Stumbling Through The Jargon... Part Four
Last month we looked at image resolution and halftones, and how to calculate scanning resolution depending on halftone screen frequency. As mentioned then, it's now time to look at the exception to the usual halftone scanning rule, and consider what resolutions to use when scanning line art.
Just what is "line art?" Well, last time we covered halftones and halftoning, where images are broken down into halftone cells, the larger the cells, the darker the area, and the smaller the cells the lighter the area. So a black and white printed image which contains shading is made up of many halftone cells of varying size, to create the shading. Line art describes images where there is no halftoning and therefore no shading, the image is one of either two colours, usually black or white. Because of this, line art images are sometimes described as 'bi-level'. The image is not broken down into halftone cells, but is simply made from individual pixels that can be either on or off (black or white).
A line art image could, for instance, be a 'pen and ink drawing', where there is no variation in the weight of each line, but lines are drawn in such a way to create an impression of shading. Last time it was established that scanning resolution was dependent on the halftone screen frequency used for printing, but if line art is not halftoned, the screen frequency would have no effect. So what to scan line art at? Well, it's best to scan any line art at the maximum resolution of the final printing device, so for a 300dpi laser printer scan at 300dpi. If output was to be printing press, via imagesetter films, the resolution would be increased up to say, 1270dpi or even 2540. However, depending upon the image, even with a 2540dpi imagesetter output, the line art could be fine at 1270dpi, any higher could greatly increase the file size, but with only a small improvement in image quality. Although the resolution of line art is very high, because these images are only two colour (black and white), the file sizes are still manageable.
What would be scanned as line art? Usually detailed line drawings, etchings, diagrams, logos etc. that contain no shading would be scanned as line art. The higher resolution captures all the detail from the original, and this is no conventional halftoning to break up the image. Try scanning a page of black and white text, as both a greyscale image and then as line art. Print both scans and the differences between the two formats will be very clear. The line art would represent the text as solid black or white, there is nothing in-between, whereas the greyscale would almost appear 'blurred' in comparison, as the text would shade from black to grey around the edges.
When scanning line art, the scanning software has to calculate which areas are black and which should be white. If a value falls in-between black or white, the software has to decide whether to make that area white or black, and this is usually controlled using a 'threshold' value. The default would usually be 50% black, so any areas from 100% to 50% black are described as black, 49% to 0% becomes white. This 'threshold' can usually be set to suit when scanning, and can compensate for overtly light or dark images.
One particular pitfall to avoid with line art scans, is resizing it in DTP packages. Always try to scan the images at the size they will be used at. If you scan a image 1-inch square, then stretch the image in a page layout program, to say 3-inch square, the image may become jagged. This is because when the image is re-sized after scanning, it's effective resolution is reduced, and in this case the resolution would reduce from 300dpi to 100dpi. If the re-sizing is set at the time of scanning, the scanner scans at a higher resolution than the one set, and uses the extra data, to increase the images dimensions, whilst maintaining the resolution required.
If the final required size is not known at the time of scanning, simply scan the image larger than will be necessary, then the image could be re-sized down, in an 'image editor' package such as Photoshop, Picture Publisher or PhotoPaint. It's better to be able to reduce them to suit, rather than having to enlarge, when re-sizing images after scanning. However, it's also preferable to do any re-sizing in a 'image editor' program rather than the final page layout or drawing program, as this avoids unnecessarily large file sizes.
Scanned line art is stored as a two colour (bi-level) image, usually black and white, but it can be printed in any two colours to suit. Most page-layout or drawing programs allow line art to be re-coloured, so that what originated as a black and white scan, can be re-coloured to say, red and black, or white and blue. The two colour data in the original scan remains the same, but the DTP software simply swaps the colours to suit, i.e. whatever was black becomes blue, white could stay the same. Another great use is that either of the colours can be made clear or 'invisible', allowing a line art image to be placed over a background, but still letting parts of the background through. As an example, a black and white line art image, could be re-coloured to change the black areas to blue, and the white to clear. This could then be placed over a background to suit. A very simple black and white image can look great once re-coloured in a page layout program, and it can be a very versatile feature.
If a line art image is re-coloured, to a colour that is either a shade or halftoned, rather than a spot colour, the line art will be halftoned. This would have no effect on the image resolution, but detail could possibly be lost, as the image would be broken down into cells, which may not be fine enough to capture the originals details. As an example if a line art image was printed with 100% black there would be no halftoning, the black being solid. But if the same image was printed at 50% black, i.e. grey, to create the grey the black would have to be halftoned, and the image broken down into halftone cells possibly degrading the image quality. Depending on the image, this may be acceptable, but if in doubt, simply run a test print to check for any possible problems.
Next month it'll be time to conclude our look at scanning and resolutions, and move onto the different colour models, including RGB, HSB, CMYK Process Colours and Spot Colours. In the meantime, please feel free to email me with any comments or queries you may have, and happy DPT'ing.