Everything You Didn't Know About Large Format Color and Image Quality
Today we have a feature written by Tony Dungan, Color Specialist with Excelsus Solutions - a large format printer in Rochester, NY. Tony's years of experience in the Rochester printing industry are invaluable in terms of making sure we keep our colors accurate, vibrant, and on brand!
Tony can be reached via e-mail - tdungan@xl-sus.com - if you have any further questions for our color guru.
Quality and the Raster Image
Image quality is most important as to whether a raster image can be used for printing, or if it might need to be replaced by a higher quality image. When we receive files for printing, our prepress department will preflight the files to see if they are fit for the particular product, size, or even substrate that they are intended for.
What is a Raster Image?
A raster image is created by pixel driven applications or captured with a digital camera.
Rasterized or bit map images and graphics are digital images that are composed of tiny rectangular pixels or picture elements. Each dot or pixel in the raster image has one or more numbers associated per dot or pixel that convey information about the image. These numbers have a value which represents a specific location, size, and color for each of the pixels.
Rasterized images are measured in dots per inch (DPI) or pixels per inch (PPI).
There are two important factors to consider when reviewing and using rasterize images for the printing process. The first factor is image resolution or the image quality. The second factor is the image color space.
The size of the file of a raster image is determined by the number of pixels being used in the image. Some of the raster file formats available are BMP, GIF, JPEG, PNG, and TIFF. Most of the images used today are created using a digital camera or by using a raster driven application as opposed to a scanner.
The raster image can be used for all types of printing and on the Internet. One of the biggest challenges when using a rasterized image is its inability to be scaled up. Trying to resize a smaller image or an image with a lower resolution may result in an image that is of poor quality and looks pixelated, this visual defect is called stair stepping. Raster images or graphics can be scaled down with no quality changes. However, when the image resolution is scaled up, quality loss will occur.
Another challenge when working with raster graphics is the large file size. This is due to the large amount information contained in the file. A raster file with a large image size can be difficult to work with on some computers and can take up a lot of storage space. Raster images when enlarged may not retain suitable quality, so it is important to supply the printer with the correct size required for the final product. The image or graphic should be the same or as close to the final dimensions you want to print the final project and at and the pixel resolution for the particular project.
Raster images can be modified, retouched and converted using both the RGB and CMYK color spaces. Different print projects require different resolutions in order for it to print properly. Most printers require images that are between 100 DPI and 300 DPI in order to avoid visual stair stepping or pixelization of the image.
If the final print project is being viewed from a distance that is 10 feet or more the image resolution can be on the lower side say 100 dpi. Raster files can have a larger file size when compared to vector graphics. When creating the raster image in a photo application it may contain several layers and you will need to make any changes to the color or design in this native format. These images can be compressed to a pdf format before being sent to the press for printing.
Once converted to the pdf format raster images that need color correction can only be modified globally. If these images need local or selective color or design modifications than you will have to go back to the original native file that was created with the photo application.
Raster or Pixel Images and the RGB and CMYK Color Space
There are two different color models found in rasterized images and they are CMYK and RGB.
The individual ink colorants found in CMYK are cyan, magenta, yellow, and black. This color model has been in use since the 20th century. CMYK is a subtractive color system and works by combining inks onto a substrate and selectively absorb certain colors of light. The light that is not absorbed by the inks are reflected back to the viewers eye thus allowing the viewer to see the intended color.
The CMYK process uses four channels or separations measuring from 0% to 100%. For areas that are colored with just the black channel small amounts of the other three colors cyan, magenta, and yellow can be added to strengthen or reinforce graphics such as type or larger black areas.
The colors contained in an RGB image are red, green, and blue. This color model based on human color perception that was developed before the electronic age. RGB is an additive color system that uses red, green, and blue light to create the colors we perceive and see on TV screens, computer monitors, and smart phones. Combinations of RGB allow us to see multiple colors.
The combination of red and green light will appear yellow, blue and green light appear cyan and red and blue light will appear magenta. When all three colors are combined in full the result is pure white. When these colors are combined in their lowest value the result is black. RGB color is used in digital applications like Microsoft Office and Adobe Creative Suite which are used in the graphic design industry.
The RGB color system is a device dependent color model meaning that each devise reproduces the RGB values differently and is limited because it does not translate well to CMYK print system. RGB does not display the same on different LED devices creating challenges for when you need to reproduce precise digital color.
Not all images are created equal when they are supplied to you from an outside source. There are many factors that come into play before images end up on your desktop. The most critical of these factors is how the image was captured, resized, and converted before you received it. Images that have been resized from a smaller resolution will retain the lower quality of the original image. In additional images that have been converted from RGB to CMYK will lose the wider color gamut from the original RGB color space and will not hold the previous color gamut if converted back to RGB.
There are times when you receive images for design and printing that are supplied to you in both the RGB and in a CMYK color space.
The large format printing process generally uses CMYK which is a subtractive color model and produces a spectrum of colors on various substrates. There are times however when it is useful to retain the RGB color space in an image when preparing the file for printing. This can be particularly important when you want to retain the wider gamut of color found in the original file that uses the RGB color space.
How these files are translated or converted from the computer to press vary depending on the method of processing the files. There are different methods that affect the translation or conversion from the design stage to sending the print ready file to the press. There are several ways to send the files to press some include the design application software, the print driver, a raster image processer.
Most large commercial printers use a software system called a raster image processor or rip.
What is the Difference Between RGB and CMYK?
RGB and CMYK colors render differently depending on whether they are used for the internet or print. The color gamut of these images allows for the range of color and density values that are able to be reproduced on a device such as a printer or a monitor.
There are times when RGB graphics and images need to be converted to CMYK before being used and depending on the printer’s workflow. After the raster images is converted from RGB to CMYK additional color correction may need to be done to enhance key color areas that have been desaturated or flattened due to the conversion process.
Traditionally the CMYK process is best used when printing signage, banners, business cards, or decals. If the graphic is only going to be used for digital and not printed than the RGB color mode should be used. The internet is set up to use RGB colors due to that fact that a digital monitor or cellular phone display is made up of RGB pixels. The RGB values are applied to the pixels setting the luminosity in each pixel. The RGB gamut is usually wider than the gamut needed to print CYMK. In order to print the file or image correctly form a CMYK printing press it must be converted from the RGB format to the CMYK format.
In the early days of desktop publishing and printing many where taught that design for digital displays should be in an RGB format and designs being used for print should be converted to CMYK color. This is still somewhat true today, however due to the expansion of technology in printing many designers and printers are using the RGB and CMYK format to send the files to the press. Note that some design to print workflows do not require the digital files to use the CMYK colors space, and today a number of workflows except or even prefer RGB color for content submitted for printing.
Before digital printing printers used CMYK because these values where specific to the operation and any reprinting was probably done at the same location often using the same press. With advances in technology many commercial printers have changed and can use files with RGB color space. The RGB allows a greater range of flexibility in the workflows. Today we have the ability to print on different kinds of printers using many locations.
With the increase use of digital presses and print on demand workflows additional inks may be used to extend the color gamut taking advantage of the RGB color space.
There are many factors that come into play when choosing and using raster images for internet design and printing. If your final project is a digital screen using computer, smart phone or tablets the and not traditional print it is work best to use the RGB color format. Regardless of the end usage of the raster image the image quality and resolution and the color space of the image will have a direct effect on the final product.
The RGB and CMYK color space of the raster image provide advantages and disadvantages depending on the workflow and projects end use.