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Why is it called "1:1 pixel mapping" when it isn't?

Discussion in 'Projectors, Screens & Video Processors' started by NicolasB, Mar 10, 2005.

  1. NicolasB

    NicolasB
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    Of course my understanding may be off (again! :rolleyes: ) but it seems to me that when people use the phrase "1:1 pixel mapping" what they usually mean is using an external video scaler (either a dedicated device or an HTPC) to scale an image to a resolution that precisely matches the resolution of the display. But that's not 1:1 mapping in any useful sense!

    Suppose we're playing a PAL DVD on a so-called high-def 42" plasma screen. If we feed the signal straight to the screen, the path is:

    DVD output (576i, analogue)
    -> A/D convertor (inside TV) - signal is now 576i digital
    -> deinterlacer (also inside TV) signal is now 576p digital
    -> scaler (inside TV) - 1024x768p digital
    -> screen

    If we use so-called "1:1 pixel mapping" then the path is:

    DVD output (576i, analogue)
    -> A/D convertor (inside scaler) - signal is now 576i digital
    -> deinterlacer (inside scaler) signal is now 576p digital
    -> scaler (inside scaler) - 1024x768p digital
    -> screen (via DVI)

    In other words, it's exactly the same process! Sure, the external scaler will quite possibly do a much better job of scaling and deinterlacing, but if the second case is no more "1:1" than the first is, why call it that?

    The only time it actually makes sense to talk about "1:1 pixel mapping" is if the source image resolution matches that of the screen. For example, a 720p high-definition signal being displayed on a 1280x720 50" plasma screen, or a 1080i signal on a 1920x1080 LC panel - that could legitimately be called "1:1 mapping".
     
  2. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Nicholas: You presume that if you feed a 720P source to a 720 display it doesn't actually overscan or scale the image......which I think you'll find it probably will unless it has a 1:1 aspect setting

    You are correct in your path listings.
    The whole point of 1:1 in the way folk talk about it here is to allow an extrnal processor to do a better job then for the displays processing to have as little affect on it as possible.

    Gordon
     
  3. soiaf

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    By "1:1" matching I always took that to mean basically getting the display to do the minimum amount of processing - usually meaning getting your scaler (or PC) to do the scaling/deinterlacing etc.

    I've never quite figured out how (and I'd love to know what people are using) people know they've got an exact (or as exact as you're going to get) 1:1 match. My understanding of 1:1 matching (and I'm happy to be corrected here) comprises of
    Step 1 - Matching the native resolution - ok, straightforward enough (well kind of :) )
    Step 2 - 'Fitting in' - By this I mean changing Front Porch, Sync Width settings etc. till the image properly 'fits' the screen
    Step 3 - Getting the exact vertical/horizontal frequencies - so you match the displays pixel clock.

    Step 1 you can 'know', step 2 you can see (black borders etc.), but never quite got step 3, what shows you've got this exact? I imagine you can tell if you've got it really wrong by judder etc. but how about fine-tuning?
    And is this regarded as 'processing' by the display?
     
  4. KraGorn

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    That use of '1:1 mapping' is certainly correct, albeit that as Gordon can tell you the display device still does some processing, the fundamental issue is to avoid the display devices scaler in the presumption that it is inferioir to an external scaler or an HTPC; which is usually the case.

    Apart from using test patterns (as Gordon showed me this afternoon with the Lumagen), with a PC it's very easy to see if you have 1:1 pixel mapping, unaliased black text on a white background clearly shows each pixel of the text occupies exactly one pixel in the display. As soon as you lose 1:1 then unaliased text gets immediately blurred as each input pixel of the font gets smeared over 2 or more pixels.
     
  5. NicolasB

    NicolasB
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    Please note: I am not suggesting that using an external scaler is pointless, I'm merely suggesting it ought not be called "1:1 pixel mapping". :)
     
  6. Liam @ Prog AV

    Liam @ Prog AV
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    I've seen it commonly be understood three slightly different ways.

    1. to describe the process of using a video scaler or PC to send out exactly the same resolution as your display uses. i.e. XGA plasma, PC set to XGA output. This term could (innaccurately?) be used to describe sending 720p component to a 720p projector, or 480p NTSC prog scan to a SD plasma panel.

    2. to describe the process of totally bypassing the video processing within the display by sending an exact resolution from a scaler or PC. Slightly innaccurate as the processing is very, very rarely entirely bypassed.

    3. to describe the process of achieving the most accurate image on a display through feeding the display a resolution which receives the least negative treatment by the displays own processing. This is usually native resolution with customised timings, but is often rescaled again by the display anyway. My understanding of it is if the display processing is carrying out a mathematical equation, it's about giving it the variables (h and V timings) that will create the same answer (resolution) as you want displayed on screen. Every slight change in porch or sync settings (variables), has an affect on the image displayed (result), but only one or two combinations will work.

    Obviously I see 3 as the only accurate meaning of the term since 1 implies 720p from an upscaling DVD player achieves the same as an accurate 1280 x 720 resolution, tweaked to fit a panel resolution (which it doesn't - see Sony HS50 and it's over/under scan fun). 2 I like to highlight as it addresses the myth that somehow the plasma can take a resolution and literally place it onto the panel without process. Which it doesn't.

    Hope that helps any confusion (and doesn't add!)
     
  7. KraGorn

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    While I agree at times there's a bit of confusion as to precisely what's happening electronically, it seems to me in the vast majority of discussions about '1:1 mapping' the intent is to indicate bypassing of the display's scaler insofar as it affects the pixel-for-pixel rendering as I described in relation to unaliased text for example.

    In my experience of 4 projectors when I set my PC to the same resolution as the projector I achieved pixel-for-pixel mapping as evidenced by text display, clearly the scaler in the display wasn't being used, or if it were then it was still keeping pixel alignment which as far as I can see is perfectly derbied as 1:1 mapping.

    Of course, this explains the stricture against using keystone, not because per se keystone adjustment is 'bad' but it destroys the pixel array, introducing softening due to anti-aliasing etc.

    I agree with your statement 2 in that there is always some processing being done, but when discussing PQ IMHO it's the ability to ensure that each input pixel occupies exactly 1 output pixel that's the key issue, this avoids softening caused by scaling operations.
     
  8. Liam @ Prog AV

    Liam @ Prog AV
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    What I want to highlight is that 720p (say from Denon 2910) to a 1280x720 display is not pixel matching, and may continue to be squished and rescaled and incorrectly scanned before being displayed.

    The same when setting your source scaler/PC to native resolution using default timings, this doesn't *always* gaurantee it will be actually matching pixel for pixel even if the picture does appear better than before (which it will on account of the improvement to deinterlacing).

    A simple text based pattern will show if it is or not (as has been your experience Robin), but my experience of some first-time scaler buyers is that they are not always aware of this and the fact that there can be great difficulty in achieving 1:1 (hence the importance of quality back-up, installation etc). There are a few who have applied the default resolution for their display, got a picture and left it at that. An improvement is there but it may not be using the full potential of the setup. None of this applies to my customers of course ;)
     
  9. KraGorn

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    You are of course a lot more experienced than I Liam and so if you say it's so I accept that it's the case, just that in my limited experience I've not see any examples of a display which, when presented with a resolution the same as itself, continued to 'mangle' the image such that the text test shows other than perfect pixel mapping.

    What is it, for example, about the signal presented by the Denon in your example that caused the display continue to fiddle with pixel placement in the display panel?
     
  10. Liam @ Prog AV

    Liam @ Prog AV
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    Many displays will be over or underscanning the signal which is the more common nasty, sending native res but not having it displayed "true" is rare I'll admit. Fujitsu and NEC plasmas will monkey around with 1366x768 resolutions, displaying them seemingly fine until you bring up the pattern (or with a 1" stripe or something), then feed them 1360x768, or 1365 rather than 1366 (different models require different slightly adjusted resolutions) and you eventually lock on. A Panasonic plasma can just about show perfect pixel matching based on test pattern results, from 720p signal fed into a 480p panel!!
     
  11. philipb

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    A simple text based pattern will show if it is or not (as has been your experience Robin), but my experience of some first-time scaler buyers is that they are not always aware of this and the fact that there can be great difficulty in achieving 1:1 (hence the importance of quality back-up, installation etc). There are a few who have applied the default resolution for their display, got a picture and left it at that. An improvement is there but it may not be using the full potential of the setup. None of this applies to my customers of course

    Liam - umm actually yes it does, because that's precisely what I've done with the Cinemateq you supplied. My ignorance I hasten to add, not your negligence.

    So how do I go about getting a pixel by pixel match on my Pioneer 43MXE1 from the Cinemateq, asopposed to what i currently do which is use the default native resolution - 1024x768?
     
  12. Liam @ Prog AV

    Liam @ Prog AV
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    With the MXE1 set to XGA, FRC Mode2 and position adjusted to "fit" the 1024x768 output to the screens actual resolution I think you'll find the default Cinemateq timings give a pixel matched image!! Get yourself a test disc to be sure, PAL DVE or Peter Finzel (German tho). Other plasmas are unfortunately not so simple, NEC for one will display what looks like a pixel matched image from NR (esp 50") until you throw test patterns at it. Hitachi, Panasonic, Fujitsu all like to be difficult. Fine tuning the res would use exactly the same technique as to how you ensured pixel matching from your HCPC setup - get the patterns up, slight adjustments of porch, sync timings etc until you get the result you are after.

    If the default timings aren't pixel matching the Pio drop me a mail/pm and I'll help you through it!
     
  13. philipb

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    Liam

    Thanks for that. I have the DVE disc so I'll see how well the current settings work.
     
  14. fabiorep

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    Dear Liam,
    i have a some problem to setting my crystalio scaler with the imput dvi of plasma nec42xm3,I know only the native resolution (1024*768), i haven't the timing for the correctly setting
    can you help me?
    thank very much!

    best regards

    Fabio repetti
     
  15. dilsher

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    Liam said
    " but my experience of some first-time scaler buyers is that they are not always aware of this and the fact that there can be great difficulty in achieving 1:1 (hence the importance of quality back-up, installation etc). There are a few who have applied the default resolution for their display, got a picture and left it at that. An improvement is there but it may not be using the full potential of the setup. None of this applies to my customers of course"

    I say

    "Thats why I am paying you ya git!!!"

    xxxx
     

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