Resolving
pale "halo" when printing semi-transparent objects from CorelDRAW
If you worked with CorelDRAW's
dropshadows or objects with transparencies you may have probably noticed a problem
printing those objects on composite printers (such as laser printers, inkjet printers and
others). Even if everything looks perfect on the screen, you still can't get your artwork
printed correctly:

Original image as seen on screen
in CorelDRAW |
 |

The same document printed to
an inkjet printer |
Why does this happen and how to cure
it? Let's try to find this out.
The way CorelDRAW treats
transparencies
In order to reproduce a transparent
object CorelDRAW creates a bitmap image from the object and everything that lies directly
underneath:

As a result, we see the dropshadow
bitmap bordering on the underlying vector object(s).
Color space mapping
Exterior areas of the dropshadow bitmap
has exactly the same color values as the vector object beneath it. Why do we see
differences on printout, then? The answer to this question is buried deeply into color
representation theory.
As we know, different output devices
have different color reproduction capabilities. Monitors emit light using three color
phosphors - red, green and blue (thus, representing RGB color space). Most of printing
devices uses other technologies, such as subtractive color inks (cyan, magenta, yellow,
black - CMYK color space) and so on. Obviously, devices built with completely different
techniques have different properties and capabilities. Modern RGB color monitor can
display wider range of colors than CMYK printers can render. Because of this we are facing
a problem of color transfer between different color models.
There are two ways to translate colors
from one color space to another:
- Replace colors falling behind the output
device's capabilities with the closest ones that can be displayed by the device. This
method reproduces colors without alterations unless they are out of gamut of output
devices.
- Scale all colors in the source image to
fit the output color range. This changes all colors in image but keeps relative color
differences between colors along the whole source color range. The advantage of this
method is to keep shades of continuous-tone images such as photographs.
The two color transfer algorithms can
be represented by the following charts:

Method 1: Colors are
clipped at the end
of color range |
|

Method 2: Colors are
mapped along
the whole source color range. |
On these drawings: S -
source color space, D - destination color space, B - the
color range that can be represented by the output device, A and C
- the color range falling behind output device capabilities, E - output device color space
scaled to the source color space.
When using method 1, colors from region
B will be printed without changes. Colors from range A and B will be replaced with the
closest match from range B. When method 2 is used, all colors printed look unique even if
they are not completely the same as the original image.
The method 1 is best for vector
illustrations because usually vector artwork doesn't have much colors and it is not likely
that clipping will be visible. On photographs, though, it is possible to see the color
distortion like shown on the illustration below:

Original Image |

Color histogram of the original image |

After applying method 1 |

Color histogram of result 1 |

After applying method 2 |

Color histogram of result 2 |
On these simulated images, method 1
just clips everything that falls behind the output color space. Both resulting images have
the same color range (as seen from the histograms above). The result #2 shows more details
even if it differs more in color from the original.
When CorelDRAW prints a document on a
composite printer, it applies one of the color mapping algorithm described above. By
default, it uses method 1 for vector images and method 2 for bitmaps. And this is the
cause for that pale bounding box around the dropshadow.
To remove the halo, you can do one of
the following:
- Convert the underlying object(s) to
bitmap. You can add the dropshadow to that combined bitmap, or print it as separate object
- this doesn't matter. The most important thing is not to have vectors and bitmaps of the
same color contact each other.
- In CorelDRAW, go to
Tools>Options>Global>Color Management>General and you will see a drop-down
list named "Mapping Mode" (in CorelDRAW 9 it is named "Rendering
Intent"). Set it to either "Illustration" or "Photographic"
("Saturation" and "Perceptual" respectably in Draw 9), but make sure
it is not "Automatic".
This should fix the problem. Note, that
some printer drivers have color correction too. For example, HP DeskJet 850C uses
automatic color mapping by default. Make sure you set it to either of the two methods,
rather than automatic. HP DeskJet's driver names method 1 as "Vivid colors" and
method 2 as "Match colors on screen". Unfortunately not all drivers allow you to
turn off its color correction...
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