CompuServe Thread

Mark, >> Does this mean that for every 16 pixels horizontally there would be 9 pixels vertically? << yes, with a square pixel output device, for example a computer monitor or a frame buffer in square pixel mode. However, with a PAR or D1 or Vista the pixels are not necessarily square, they are rectangular and you have to compensate for this in your calculations.

I find I often get asked about aspect ratio concepts. Why, I don't know. But I'll hit you with the following essay, and see if it helps. (A copy will be uploaded to the file libraries for permanent reference.) -Martin -=-=-=-=-=-=-=-=- Aspect Ratio Primer for 3D Studio Users By Martin Doudoroff There are two interrelated issues regarding aspect ratio which 3D animators face. The first, and most fundamental, is a description of the proportions of the rectangular image. The second, and most technical, describes the proportions of a picture-screen element (the pixel) as produced by a given computer display device. The former case I will refer to as "image aspect ratio", and the latter as "pixel aspect ratio." Image Aspect Ratio —– —— —– Image Aspect Ratio compares the width of an image to its height. Mathematically, this ratio is calculated by dividing the width by the height: w — h Formally, aspect ratio is written in ratio notation, with numerator and denominator separated by a colon, and is left in simple scalar form; either the width is written as a decimal value relative to one, or both numbers are multiplied into an integer fraction (a less intuitive representation for most people). For example, NTSC television standard aspect ratio (which also happens to be the Academy Aperture standard in the film world) is: 1.33:1 (or 4:3) These two equivalent ratios indicate that the NTSC video image is exactly one third wider than it is tall. Television manufacturers do some fairly peculiar things when they design the plastic frame around a tube: a nice exercise is to take a measuring tape and calculate whether your own TV is showing you a 1.33:1 image. Our Sony BVM-1910 broadcast monitor is accurate. (Whew!) Measuring the image area of the BVM, I found that its dimensions to be 15.25" by 11.5". 15.25 1.33 ——- = 1.33 (the same as —— ) 11.5 1 Television in the U.S. has, for practical purposes, always been based on a 1.33:1 image. Film, on the other hand, has never conformed to such specifications. With the somewhat eclectic exception of the projection television, TV's have always been direct-view tubes designed to produce an image based on a standard electrical signal. The ubiquitous tube and its plastic border reflect this standard. Film is an exclusively projection-based medium which is not constrained by electrical signals or glass screens: any image you can expose on the celluloid that fits between its edges and the vertical spacing of the frames can be projected. What's more, one can rearrange the curtains which customarily adorn a movie screen to perfectly matte any aspect ratio you choose to project. Consequently, the image aspect ratio in film is a creative decision. Some common aspect ratios found in film are: 2.35:1 Anamorphic Scope/Panavision/CinemaScope 2.05:1 70mm (maximum frame size with soundtrack) 1.85:1 Academy Standard Flat _________ Widescreen Review, Issue 12, page 30. These ratios are common, but many variations exist. There is no "standard", though the 1.85:1 ratio is statistically the most common. Cinematographers and their directors have the luxury of framing any way they choose, and they do. In film, image aspect ratio is a question of aesthetics. Coming from left field is the new aspect ratio attached to HDTV and Widescreen NTSC technology. For no particularly good reason, video engineers adopted the 1.78:1 (16:9) ratio for use in HDTV, and that ratio is also appearing in DVD (the new digital video compact disc standards proposed in early 1995) and digital film technologies (such as Digital Betacam and Sony's HDVS RGB monitors). This decision is motivated, as in the case of the NTSC standard, to accommodate a direct view tube display which doesn't change shape like a movie screen. Some have criticized the 1.78:1 ratio being adopted for HDTV as ludicrous, since no program material has ever been recorded in this ratio, while a hundred years of motion picture film exists in other aspect ratios and fifty years of television programming exists at 1.33:1. This argument raises an important issue which has faced television from its inception: how to display film images of aspect ratios other than 1.33:1 on a 1.33:1 television tube. The options, of which laserdisc owners are highly conscious, are: 1) Place the entire film frame within the television image and matte the top and bottom (letterbox) or sides (windowbox) accordingly. This approach places black bars across the top and bottom (or sides) and shrinks the image spatially, but it accurately preserves the aspect ratio and composition intended by the director. 2) Place most of the film frame within the television image and apply a more mild letterboxing or windowboxing. This approach is common in "Widescreen" video releases and is a compromise to include more of the frame without shrinking the image as much. 3) "Pan & Scan" the image; this method throws out the original composition of the film entirely, and fills the entire television screen; in this case somebody (usually NOT the director) has to make the decision how to re- frame the film on a shot-by-shot basis. This approach is used in most video releases of films, and causes great consternation among film purists. Letterboxed imagery is becoming more common on television in music videos and some advertisements as programmers look for more "cinematic" aesthetics. Letterboxing is irritating to others, and is certainly not normally accepted in video. If you choose to "letterbox" your animation, you'll find it very easy to do in 3D Studio. For letterboxing, the horizontal resolution is a constant. To calculate the vertical resolution, use the following equation: 1.33 ———————- * [normal y value] [desired aspect ratio] If I wanted to convert CCIR-601 resolution (720 x 480) to a letterboxed image at 1.85:1, then 1.33 / 1.85 * 480 = 345 I would plug in 720 and 345 into the render config dialog. Note that by doing this, 1) I'm trading 28% of my image space for the 1.85 ratio I prefer aesthetically 2) Risking rejection from viewers who dislike letterboxing and broadcasters who don't accept letterboxed material 3) Reducing the maximum byte count of my image files by 28% Pixel Aspect Ratio —– —— —– Pixel aspect ratio is a different ball game. If you apply the aspect ratio formula at the top of the last section to the common device-dependent video resolutions in computer graphics, you'll note that they never divide out to 1.33:1. D-1/CCIR-601 720/480 = 1.5 ATVista 756/486 = 1.56 DPS-PAR 752/480 = 1.57 Targa (16/32/64) 512/480 = 1.07 Nevertheless, the images these devices display (on properly calibrated monitors) have 1.33:1 image aspect ratios. The problem is that the pixels they paint are not square. In the first three examples above, the pixels are horizontally thinner than they are tall; in the case of the Targa display cards, the pixels are wider than they are tall. These characteristics are unique to each device, and they are typically constant: regardless of what resolution image you are displaying on an ATVista, it so happens, the pixel aspect ratio should always be 0.8816. (This includes "letterboxed" images.) Likewise, images for output on Targa display cards require pixel aspect ratios of 1.23, and so on. Note that if you do your math, you'll find that these values STILL don't get you to a perfect 1.33:1 image aspect ratio in mathematical terms. To take the ATVista as an example, (756) * (.8816) ————— = 1.37 486 You can amuse yourself the next time you're on the phone with your manufacturer's technical support by asking about these little discrepancies. Just make sure it's on their dime, since it will probably take them some time to figure it out. 3D Studio has this pixel aspect ratio parameter so that the 3D scene can be rendered "accurately" for any given display device. If you render an image at the wrong aspect ratio for one of these video adapters, the image will be stretched or squashed. Therefore, leave your pixel aspect ratio set appropriately for your output devices. If you are rendering for the computer screen (such as in multimedia), print, or a film recorder, your pixel aspect ratio should always be set to 1. HDTV uses square pixels, I am told, so if that technology enters your life, render at pixel aspect ratio 1 then as well.

David, >> What a great time to be alive ( and in this business) <g> << I'll say. I'm enjoying it greatly 🙂

>>I think it's around 2k.<< really? I thought it would have been around 4k, but what the H*** do I know?

jeez, ask a simple question… (g)

We just finished some shots for a major motion picture using 3DS. We rendered at 972×486. We got away with that because it was supposed to be "virtual reality" and so in fact they wanted to introduce "glitches" so that people would not be confused with "reality". Even at that low res it took a p90 with 192 megs of ram aprox between 20 to 30 min/frame. Considering that the more modest sequences had 700 frames, well, you do the math… I would say that film res is about as subjective as "definition". It has to do with comparisons more than absolutes. In other words, to the viewer a scene shot in 16mm or 35mm will look the same if it is lit properly taking into account contrast ratios and the such. I know I am not referring to a scientifically measured "granularity" but rather to a real world subjective appreciation. I think that the fact that we did this job for the big screen at a more competitive price with equivalent results as if we had use Silicon Graphics-Alias-Wavefront technology shows that PC's are in the border of becoming a familiar tool for the film industry. The crucial step on this process I think is the digital-analog transfer where pixel interpolation and other techniques effectibly double the "resolution" (read -appearance) of the image. ———————- Now for the very low budget homemade pixel-interpolation here goes a tip. Place a filter such as ROSCOE 3040 (powder frost) or LEE 253 (Hampshire frost) against your screen (I assume you have at least a 17 or 21 inch monitor!) and shoot the image frame by frame with a 16mm camera. The filters will diffuse the pixels (interpolation!) just enough so that they will not be visible while at the same time lowering the contrast of the image. The result is very acceptable and it does not have that horrible "video to film transfer look. For correct exposure of a video monitor image check an American Cinematographer manual and use a spot meter to establish your contrast zones. Thanks Alan for your thorough technical responses. Arturo

Hi David, Listen, if you're passing up the chance to work on that film project. Could I learn a little more about it? I'm REALLY interested in film. Thanks! Alex K. NGA