DPI, PPI, Pixels - Facts and Fallacies

© 2002 by K.N. Pepper

Every three months or so, a novice comes along and asks about DPI (dots/inch), PPI (pixels/inch), and pixel dimensions, with respect to scanning, digital photography, exporting for web display, and printing. Experts join in the discussion. Invariably, confusion follows. What is truly remarkable is that even some of the veterans are confused. How much understanding does the novice leave with? Less, I fear, than he came in with.

This series of articles attempts to clarify the issues: PPI, DPI, pixel dimensions, display size, and print size.

Screen Metrics - DPI fallacy

Question:

I have been told that the "standard" screen resolution for Macs is 72 dpi, while for PCs it's 96 dpi. Is this correct?

Answer:

I have no idea where these things start. This is pure nonsense propagated by people who know less than the student asking the question.

Consider a 21-inch monitor running at the following resolutions:

1. 1024x768 pixels: This is equivalent to 61 dpi, vertically and horizontally.
2. 1280x1024 pixels: 76.2 dpi horizontally, 81.3 dpi vertically.
3. 1600x1200 pixels: 95.2 dpi, vertically and horizontally.

Do you see 72 or 96 anywhere? Pure superstition!

For images intended for screen display, plan your picture through total pixel count horizontally and vertically. For example, a picture 512 pixels wide and 384 pixels high would occupy exactly a quarter of the screen in (1). This holds true for any monitor size (e.g. 14, 17, 19, or 21-inch).

Pixel Dimensions and DPI in bitmaps

For screen display dpi means nothing. It 'may' have some meaning at print time, as we shall see a bit later.

Points to note:

1. Not all file formats carry dpi information (pixel density along X and Y). For example, GIF doesn't and JPG does.
2. To verify experimentally, use a third-party viewer that provides internal information (e.g. IrfanView). Don't use the same software you used to generate the file (e.g. PP). The generating software may place proprietary info in the header, which *it* can read but no other program can. This would be misleading.

Experimental Demonstration:

1. A precision ruler was scanned at 100% yielding a one-inch segment that is 1000 pixels wide.
2. It was resampled down in PP to 100 pixels wide, but with different dpi settings - 10 dpi and 10,000 dpi.
3. They were exported to gif and jpg formats.

Observations (see attachments):

1. Viewing the attached files in a non-scaling viewer, all four images appear the same size.
2. Opening them in PP show different sizes with respect to the ruler.
1. The 10 dpi jpg is 10 inches wide, as expected, and will 'print' 10 inches wide. The 10,000 dpi jpg is 0.07 inches wide. (It should have been 0.01 inches wide, but PP maxes out at 1410 dpi on opening. However you can create a new drawing at 10,000 dpi, then import the 10,000 dpi jpg.) The high dpi jpg prints the size of a tiny dash.
2. Both gif files are about 1.4 inches wide. This is because they did not retain the original dpi information. However PP defaults to the legendary 72 dpi. The image was 100 pixels wide which when divided by 72 gives approx 1.4.

Conclusion:

For screen display, dpi is of no consequence. However, if you expect your audience to print the image, then select a correct format and specify the correct dpi when resampling.

Example: You want to post a picture on a Web site that will be viewed and printed. You want the image to be 400 pixels wide on the screen and to print 2 inches wide.

1. Select a suitable file format (e.g. jpg)
2. Calculate the dpi thus: 400/2 = 200
3. Resample the image thus: Width = 400 pixels, Resolution 200 dpi

An interesting aside - Fonts:

In a Corel PHOTO-PAINT document *of any dpi*, a 72 pt font occupies approximately 1 inch of vertical space with respect to its ruler. (Try accented character "É" followed by a descender character "p" in a standard font.) The pixel count however will depend on the document dpi, which governs the ruler.

DPI, PPI, Pixel Dimensions - Scanning

Digital device manufacturers are major contributors to the confusion that abounds. We'll use a typical high-end flatbed scanner and its software as an example.

The scanner has a linear array CCD, eight inches long with 8000 sensors. This generates 1000 PPI (pixels per inch) of raw data. But this data does not arrive directly. The scanner has firmware. As well, the PC has scanning software. The two in concert, manipulate the raw data extensively before offering it to you. The worst offender is interpolation for which there's no easy bypass. The attached shows the perplexing resolution question the scanning software asks before scanning. This is where the "per inch" confusion arises. (Remember, the scanner only has the one real optical resolution.)

So in order to get close to the real number, one should specify the real optical resolution of the device, disk space permitting. In the "Inch" scans earlier, the resolution was set to 1000 ppi. (One can only hope the firmware and the scan software will allow a true pass-through without arithmetical, round-off, and truncation errors.) One can subsequently resample down using reliable software (e.g. Corel PHOTO-PAINT).

There are several reasons for scanners manipulating data - some good, some bad.

1. Good: Keeps file size down. A full-blown 8"x14" scan could easily exceed 2.5 GB.
2. Good: Allows pre-corrections (e.g. dynamic range, tone curve, etc.)
3. Bad: Interpolation gives high pseudo resolution numbers, making the device sound better than it is.
4. Bad: Interpolation of data conceals defects in the CCD array (dead spots).

Example - Dimensional Control:

You want to scan only a part of a photo, but want the scan to be at a certain pixel width. Measure the subject with a ruler. Say it is 1.5 inches wide. You want the scan to be 300 pixels wide. Assuming your scanning software allows it, define a tight bounding box (scan area) around the subject. To the scanner's question of 'resolution', answer "200 ppi". (300/1.5=200.) Then scan. Of course, when you view the scan in Corel PHOTO-PAINT in its full glory, you'll find yourself having to trim the picture a bit. Your original intent of 300 will be slightly compromised. A better method is to scan the picture at the true optical resolution, trim it in Corel PHOTO-PAINT, then resample to an exact pixel width.

Digital Camera - Printing

The Mission:

You want to take an excellent picture. You want to print an excellent print. You want to control the print size precisely. You want to economize on material.

First shoot at best res. Let's assume 1600x1200 (quite common).

Scenario I - You want to print the whole frame, but you want it to be precisely 6 inches wide. This will make the height 4.5 inches. (This is non-standard but heck, you wanted the whole frame.) Now, where did the 4.5 come from? If 1600 shrinks to 6 then 1200 shrinks to 6*1200/1600 = 4.5.

What should the dpi figure be? (Never mind the default it came in with.) Simply 1600/6 = 266.67. So pick 266.

Action: Resample your image at 266 dpi, with size showing in Pixels, Maintain aspect ratio, Maintain original size. Anti alias shouldn't matter as there's none involved. Observe that the pic is still 1600x1200 pixels. No loss. Observe that the rulers now indicate 6 inches wide and 4.5 inches high. (Actually it's 6.01). Print it on your fine inkjet printer.

Scenario II: Advanced. You want it to have the size/aspect ratio of a drugstore print. Measure one if you have one handy. (Approx 6" x 4". You can be more precise if you want.) Create a rectangle of this size. Stretch it diagonally (to maintain the ratio) to cover your picture as you see fit. Trim the picture to this rectangle. If you go full width in this instance you'll lose half an inch vertically. Resample to 6 inch wide with Maintain original size selected.

Note 1: Pixel count is gold. "Maintain original size", which could have been labelled "Maintain my wealth", preserves your gold. Dpi is just a last minute packaging instruction to the printing device. Essentially it indicates how large a crate to lay your gold bars in.

Note 2: Create rectangles of whatever ratios your clients commonly ask for (4x6, 8x10, whatever) in CorelDRAW. Fill yellow. Clip them into PP. Adjust transparency so you can see the picture through it. Place over area of interest and stretch/compress diagonally. Convert to mask - trim. Resample as explained above. Save.

Note 3: Hi quality photo paper can run up quite a tab. Create a layout in CorelDRAW to fit as many finals on the sheet as possible (a mix of portrait and landscape), with appropriate trim lines. Import your final images into this layout. Place them. Print and trim on your cutter.

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