Aspect Ratio?

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  • #23414
    Cae
    Participant

    Hello everyone. So, I use the OSSC with a 16:9 computer monitor that has two settings for aspect ratio: “aspect,” which preserves the natural aspect ratio of the image by assuming it’s made of square pixels, and “full,” which fills the entire screen. I assumed initially that everything would have the correct aspect ratio if I use “aspect” for most games and that I should use “full” for any game that calls for anamorphic widescreen. (Side note: anything wider than 16:9 will fill the screen both vertically and horizontally no matter what; it treats 240px1 and 480ix1 both as 720×240, 480ix3 as 1440×720, and 1080ix1 as 1920×540, all of which obviously look wrong, except for 1080i, almost by accident.) I’ve noticed a few inconsistencies, however, that make me question whether or not I’m getting the correct aspect ratio for everything. Matching settings as described above, games that support anamorphic widescreen seem to show objects within the image as being slightly wider when I set everything to not take advantage of the widescreen option. I had assumed that 4:3 was achieved by only using 640 pixels out of each 720-pixel-wide line, but maybe that black space simply accounts for overscan on a CRT like the black space at the top and bottom? Aditionally, the nature of the different 240p modes seems wildly inconsistent when it comes to aspect ratio.

    In short, how is the display meant to handle the aspect ratio of each of the following resolutions when being output from the OSSC?

    720×240
    720×480
    1280×720 (specifically as a result of line-triple mode)
    1280×960
    1440×960
    1600×1200
    1920×1200

    It basically boils down to whether 720-pixel-wide resolutions are meant to be distorted into 4:3 or whether they’re meant to contain square pixels (once converted from 15 to 31 kHz if necessary). But I also wouldn’t mind knowing why 240px3, x4, and x5 behave the way they do (as far as aspect ratio) in tandem with everything else.

    #23416
    nmalinoski
    Participant

    Which aspect ratio the image needs to be distorted to depends on whether or not it’s anamorphic widescreen, but, I think, in both cases, you’ll end up with non-square pixels, which makes proper display on LCD/LED panels difficult without something like interpolation. This is why using the Pixel-Perfect mode on the NES and SNES Classic makes games look horizontally squished.

    As for your monitor, it sounds like it’s not really compatible with 240p or interlaced video modes, and a lack of proper aspect ratio controls is an unfortunate reality of cheap monitors (my Asus 22-inchers have zero aspect ratio control–full screen or bust–but my Dell U2410 gives me 5 or 6 options). If that 1080i is from a PS2, from something like Gran Turismo 4, I believe the internal rendering resolution is 540p, so squishing it back down to that probably looks okay.

    Line 3x, 4x, and 5x for 240p default to 1280×720, 1280×960, and 1920×1080 frames respectively, which are all pretty standard PC monitor resolutions, so that may be why your monitor displays them correctly. 240p was never really accepted as a standard in terms of video equipment (it’s considered by some to be a perversion of 480i), and 720×480 is a EDTV thing and isn’t really seen in the PC world.

    #23440
    marqs
    Participant

    In short, how is the display meant to handle the aspect ratio of each of the following resolutions when being output from the OSSC?

    720×240, 720×480, 1440×960: these should be scaled to 16:9 or 4:3 depending whether source is anamorphic widescreen or not. 720px default active width is selected for max compatibility – if your display supports 704 (via adv. timing tweaks), it should result to most correct aspect ratio.

    1280×720, 1280×960 (l3x/l4x generic 4:3): display should treat these as 1:1 pixel aspect ratio. The default settings should be good for correct aspect ratio for 4:3 sources.

    1600×1200, 1920×1200 (l5x generic 4:3): these should be also treated as 1:1 pixel aspect ratio, but default sampling rate is selected for compatibility at the price of slightly wrong aspect ratio. H.samplerate 1950 should give correct aspect ratio but is not supported by many displays.

    #23471
    Cae
    Participant

    That makes a lot of sense of things. Tweaking advanced timing to change 720 to 704 fortunately does work with my monitor, but that does seem to require increasing horizontal backporch in order to re-center the image. Is default +8 the correct value for the backporch in that scenario?

    So if I use 3x or 4x for 240p (or mess with advanced timing for 5x) and adjust horizontal active and backporch for 480i and 480p, I can get the correct aspect ratio for 240p games and widescreen 480i and 480p games, but I’m still stuck only being able to display 4:3 480i/p games as being 10% too wide, unless I get a new monitor. With a bit of research, I’ve found that Asus, Dell, Samsung, and ViewSonic make monitors that provide 4:3 distortion as an option, but a new monitor seems like a big investment over a small error. What would be more helpful is if the OSSC itself had modes for adding blank pixels to the left and right sides of a 480i or 480p signal, just like 240px3 in generic 4:3. I tried achieve something like that using advanced timing, but I didn’t have much luck. Perhaps this is something I should post about in the “feature requests” section?

    One last thing that seems curious to me is that the 240p optimal modes seem to operate around a different set of ideas about how aspect ratio should be handled, at lest for a 240p source. These modes seem to be designed to completely fill a 4:3 frame (or alternatively 8:7 for 256×240), but using the 240p test suite on the Wii, I get a narrower image in 3x and 4x generic than when using 320×240 optimal. If the optimal modes mess up the intended aspect ratio, that’s a bit of a shame, considering how fantastic things look with optimized settings.

    #23480
    Harrumph
    Participant

    One last thing that seems curious to me is that the 240p optimal modes seem to operate around a different set of ideas about how aspect ratio should be handled, at lest for a 240p source.

    Yes, this is by design. The purpose of optimized mode is to accurately digitize the source on a dot-by-dot basis, and output them as-if square pixels. As the source material in almost all cases does NOT have square pixels, the output image is a compromise in regards to image aspect ratio.
    So 320×240 optimized output will normally be too wide, and 256×240 will be too skinny (8:7 mode) or too wide (4:3 mode, and to varying degree depending on line multiplication mode).

    It can be corrected in a second step by your display (if it has advanced controls enough, which most won’t), a secondary scaler, or by capture software. (see another currently active thread on this subject)

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