural.</p>

<p>Remember that high-definition TV sets were on the market well before much HD material was actually widely obtainable, and even then it was a substantial quality improvement over the old standards.</p>
 2836: <p>It depends on which resolution of 4k your display supports.  Displays have what is known as a native resolution.  Even when you increase the number of pixels significantly, if it isn't an even multiple of both horizontal and vertical resolution as well as frame rate, then the output video must be interpolated to create pixels that fill in the gaps.  This results in distortion and softening of the image as well as potentially producing artifact from trying to guess at what the pixels in between should be.</p>

<p>If the screen is an exact multiple, then it can simply use 4 pixels for each of the previous pixels and there are no impacts on sharpness.</p>

<p>That said, as leftaroundabout mentioned, there are also far more details than simple sharpness that "crisp" could refer to.  Contrast ratio and overall brightness also contribute to how crisp an image looks and that varies based on the quality of the TV itself, independent of resolution.</p>
