A raster graphic or bitmap (e.g. PNG or JPEG) consists of individual image points (“pixels“), which can have different colours. The image points or "pixels" collectively represent a colourful image. The smaller the image points and the more image points there are, the better the quality (resolution) of the image. However, a raster graphic has the disadvantage that it cannot be enlarged arbitrarily, without being able to see the individual pixels as small squares. The term “raster graphics” is also based on this effect, as the image has different points arranged in a raster.
During laser engraving, the grayscale or colour images are converted to black/white images during rasterising. The impression of a grayscale image is generated through different point sizes and point distances. However, the human eye perceives the raster as a mixture of colours.
How does this effect laser engraving?
During rasterising, binary information is calculated from image data as: "1 = lasering / 0 = not lasering." Grayscale or color images are then converted to a black-white image that can be laser engraved. There are different algorithms for the arrangement of the individual image points. In JobControl® laser software, you can simply select the desired raster algorithms.
The following raster algorithms are available in JobControl® laser software:
- Ordered dithering raster
- Error diffusion raster (Stucki, Jarvis, FloydSteinberg)
"Ordered Dithering" is an ordered raster structure. The individual points are placed along a virtual raster and differ in size depending on the grayscale value. The darker the grayscale value, the larger the point. However, the density and the position remain unchanged.
This raster can be compared with the print of large billboards. The four primary colours are presented as points and are printed above each other. The impression of an arial image results when they are viewed from a distance.
These raster algorithms are a chaotic raster arrangement. JobControl® laser software contains the following three error diffusion rasters: Stucki, Jarvis and FloydSteinberg.
The principle of each algorithm is the same: the darker the grayscale value, the denser the points are set. The point size remains unchanged. They only differ minimally in their coarseness and depth sharpness.
The error diffusion algorithms reach a better detail accuracy than the ordered dithering raster.
Colour and black/white
The two “rasters” colour and black/white are used for engraving vector data. In contrast to ordered dithering and error diffusion, the data is not converted to grayscale. Instead, the objects with the 16 predefined colors in the JobControl® are directly transferred to the laser software, where individual parameters can be defined for each colour, thus allowing for different performances, speeds and offsets to be processed in an engraving job.
It is mostly your personal preferences in material and design will determine which raster algorithms you should use, but there are some general guidelines:
- Error diffusion algorithms let photos with lots of details become clear, such as buildings or animals with thick fur. Images with little contrast such as faces of babies or low-resolution photos can also be optimised in this manner.
- The ordered dithering raster is particularly suitable for images with fine grayscale courses, such as people or images which must achieve maximum visibility.
- Black-white is used for vector graphics where only a black engraving colour is present in the graphics.
- Colour: As soon as a second engraving color is assigned in the vector graphics, the raster color must be used; otherwise the parameter for black is automatically used for all engravings.
These rasters do not have an effect on the cutting lines.
Photo engraving is made easy using Trotec's JobControl® laser software. Achieve the perfect photo engraving in 3 steps.