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A Photoshop document‘s physical size, pixel density (DPI/PPI) and pixel dimensions are all linked.

Bitmap images contain bitmap data, which are a specific size (pixel dimensions). They also contain a DPI/PPI tag, which is usually a single value or pair of values (one for X, one for Y). The image's physical size is calculated using the pixel dimensions and DPI/PPI. This is how some print design apps place images at “100%” optimal size, which will typically be a different physical size to viewing an image at 100% in Photoshop.

Questions

Please note that the questions below relate to changing a document's DPI/PPI without changing the pixel dimensions of the image.

  1. Given that DPI/PPI is just a single value contained within a Photoshop document, which — if any — features in Photoshop change depending on the document's DPI/PPI?

  2. Do gradients and gradient dithering look different with different DPI/PPI values?

  3. Do other layer styles look different with different DPI/PPI values?


Additional info:

Sometimes metric measurements are used instead of inches, like pixels per centimetre.

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3 Answers

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Given that DPI/PPI is just a single value contained within a Photoshop document, which — if any — features in Photoshop change depending on the document's DPI/PPI?

Appearance of all elements is preserved when a document's DPI/PPI is changed (just the DPI/PPI, not the actual pixel dimensions). However, the values of anything measured in non-pixel units will be affected. This includes text, if type units are set to anything other than pixels (points or millimetres). It also includes shapes and other items, if ruler units are set to anything other than pixels (points, picas, percent, millimetres etc).

At 72DPI (Pixels/Inch under Image Size), text size will be the same in pixels or points. If the document DPI tag is changed, text will remain visually the same, but the values will have changed. These text sizes all look identical and take up the same amount of pixels.

  • 10 pixels with any document DPI.
  • 10 point with a 72DPI document.
  • 20 point with a 144DPI document.

As far as I'm aware, nothing else changes with document DPI, within the one document (that last point is important).

You can change a document's DPI without altering pixel dimensions by opening Image Size and turning off Resample Image.

Resample Image in Photoshop

Do gradients and gradient dithering look different with different DPI/PPI values?

No. Gradients look the same, and are rendered based on their pixel dimensions. Gradient dithering is done on a per-pixel basis and is on the least significant bit — it works the same no matter what the DPI setting is.

Do other layer styles look different with different DPI/PPI values?

Layer Styles use pixel units, so they are unchanged by document DPI. However, if you change a your document DPI and you resize the document, you can choose to also scale styles.

Scale Styles in Photoshop

Also, if you drag an element between documents, Layer Styles will be scaled. This may be what you're after, or it might not. It is for this reason that I recommend all web, app and screen designers use 72DPI for all document they create, even if they're designing for a known display (like a 326PPI Retina iPhone display).

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You have a good question, Marc, on a subject that is fundamental to executing any and all all design concepts. However, I think you're focusing on the wrong end of the DPI elephant.

In this post, I will use DPI and PPI interchangeably. All linear units are for discussion purposes only.

Resolution, as stated by DPI is a device construct. The device could be a printer dealing with ink-on-paper or a flat panel monitor with glowing LEDs arranged in a matrix. A device needs to match the virtual components of a file with the real-world dimensions of the device. On the device side, resolutions expressed in dpi do not always transfer equally between devices.

iPad with Retina display

For example, Apple's original iPad display had 1024 pixels by 768 pixels. The Retina iPads display 2048 pixels by 1536 pixels, but the device viewport is "still" 1024x768. "Normal" resolution artwork (132ppi) is doubled both horizontally and vertically, while text is rendered at the higher (264ppi) resolution. "Retina" resolution artwork is displayed at the native 264ppi.

My Big Laser Printer

Most of my design work is destined for paper, printed at 1200dpi on a $75,000 laser printer... but the printer's 1200dpi is dramatically different than Photoshop's dpi. The printer uses a 16-dot square (4 pixels on a side) composed of 16 dots each of cyan, magenta, yellow and black to express a single RGB Photoshop pixel. Even here there is no apples-to-apple comparison!

Trade Show Banners

I occasionally create vinyl banners for trade shows. The wide inkjet printers might have print resolutions higher than my big laser printer, but they only need 100dpi files to produce a good-looking banner.

Photoshop

In the end, Photoshop really doesn't care about devices, resolution or inches. Photoshop's engine is pixel-based. All the functions really compute pixels. Nothing else. The pixel-to-inch ratio only works when you use the file somewhere else, and usually only exposes a lack of understanding... when you do it right, almost no one sees the "good".

Back to my print design... let's assume I'm building a two-page spread measuring 17.25" wide by 11.25" tall. I need to supply the big laser printer's 1200dpi print engine with raster files at 300dpi. We'll discuss two files:

Raster File A

  • I design a "synthetic" paper color and texture to print on white paper. The paper file in Photoshop is 17.25"*300dpi x 11.25"*300dpi, or 5,190 pixels x 3,375 pixels.
  • When I save the finished background file, the file header contains the hard resolution in pixels, but only a "suggestion" for the dpi of the file.
  • When I place the file in InDesign, it reads the header and sizes the file to 17.25"x11.25".

Raster File B

  • I shot a picture with my iPhone 5. The 8MP image's pixel dimensions are 3,264 x 2,448. Digital cameras default the "resolution" to 72dpi.

  • I drop this file into InDesign as a 45" x 32" photo.

  • Had I set the resolution to 300dpi in Photoshop, the file would have placed as a 10.8" x 8.2" image instead.

  • Regardless of the resolution I set in Photoshop, I can scale the file to whatever size I desire... let's say just about 5" x 4", or about 650dpi.

Answering your question

"Ok, Tom. I'll cry 'Uncle!' What's up with this dissertation?"

Correct me if I'm wrong, but your question is about perceived quality--right, Marc? Within reason, I can make a blend look good on any device. Perception is all in the output. My definition of a good blend is a smooth transition of color and does not display any visible banding.

Creating a good blend requires

  • an image in RGB colorspace. Even with "one fewer color", RGB utilizes a larger gamut (total number of colors) than CMYK.
  • a minimum of a 25% difference between the start and end values
  • the blend is in at least one CMYK color channel (I'm not good enough to think through individual colors in RGB!)
  • the blend covers approximately 300 linear pixels
  • and can be assisted or "fudged" by introducing a little anarchy (3% to 5% Gaussian noise, with or without a 0.75 pixel to 2.25 pixel Gaussian blur in the blend layer)

There's a lot there. Following the guidelines produces a blend that exhibits no visible banding for about 300 pixels to 400 pixels.

If you are trying to blend a 35% black to "white" (we'll end at 10%K for a bit more "ummph!), you can extend the blend's length by varying the following...

  • replace your original K35 with C10M10Y20K35 is "still" gray
  • replace your original K10 with C3M3Y0K8 is "still" almost white
  • You end up with more overall color data to calculate each step of the blend!

Adapting the recipe for a longer blend follows these rules...

  • increasing the difference from the start color to the finish color
  • increase the difference across multiple color channels in the start color tithe finish color
  • increasing the Gaussian "fudge" factor

Making Photoshop Work Better

Regardless of resolution, file size or DPI, Photoshop works best when you throw more pixels at it. Anything you can do to a file that is 72 pixels on a side will look better when processed on comparable visual information that is 720 pixels on a side.

Photoshop is an extremely complex beast of a program. What starts out looking simple may have many intricate dependencies that can affect the outcome. Controlling theses interdependent variables is where the true magic happens.

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I don't think this really addresses the simple question being asked. –  e100 Jan 7 '13 at 10:33
    
Thanks for the great and very detailed answer, TomUnderhill. Unfortunately e100's right... I'm asking about changes to DPI/PPI where the pixel dimensions remain the same. I've edited the question. Sorry for any confusion. –  Marc Edwards Jan 7 '13 at 10:39
    
Not a problem... As I read your question I had a chance to think through an issue facing me with a long gradation in web application. –  TomUnderhill Jan 7 '13 at 16:46
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In all my years of working with Photoshop, the only one that really stands out to me (aside from "print" and saving to other PPI-sensitive formats like PDF, obviously) is the type tool. But that's primarily because the type tool always defaults to points (even if you pull up the type dialog and type in pixels, it'll just convert it to points for you). Still, for a new Photoshop user who creates a 300 dpi image for the first time, they might be surprised that 12pt font suddenly looks huge on their screen.

Aside from that, I haven't noticed PPI mattering much. After all, it doesn't really make sense for Photoshop to make any of the other tools/features behave differently based on PPI resolution.

At the end of the day, a raster program has to render everything onto a pixel grid. PPI is just meta data for converting to physical units. There's no way for it to affect gradients or dithering. Whatever PPI you use, it's still 1 pixel per pixel.

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