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...But If You Do

In what follows we will call the file to be converted (which already exists in one format) the original, and the file produced in the second format (after the conversion operation) the converted file. A conversion application acts upon an original file to produce a converted file.

If you remember our terminology, an application renders a graphics file to an output device or file. By extension, then, we will say that a conversion application renders an original file to a converted file. We will also use the term translation as a synonym for conversion.

Bitmap to Bitmap

Converting one bitmap format to another normally yields the best results of all the possible conversions between format types. All bitmap images consist of pixels, and ultimately all bitmap data is converted one pixel value at a time. Bitmap headers and the data contained in them can vary considerably, but the data contained in them can be added or discarded at the discretion of the conversion software to make the best conversion possible.

Usually, some rearrangement of the color data is necessary. This might take the form of separation of pixel data into color plane data, or the addition or removal of a palette. It might even entail conversion from one color model to another.

Unsuccessful bitmap-to-bitmap conversions occur most often when translating a file written in a format supporting deep pixel data to one written in a lesser-color format, for example, one supporting only palette color. This can occur when you are converting between a format supporting 24-bit data and one supporting only 8-bit data. The palette color format may not support the storage of enough data necessary to produce an accurate rendering of the image. Usually, some image-processing operations (quantization or dithering, most likely) are needed to increase the apparent quality of the converted image. Operations of this sort will necessarily make the converted image appear different from the original, and thus technically the image will not have been preserved in the conversion process.

The other main problem comes about when the converted image must be made smaller or larger than the original. If the converted image is smaller than the original, information in the original image must be thrown away. Although various image-processing strategies can be used to improve image quality, some of the information is nonetheless removed when the original file is shrunk. If the converted image is larger than the original, however, information must be created to fill in the spaces that appear between formerly adjacent pixels when the original image is enlarged. There is no completely satisfactory way to do this, and the processes currently used typically give the enlarged image a block-pixel look.

An example of a bitmap-to-bitmap conversion that is almost always successful is PCX to Microsoft Windows Bitmap.

Vector to Vector

Conversion between vector formats--for example, from AutoCAD DXF to Lotus DIF--is possible and sometimes quite easy. Two serious problems can occur, though. The first comes about due to differences in the number and type of objects available in different vector formats. Some formats, for instance, provide support for only a few simple image elements, such as circles and rectangles. Richer formats may also provide support for many additional complex elements, such as pattern fills, drop shadowing, text fonts, b-splines, and Bezier curves. Attempting to convert a file written in a complex format rich in elements to a simpler format will result in an approximation of the original image.

The second problem comes from the fact that that each vector format has its own interpretation of measurements and the appearance of image elements and primitives. Rarely do two formats agree exactly on the placement and appearance of even simple image elements. Common problems are those related to line joint styles and end styles, and to centerline and centerpoint locations. For example, the Macintosh PICT format assumes that lines are drawn with a pen point that is below and to the right of the specified coordinates, while most other formats center their pens directly on the coordinates. Another example is the GEM VDI format, which assumes that a line should be drawn so that it extends one-half of the width of the line past the end coordinate. Lines in other formats often stop exactly at the end of the coordinate pixel.

It is quite difficult to generalize further than this. If you write a vector-to-vector format converter, you must be aware of the peculiarities of each vector format and the problems of conversion between one specific format and the other.

Metafile to Metafile

Because metafiles can contain both bitmap and vector image data in the same file, problems inherent in bitmap and vector conversion apply to metafiles as well. Generally, the bitmap part of the metafile will convert with success, but the accuracy of the conversion of the vector part will depend on the match to the format to which you are converting. An example of a metafile-to-metafile conversion is Microsoft Windows Metafile (WMF) to CGM.

Vector and Metafile to Bitmap

Converting vector and metafile format files to bitmap format files is generally quite easy. A vector image can be turned into a bitmap simply by dividing it up into component pixels and then writing those pixels to an array in memory in the memory equivalent of a contrasting color. The array can then be saved in a bitmap format file. This process is familiar to users of paint programs, where a mouse or stylus is used to draw geometrical shapes which appear as series of lines on the screen. When the data is written out to disk, however, it is stored in a bitmap file as a series of colored pixels rather than as mathematical data describing the position of the lines making up the image. The ultimate quality of the resulting bitmap image will depend on the resolution of the bitmap being rendered to and the complexity (color, pixel depth, and image features) of the original vector image.

The most common problem occurring with this type of conversion is aliasing, sometimes known as the jaggies. This is where arcs and diagonal lines take on a staircase appearance, partly due to the relatively low resolution of the output bitmap compared to that necessary to adequately support rendering of the output image.

The conversion of ASCII metafile data to binary bitmap data is usually the most complicated and time-consuming part of metafile conversion. As mentioned above in the section discussing the three basic formats, many metafile formats also contain a bitmap image. If conversion from vector to bitmap data achieves poor results, then converting the bitmap data to the desired format may not only result in a better job, but may also be a much quicker process.

A metafile-to-bitmap conversion that is almost always successful is Microsoft Windows Metafile to Microsoft Windows Bitmap.

Bitmap and Metafile to Vector

Converting bitmap and metafile format files to vector format files is usually the hardest of all to perform, and rarely does it achieve any kind of usable results. This is due to the fact that complex image processing algorithms and heuristics are necessary to find all the lines and edges in bitmap images. Once the outlines are found, they must be recognized and converted to their vector element equivalents, and each step is prone to error. Simple bitmap images may be approximated as vector images, usually as black-and-white line drawings, but more complex photographic-type images are nearly impossible to reproduce accurately. Nevertheless, commercial applications exist to provide various types of edge detection and vectorization. Edge detection remains an active area of research. Two examples available for Microsoft Windows are Corel Trace and Adobe Streamline.

Another problem inherent in the conversion of bitmap format files to vector is that of color. Although most bitmap files incorporate many colors, vector formats seldom provide support for more than a few. The conversion of an original bitmap file to a vector file can result in a loss of color in the converted image.

Metafiles also have the same problems associated with the conversion of their bitmap components, although many metafile formats are capable of handling the colors found in the original raster image data. Close vector reproductions of bitmap images are not usually possible unless the bitmap data is very simple.

Bitmap and Vector to Metafile

Converting bitmap format files to metafiles can be quite accurate because most metafile format files can contain a bitmap image as well. Vector format source files have a more limited range of metafile target formats to which they can successfully convert. Problems encountered in this type of conversion are the same as those occurring in bitmap-to-vector conversions.

A common process conversion of this type is the conversion of binary bitmap or vector image files to an ASCII metafile format such as PostScript. Although PostScript is covered only briefly in this book, it is widely used for file interchange, particularly on the Macintosh platform. Such conversions lend portability to image data designed to be moved between machines or which may be directed to a number of different output devices.

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This page is taken from the Encyclopedia of Graphics File Formats and is licensed by O'Reilly under the Creative Common/Attribution license.