Resizing difficulties – Convert 300dpi images to 72dpi to increase pixel aspect ratio size

[David_Bryant]

My objective:
Convert 300dpi images to 72dpi to increase pixel aspect ratio proportionately

Back in the early days of digital video editing it was frequently necessary to convert print images to video resolution (for example, 300dpi to 72dpi). The math involved calculating the size percentage (300/72 = 4.1) and then multiplying the dimensions of the image by the same percentage (640 x 4.1 = 2624, and 480 x 4.1 = 1968).

This method was extremely useful when dealing with archival sources that varied in resolution from 300dpi to 600dpi, especially when importing into After Effects for photo animations. But something has changed in Photoshop or I'm not remembering what I used to do and, after dedicating an hour to it, I can't repeat the process I remember using in Image Resize that did this automatically.

The project:
I currently have several thousand photos to be used in an animation that were shot on a still camera at 300ppi. Their current aspect ratio is 2992x2000. By my calculations, when converted to 72dpi the pixel aspect ratio should change to 12,267x8000. But photoshop insists on resizing them to 718x480. 

How do I simultaneously convert from 300 to 72 and see the aspect ratio increase rather than decrease? 

If anyone is wondering why I don't just work with the 300ppi sources it is because I want to downsize them to 4K for a 2K project while also setting up a scenario where scaling functions at 1:1 in Premiere and After Effects. I want to do this to insure I don't scale beyond 100% if we do any resizing in editorial and compositing.

[melissapiccone]

Why do you want to upscale that much? You are going to lose quality - a lot of quality. That is a huge file size. Dpi literally has nothing to do with pixel size, by the way. If you have resample checked, PS will resize your image - change the pixels. If you uncheck resample, it will just change the ppi (dpi is out of date). If you really want them that large, you can just type in the pixel size and turn off resample. 

[D Fosse]

You're working from a long list of wrong assumptions, so your conclusions are off on the wrong track.

This is very much simpler than you think.

Ppi, pixels per inch, doesn't apply on screen. Disregard it completely.

On screen, the image pixels align to the physical screen pixels. If your image is 1920 x 1080 pixels, and it's displayed on a standard HD screen of 1920 x 1080 screen pixels - then the image fills the screen edge to edge.

If the image is 960 x 540 pixels, it fills exactly a quarter of the screen. See the direct relationship? Image pixels to screen pixels.

Whether that image is 300 ppi, 34 ppi or 25786 ppi makes no difference whatsoever. It doesn't apply. Ppi is a print parameter, a measure of pixel density on paper. The paper doesn't have a native pixel grid, so one has to be invented. That's what ppi is.

[Conrad_C]

This is a challenging question, because there are multiple ways to end up really confused…

There is no “72 dpi” for video. “Dots per inch” or “pixels per inch” both require that inches are the final unit of measure, but for video, inches are not a fixed value like print: The same number of pixels end up with a different ppi value depending on whether your screen is 6, 16, or 60 inches diagonal.

Your stated goal is 100% scaling in Premiere and After Effects. In video, 100% scaling actually means one image pixel shown for each display pixel, and 100% scaling is always relative to your frame size in pixels. For 16:9 1080, full frame 100% = 1920 x 1080 pixels. For 16:9 4K, full frame 100% = 3840 x 2160 pixels. PPI and DPI obviously have nothing to do with this, it’s all about the frame size in pixels.

The second point of potential confusion is aspect ratio. You keep saying “pixel aspect ratio” but I suspect you are really talking about “frame aspect ratio.” For the most common current digital video standards, the pixel aspect ratio is square (1:1). If you are not talking about that, because you are working with the aspect ratio of the entire picture, then you are talking about frame aspect ratio. For example, if you are working with standard 2K or 4K and you start using the Photoshop commands on the View > Pixel Aspect Ratio submenu, the images would become totally screwed up, because your concern is not pixel aspect ratio, it’s frame aspect ratio.

Let’s walk through all of this:

Your original is 2992 x 2000 pixels. That equals a frame aspect ratio of 3:2 (or 1.5, depending on how you like to express it). 3:2 is expected if the frames were shot as stills with an interchangeable lens camera, since most of those have a 3:2 aspect ratio sensor. If you want that to fit a standard 4K or 2K HD frame which has a frame aspect ratio of 16:9, it doesnt fit, 3:2 has different proportions. You have several choices:

Option 1: Fill the 16:9 frame completely, but the top and bottom of the 3:2 image must extend beyond the top and bottom of the 16:9 frame and will not be visible. To achieve 100% at 4K, the correct Image Size for this option is 3840 x 2567 pixels.

Option 2: Fit the 3:2 image completely within the 16:9 frame so that the entire 3:2 image is visible, nothing is lost. However, this requires black bars or some other background along the left and right sides that cannot filled by the narrower 3:2 image. The correct 4K Image Size for this option is 3231 x 2160 pixels.

Option 3: Stretch the 3:2 image (scale the X dimension) to fill the wider 16:9 frame exactly. Obviously the disadvantage here is that the stretched image is visually distorted, which is typically considered unacceptable. But if you want this, start with the Option 2 size, but in Image Size, disable proportional scaling and enter 3840 x 2160 px.

Option 4: Use Option 1 and in the video editor, animate the Y axis to pan the image vertically, so that the entire image can be shown over time, just not all at once.

Important: All of the above assume you are aiming at 2160p 4K (i.e., the UHDTV1 standard). If your project actually targets one of the other 4K frame sizes, adjust your frame size accordingly.

[TheDigitalDog]

The "DPI metadata" has nothing to do with the number of actual pixels or aspect ratio of your images and best be ignored. What you always want to do is work with pixels themselves over the length and height. This is very old, but still applies to the basics of this difference:

http://digitaldog.net/files/Resolution.pdf

[Bojan Živković]

"I currently have several thousand photos to be used in an animation that were shot on a still camera at 300ppi. Their current aspect ratio is 2992x2000. By my calculations, when converted to 72dpi the pixel aspect ratio should change to 12,267x8000. But photoshop insists on resizing them to 718x480. "

Wrong calculation. When you change 300ppi to 72ppi pixel dimensions (not pixel aspect ratio, that is something diofferent) will not change(uncheck Resample while changing PPI). The only thing you can achieve is to increase print dimensions if you are sending image to print although that is not good idea.

Aspect Ratio is ratio betwen long and short photo side like 1200x800px > aspect ratio is 3:2.

[Conrad_C]

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@David_Bryant wrote:

By my calculations, when converted to 72dpi the pixel aspect ratio should change to 12,267x8000. But photoshop insists on resizing them to 718x480.

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Photoshop does that because it is the only possible answer, based on hard math and how Image Size was used in this case.

The original is 2992 x 2000 pixels at 300 ppi. By using ppi to change the dimensions, you have tied the pixel dimensions to specific dimensions in inches (which are irrelevant for video, as we are all saying).

2992 x 2000 divided by 300 ppi equals 9.973 x 6.667 inches. If you then change the ppi to 72, while dimensions are locked to inches, and you multiply the inches by the ppi, you get:

9.973 inches * 72 pixel per inch = 718 pixels wide

6.667 inches * 72 pixel per inch = 480 pixels wide

 718 x 480 pixels, just like Photoshop said.

The way to get out of that trap is to remove inches (and therefore pixels per inch, since it involves inches) from the equation, and work only with the pixel dimensions, using your project’s frame dimensions in pixels as your reference, as I showed earlier.

So what about the 12267 x 8000 that you got, how can we get to that? That sounds more like what you would get if your image started out 72 ppi and you changed it to 300 ppi with Resample enabled (see below), although Photoshop says it would be 12467 x 8333 pixels. But in addition to being the reverse calculation from what you want, all that does is artificially create pixels (by resampling up), bloating the image’s file size and memory requirements. And if you were to import it into a video application and set it to 100% scale, a 12467 x 8333 image would extend far beyond the borders of the 4K or 2K frames you want to work with, so the formula used in your example clearly is not the right one.

[David_Bryant]

I think Conrad is getting closest to what I was trying to express. Thanks to all the comments I realize I misstated several terms and did a poor job of describing what I'm after. I see that as a reminder to not post my questions after a long day of editing.

I've had some time to experiment since asking my question and I think I am better prepared to describe what I discovered and the solution I am pursuing.

Correction: Pixel aspect ratio: incorrect use of term on my part. I meant frame aspect ratio as Conrad pointed out. I am working with square pixels, but at a 2992x2000 picture aspect ratio.

dpi vs ppi: I realize these seem irrelevant but they are not, or so it seems to me. Here's why:
– when I drop a frame to create a new sequence that defaults to the source settings, I get a sequence at 2992x2000. When I check the effects controls for scaling, the image is at 100%. This is one of the 1:1 ratios I was looking to achieve (meaning if I scale the image to 50% it fills half the screen).
– AND, if I create a 4K sequence and drop my 2992x2000 300ppi source or nest the 2992 sequence in the 4K sequence, it fills the screen about 90%. When I check the effects controls for scaling it is at 100%. This is another way I can see 1:1 scaling (meaning if I scale the image to 50% it fills half the screen).

BUT, if I create a 3K sequence (3072x1728) and drop my 2992x2000 300ppi source into the sequence it is cropped top and bottom. How is it that an image with a pixel height of 1728 be larger than an image with a pixel height of 2000 if ppi doesn't matter? When I select "Set To Frame Size" and check the effects pannel the image is scaled to 86.4% How is a 1728 image 13.6% larger than a 2000 image? This is why I remain rather baffled by the assertion that ppi doesn't matter in video.

My original concern with scaling accuracy in editorial remains, especially in regard to overscaling. Maybe this is how I should have been framed my original question. This film will probably be distributed in HD. This gives us plenty of room for scaling if we want to enlarge parts of the frame. The solution I plan on pursuing to avoid overscaling is to use use "Set To Frame Size" rather than "Scale To Frame Size." And otherwise leave the pixel math to Premiere.

Does my question make more sense now? Am I on the correct path to avoid overscaling?

[Conrad_C]

Yes, all that helps. If I understand that you’re trying to avoid “overscaling” and by that you mean avoid scaling an image up so far that it gets pixelated or jaggy, then the answer is simple: The image must meet or exceed the maximum pixel width and height of your final frame size. If that is 3840 × 2160 4K, then the images must be at least 3840 × 2160 pixels. That equals about 8.3 megapixels, which is easily achieveable with almost any still camera on the market today, if you don’t limit the original capture size.

However, that does not account for if you want to, as you say, “enlarge parts of the frame.” If you want to do that, you must add in that multiplier. So if you have a 4K final frame size requirement and you need to be able to enlarge an image 200%, then to make every image pixel fill a pixel in the 4K frame when the still is scaled to 200%, the image must be 7680 × 4320 px. That is around 33 megapixels, achievable with some pro level still cameras.

The easiest best practice here is to shoot the stills at the full pixel dimensions of the sensor of the camera you have, so that you hopefully have enough pixels for maximum scaling flexibility in video with no loss of detail. But if at some point the image should be scaled down to fill the frame but the aspect ratio does not match the frame, it will end up cropped along one dimension or the other to avoid empty areas/black bars. This is hard to avoid, because both pro camera sensors and printed originals do not typically match the 16:9 aspect ratio of a current standard video frame.

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@David_Bryant wrote:

– when I drop a frame to create a new sequence that defaults to the source settings, I get a sequence at 2992x2000. When I check the effects controls for scaling, the image is at 100%. This is one of the 1:1 ratios I was looking to achieve (meaning if I scale the image to 50% it fills half the screen).

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Yes, that one is easy. Creating a Premiere Pro sequence from a media item always creates it at the same size as that item. But this is not useful to you because your project’s final frame size requirement is not 2992 × 2000 px.

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@David_Bryant wrote:

– AND, if I create a 4K sequence and drop my 2992x2000 300ppi source or nest the 2992 sequence in the 4K sequence, it fills the screen about 90%. When I check the effects controls for scaling it is at 100%. This is another way I can see 1:1 scaling (meaning if I scale the image to 50% it fills half the screen).

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Yes, but because neither the width nor the height of the still meets or exceeds a 4K frame size, because it can’t even fill the frame at 100% scale, it will start to break down when enlarging beyond 100%. The original still images were photographed with too small a width and height in pixels to fill a 4K frame.

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@David_Bryant wrote:

BUT, if I create a 3K sequence (3072x1728) and drop my 2992x2000 300ppi source into the sequence it is cropped top and bottom. How is it that an image with a pixel height of 1728 be larger than an image with a pixel height of 2000 if ppi doesn't matter? When I select "Set To Frame Size" and check the effects pannel the image is scaled to 86.4% How is a 1728 image 13.6% larger than a 2000 image?

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The still has to be cropped because 2992 × 2000 px is 3:2 aspect ratio, with is taller but not as wide as the 3072 × 1728 frame, which is 16:9. When the image is at 100% scaling, cropping cannot be avoided since the frames are different heights. You ask how the 1728 px video frame is “larger” than 2000 px; well, it’s larger in width. But the still image is larger in height. To fit the taller still image into the wider frame to avoid cropping the top and bottom, you applied Set to Frame Size, which scaled down the image to match the height of the 1728 px tall frame. Which leaves empty space on the sides, since they have different frame aspect ratios.

Some of this might be cleared up by seeing what happens in this animation, where all frame sizes are shown 1:1 relative to each other:

Again…ppi never comes up in the problem or the solution. In fact part of the reason there are problems is because of introducing ppi into the math. Because again, to understand size with pixels per inch, you have to say what the number of inches are on output. But there is no number of inches in video – you will not find any place in any video editor to enter inches or ppi – so anything involving inches is a wrong turn that does not lead to a solution.

You should see that what solves the problem is first stating the target final frame size, which appears to be 3840 × 2160 4K. Once you have that, you know all your original media must meet or exceed both the width and height of that in pixels in order to fill the 4K frame. If the stills exactly match both the width and height of the video frame, that will of course also match the 16:9 video frame aspect ratio. Then, to accommodate enlargement, the required width and height in pixels of a still image must be increased proportionally to how much you are required to enlarge the still images in Premiere Pro.