ppi or Pixels per Inch

Scanning, Painting or Exporting a new Image without first establishing the various ways the bitmap is to be printed will often waste many hours of work. What resolution to use?, must be decided by knowing which will be the highest quality device used to output the image, immediately AND possibly sometime in the future.

When you read the 'Image Output' subjects you will learn about the definite relationship between the output device resolution and the number of grey levels available at a required halftone screen dot resolution. The ideal halftone cell (maximum grey levels) is made up of a matrix of 16 X 16 output device dots.

The Boffins and technical people who create high quality output devices like imagesetters etc. have established the golden rule, the 2X rule, for calculating the required resolution of a greyscaled bitmap (grey or CMYK picture). This equates to 4 bitmap pixels underlying or being used to calculate (the average of the four) the single channel colour value of each halftone dot, OR, horizontally 2 pixels per halftone cell - as displayed in the simple graphic above.

The 2X rule can be varied by a couple of decimal points up or down without affecting the output, but 2 is the preferred and easiest remembered number. (note that the graphic above uses the simplest scenario of a screen angle of 90 degrees for easy understanding, and the blue grid represents the ideal 16 X 16 array of output device laser dots)

Therefore, very simply, if an image has to be output at #150 (150 halftone dots per inch) then our best quality bitmap image file should contain 300 pixels for each inch of that image horizontally = 150 X 2. (if the image is to be 4 inches wide, then there will be 1200 pixels in each row of pixels in that image).

Because of the method of pixel averaging per halftone dot cell, there will not be any noticeable improvement in picture quality if higher resolutions are used, and a lower resolution will begin to reduce sharpness to a point where pixalisation will become evident. The bigger file size from a higher resolution will also take much longer for the RIP (Raster Image Processor) to rasterize the page the image is placed in. (also read the observations later in the Text & Drawing - 'Page/Image Errors' tutorial)

Now for something completely different!

We literally throw this theory out the door when considering the resolution required for a scanned or painted Line bitmap (black OR white pixels only - a 1Bit bitmap) and here are the reasons.

The following comments must be fully understood and answers followed if quality is the main objective.

• The normal human eye will notice the lack of picture quality when the resolution used to print the Line picture is around or below 700 to 800 pixels/inch.

If the graphic is to be output via an imagesetter - to film or bromide - and then reproduced on a printing press, the MINIMUM resolution used for the creation of line pictures should be at least 900 to 1000 pixels/inch because of the extremely high definition of both the imagesetter and press.

If the graphic is to be output via a conventional laser printer the same situation should be kept in mind, for if the laser pixels/inch resolution is 600, then the resolution of a scanned line image should be more than 600, maybe 700 to 800. Even when using the common 300 ppi laser printers, tests must be carried out to determine the minimum bitmap resolution that is both acceptable and at the limit of detectable control.

One variable that changes from laser printer to laser printer is the edge definition of the output image on the paper, which can even be affected by the quality of the paper used. Laser printers use an extremely fine powder to produce the image and around each object (i.e. character, halftone dots) will be a sprinkling! of powder residue quite noticeable under a 10X or 20X magnifier. This variable is unpredictable between machines and will affect just how much change you see between tests.

• If your line image is to be finally output via a high resolution device like an imagesetter, NEVER use a laser printer to determine quality, just use the high resolutions described.

• The size in bytes of good Line scans can be very large indeed and operators with slow computers or computers with small amounts of memory will often try to cut corners, but quality always takes a big tumble.

• The picture above shows one very common telltale of poorly scanned and low resolution line drawings. Any areas of image that are close to but not quite horizontal or vertical, always print as uneven lines. At the high resolutions of 900ppi+ this does not show as the jumps and bumps are very small and spread further up and down the lines.

• Increasing the line scan resolution above 1000ppi should only be an advantage if fine detail is included on the original, fine white detail in black areas and fine black detail in white areas. To understand the importance of and how to capture extra fine detail, refer to the 'Contrast and Brightness' page in the 'Flatbed Scanning Hints' topic, plus read the 'Threshold Explained'  topic in this section.

This is an enlarged image clearly displaying the "jaggies" or pixelisation at low resolutions that must be avoided.

Summary

Although proofed by a Laser printer, the image may be output by an Imagesetter at a later date, so the resolution will therefore be based on the halftone resolution (Screen Ruling or #??) for the printing press. For #150 lpi, multiply that number by 1.7 to 2.4 and the result will be the pixel resolution required for the original bitmap. The current standard for #150 lpi is 300 pxl/inch (or 120 pxl/cm) using a multiplier of 2.

Remember: The resolution required for line drawings is established on totally different principles.

• If you have an MS Windows computer, download 'Scan-Res' from the Scanning Lab.

The Chart

This chart shows *Halftone dpi / Image ppi / grey values possible* combinations using various output device laser dot values, i.e. Laser Printers and Imagesetters.

If printing vignettes causes banding, refer to the grey levels below, and read the 'Image Output' pages for further details. • The ideal number of grey levels available is 256. • 1270 device laser dots is ok to use for general line and text output (faster), 2450 is the minimum for all quality work.