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Bitmaps

Type 1 and True Type® fonts still present design and output problems for many users. The following outlines several methods of decreasing the possibility of encountering unexpected results when documents are sent to a service bureau for film output.

Color Modes

CorelDRAW 12 supports the following color modes when dealing with bitmaps:

Black-and-White (1-bit)

This is the simplest of color modes and is seldom used when dealing with photographs. 1-bit bitmaps contain the lowest number of colors per pixel in which a graphic can be stored. In a 1-bit bitmap, pixels are either black or white. This color mode is usually used when working with line art or other graphics which require only one shade of black and no other colors.

Grayscale (8-bit)

Supporting up to 256 different shades of black, this color mode is often used when a bitmap must contain more than one shade of black but requires no color. Because it contains only one color channel (black) it makes for a smaller file size than an all-black bitmap which uses a 24-bit or 32-bit color mode. Usually when people have a photograph and they say that they would like to convert it to black-and-white, they are usually referring to Grayscale (8-bit).

Paletted (8-bit)

This color mode also supports a maximum of 256 colors, however those colors can consist of any of the colors available in the RGB color spectrum. By default, converting a bitmap to Paletted (8-bit) will result in the color palette being automatically generated based on the colors provided in the source document. If the document contains colors that are not found in the RGB color spectrum (i.e. Pantone), it will convert them to the closest available RGB value. In the event that the source document contains more than 256 colors, the color palette will consist of the most used colors in the image.

Duotone (8-bit)

There are four different types of duotones: monotone, duotone, tritone and quadtone. Monotones support only one color, duotones support two, tritones support three and quadtones support four. When a bitmap is converted into a duotone (8-bit), the user is prompted to first select the type of duotone that will be created and then choose the colors that will be used in the bitmap. Duotone bitmaps must use Pantone colors and will always output grayscale as Pantone colors cannot be reproduced using CMYK inks. In addition to this, Duotone bitmaps are typically used when color separations will be created from the document. As such, color is not required when the separations are output.

RGB Color (24-bit)

This is the most commonly used color mode as it uses three 8-bit channels (Red, Green and Blue) which allow 256 shades of each color to be displayed, resulting in the possibility of 16.7 Millions different shades, representing of up to 16.7 Million colors (256 x 256 x 256). Color depth such as this is typically used in photographs where 8-bit color simply won't do. Computer monitors display color using RGB Color (24-bit) and as such, this color mode is typically preferred among those who design graphics for the web or primarily for display on screen.

LAB Color (24-bit)

This color mode was developed based on how the human eye perceives color. L represents Lightness while A represents the color along the red/green axis and B represents the color along the blue/yellow axis. This color mode can consist of all colors in the RGB color spectrum and is device-independent.

CMYK Color (32-bit)

CMYK is the color mode used by most color output devices and is usually used when documents that contain more than four colors are to be separated (less than four colors will often use Pantone colors instead). Because colors are displayed by combining Cyan, Magenta, Yellow and Black, a color with a CMYK value will usually shift slightly if converted to another color mode (such as RGB Color (24-bit)). For this reason, most designers will convert bitmaps to CMYK prior to printing or will specify that CorelDRAW convert them when printing.

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Resolution

Bitmap resolution is defined as the number of dots (or pixels) present in a given area within a bitmap (usually per inch). The higher the resolution, the greater the number of dots and therefore the better the quality of the bitmap.

Many individuals interpret the above definition as meaning that bitmap resolutions should be as high as possible, however this is almost never the case. When determining what the resolution of a bitmap will be, one should consider how the bitmap will be output. For example, a bitmap that will be printed at 100% scale rarely needs to be more than 300dpi as this is the recommended resolution for use in professional output. If the bitmap is monochrome (black-and-white), 1200dpi is recommended.

Bitmap resolution cannot be effectively increased beyond its original value without the use of specialized software. If a bitmap is obtained at 72dpi, changing the resolution to 300 dpi will result in more pixels being added to the bitmap, however image quality will likely degrade as a result. The reason for this is because when the resolution is increased, pixels size is reduced and extra pixels are added to the bitmap to "fill" the extra space. These pixels are based on existing pixels an do nothing to improve the bitmap. If a bitmap's resolution must be increased, the only way to accurately do this is by decreasing the bitmap dimensions. If bitmap dimensions are reduced for the purpose of increasing resolution, the bitmap must remain at its new size in order to maintain this resolution. Increasing the dimensions afterward will result in a decrease in resolution.

If a bitmap is imported into CorelDRAW and its physical dimensions are too large, many will reduce the bitmap size using the Pick Tool to drag the sizing handles until the bitmap is at the desired size. While this works fine for resizing images, it is important to adjust the bitmap resolution afterwards as resizing a bitmap this way constrains the size and will increase the resolution as the size is decreased. This can result in a 300dpi bitmap having a much higher resolution after being re-sized. To adjust the resolution, select the bitmap and click Bitmaps | Resample.

Fig. 1
Reduce the resolution as desired using the Resolution section of the Resample dialog and click OK.

Figure 1

The bitmap used in Fig. 1 was 300dpi to begin with and was 800dpi after having its dimensions reduced. Note that by resampling the resolution back down to 300dpi, image size goes from 46MB to 6.4MB uncompressed.

Ensuring that all bitmaps contained in a file are resampled properly can reduce file size considerably when compared to a file that contains improperly prepared bitmaps.

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Compression

Compression is often used when saving bitmap images so that they're file size is reduced. This results in the image consuming less disk space and often increases transportability (for the purpose of emailing, uploading, etc.). Compression algorithms generally use mathematical formulas to locate repetitive patterns in the data contained in a bitmap image and replace this data with code which occupies less space.

The two main types of compression in use today are Lossy compression and Lossless compression. Lossy compression eliminates information contained in a bitmap image which is deemed redundant. When the bitmap is uncompressed for the purpose of viewing, only a portion of the original image is displayed, despite the fact that it will usually appear unchanged. Lossy compression is most commonly used in JPG images and provides the user with the ability to specify how much compression is used at the expense of image quality and allows for greater compression than if using lossless.

Lossless compression maintains all data that was contained in the original file so that when the file is uncompressed, everything is exactly the same. The form of compression often provides less compression than with lossy, however it is the compression type of choice when sending information for professional output or when working with text. GIF is the most commonly used file format that provides lossless compression, however this file format supports a maximum of 256 colors. If a bitmap is being used which contains more than 256 colors, lossy compression can still be obtained by saving the bitmap as a TIF with LZW or CCITT compression.

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