Vad betyder pixelmappa?

Pixel mapping explained

The Digital Video Path
In order to understand the concept of pixel mapping it is necessary to understand the path taken by the video from the source to the display. Typical sources are analogue PAL TV, digital satellite, cable or terrestrial TV(DVB-S, DVB-C, DVB-T respectively) and DVD. The only non-digital example mentioned is analogue TV which is digitized in a TV tuner before being passed on. Up until recently all of these sources typically yielded a video signal where each individual image was made up of a 720 pixels times 576 pixels. 720x576 is called the resolution of the image. As a remnant of old times most video signals are interlaced, meaning that a single frame of video consists of a halfimage, containing only even or odd lines. Interlacing is a bit besides the point of pixel mapping but there is an excellent explanation of it here. In PAL a TV/DVD signal is 50 half-images per second. In a HTPC setup the video is de-interlaced to produce 25 full images per second, called a progressive video signal. Since we want to view the video fullscreen it is necessary to scale it in order to make each frame the same resolution as the 'desktop' on the computer. Furthermore its important to note that each scaling step causes artifacts and decreases overall image quality. Below is shown a very popular test pattern used for testing pixel mapping. The first image is the original, the second has been scaled up by a factor of 1.33(same factor as PAL is scaled horizontally on a 1366x768 desktop) using the best scaling method available in GIMP(scaling artifacts on real TVs doesn't look like this but the principle is the same, real examples are shown further down).

I hope its obvious that scaling is not something you want to repeat too often! However, assuming that the desktop resolution matches the display panel this scaling step must take place at some point in the video path, unless you want to only use a very small part of the display panel.

In the next step the resulting digital image is transmitted to the HDTV over for example DVI->HDMI. What happens now is entirely up to the HDTV. Since the images sent match the resolution of the HDTV display panel, the image should theoretically display fine by using one panel pixel for each image pixel. However, for various reasons a HDTV might do any of the following:

Perform digital zoom overscan and other non 1:1 mappings
Because some video signals contain image data which is not meant to be seen (Text TV data for example), the TV might decide to hide approx. 5% of each image edge. The digital zoom approach to overscanning involves scaling the incoming video signal. Lets try to the same to already scaled image show above. It now looks like this:

Notice that new scaling artifacts have been introduced compounding the effects of the previous scaling. Even though the TV doesn't overscan it might still scale the image for various reasons resulting in something very similar to the above. One typical problem is the signal processing chip in the TV which will try to 'improve' the video signal, causing some interesting problems with for example edge enhancement/sharpness. Another problem is that the video card might be forced to output an image that is slightly smaller or slightly larger than the physical panel resolution, in which case a good display will support under/overscanning it, while a bad one will always scale it to fit.

Refuse the signal - "No signal" message
In this case the TV might refuse the TV signal. This can happen with 1366x768 resolutions because they are not HDTV standard resolutions. It shouldn't happen with 1280x720 and 1920x1080 resolutions. But note that even with a 1920x1080p output you might still suffer from the above effects.

But how does this look with normal images? Surely the above test pattern is made to make the worst of it? Video is generally much less affected by scaling, while its quite apparent on a computer desktop. For examples of the effect of incorrect pixel mapping on text see this . Another example showing an edge enhancing effect induced by the TV on the Windows start menu can be seen here .

But lets take a look at a real life image of the London tower bridge. The first image is the original image, the other has been upscaled 10% and then back again (2 scaling steps). The differences are much more subtle than with the test pattern but note the loss of sharpness and detail in the image( compare for example the bricks in the foundation, the water etc ).

To be fair its possible to counter some of the fuzziness shown above by applying some artificial sharpness to the image. Doing that on the scaled image results in this:

At first glance this image looks much better, the details seem to have been recovered. However if you look closer you will find several edge enhancement artifacts. For example, take a look at the houses in the background or the blue rails leading to the first tower from the right. Furthermore such sharpening can disturb any video filters you HTPC might have applied( for example using Nvidias PureVideo or FFDShow). (NOTE: THIS IMAGE NEEDS TO BE REPRODUCED. JPEG ARTIFACTING INHIBITS PROPER DEMONSTRATION!)

I hope this demonstrates successfully that pixel mapping IS important! But as the above sharpened image shows, the effect is limited in video/film. If you have no intention of using the HTPC for displaying anything other than video, you should still look for a HDTV that supports 1:1 pixel mapping, but you shouldn't compromise too much on overall image quality to get it (that is, do not buy a bad HDTV just because it supports 1:1 pixel mapping). If you plan to surf and similar, you will have to judge for yourself if scaling is acceptable.

Can the problem be avoided in any way on non-1:1 displays?
Often this is possible, though it tends to be very impractical. With 720x576i/p (PAL) material, one option is to have a 720x576 desktop, effectively delaying the scaling and leaving it to the HDTV. This introduces only the one essential scaling step. Of course its annoying and complicated to automatically change resolution depending on the video you play, but possible.

However if you are trying to display 1280x720p material on a 1280x720p HDTV or 1920x1080p on a 1920x1080p HDTV no scaling is needed at all, but the TV will still scale the input from the HTPC!

Does all this evil scaling also happen with my stand-alone DVD player?
That depends. A DVD player connected over SCART will output 576i/p, leaving the scaling to the TV as described above. But a more expensive upscaling DVD player might actually suffer from this. (of course the scaler in the DVD might be so much better than the one in the TV that result might actually be better because the second scaling is small). HD-DVD and Blu-Ray player could very well be affected by this, which is another reason to buy a 1:1 pixel mapping TV if you are shopping for a 1280x720 or 1920x1080 display!

How can i test if i have 1:1 pixel mapping?
There is a test program here here which can help detect pixel mapping problems as well as other display issues. You can also get the above test pattern along with instructions at this location: http://archive.avsfo...&threadid=90884 .

Why doesn't all TV's support 1:1 pixel mapping?
First off, if the TV resolution doesn't match a standard HDTV resolution (such as 720p/1080p), it might not accept its own native resolution, but only HDTV standard resolutions.

Furthermore many LCD TV's have a resolution of 1366x768, which is a resolution that many PC display adapters have a problem outputting because 1366 is not a multiple of 8 ( for an explanation http://forums.entech...wtopic.php?t=20 ). However, most Nvidia cards and the recent ATI X1k series are capable of outputting 1366x768 over DVI-HDMI(DVI-HDMI being more future proof than VGA.

If you card doesn't support 1366x768 you can either replace it or shoot for 1360x768(underscan) or 1368x768(overscan). It is possible that TV's might reject or scale 1360x768 while still accept 1366x768 1:1.

Finally many TV manufacturers frankly seem not to care, some even putting in signal processing units that perform multiple scaling steps no matter what.

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