with transparent background, "shades of gray" require an image format allowing partial transparency. Scaling Bitmap fonts look best at their native pixel size. Some systems using bitmap fonts can create some font variants algorithmically. For example, the original Apple Macintosh computer could produce bold by widening vertical strokes and oblique by shearing the image. At non-native sizes, many text rendering systems perform nearest-neighbor resampling, introducing rough jagged edges. More advanced systems perform anti-aliasing on bitmap fonts whose size does not match the size that the application requests. This technique works well for making the font smaller but not as well for increasing the size, as it tends to blur the edges. Some graphics systems that use bitmap fonts, especially those of emulators, apply curve-sensitive nonlinear resampling algorithms such as 2xSaI or hq3x on fonts and other bitmaps, which avoids blurring the font while introducing little objectionable distortion at moderate increases in size. The difference between bitmap fonts and outline fonts is similar to the difference between bitmap and vector image file formats. Bitmap fonts are like image formats such as Windows Bitmap (.bmp), Portable Network Graphics (.png) and Tagged Image Format (.tif or .tiff), which store the image data as a grid of pixels, in some cases with compression. Outline or stroke image formats such as Windows Metafile format (.wmf) and Scalable Vector Graphics format (.svg), store instructions in the form of lines and curves of how to draw the image rather than storing the image itself. A "trace" program can follow the outline of a high-resolution bitmap font and create an initial outline that a font designer uses to create an outline font useful in systems such as PostScript or TrueType. Outline fonts scale easily without jagged edges or blurriness. Bitmap font formats Portable Compiled Format (PCF) Glyph Bitmap Distribution Format (BDF) Server Normal Format (SNF) DECWindows Font (DWF) Sun X11/NeWS format (BF, AFM) Microsoft Windows bitmapped font (FON) Amiga Font, ColorFont, AnimFont ByteMap Font (BMF) PC Screen Font (PSF) Scalable Screen Font (SFN, also supports outline fonts) Packed bitmap font bitmap file for TeX DVI drivers (PK) FZX a proportional bitmap font for the ZX Spectrum Outline fonts Outline fonts or vector fonts are collections of vector images, consisting of lines and curves defining the boundary of glyphs. Early vector fonts were used by vector monitors and vector plotters using their own internal fonts, usually with thin single