Segger - emWin - Antialiasing

Lines are approximated by a series of pixels that must lie at display coordinates. They can therefore appear jagged, particularly lines which are nearly horizontal or nearly vertical. This jaggedness is called aliasing.
Antialiasing is the smoothing of lines and curves. It reduces the jagged, stair-step appearance of any line that is not exactly horizontal or vertical. emWin supports different antialiasing qualities, antialiased fonts and high-resolution coordinates.
Support for anti aliasing is a separate software item and is not included in the emWin basic package.

Introduction

Antialiasing smoothes curves and diagonal lines by "blending" the background color with that of the foreground. The higher the number of shades used between background and foreground colors, the better the antialiasing result (and the longer the computation time).

Quality of anti aliasing

The quality of antialiasing is set by the routine GUI_AA_SetFactor , explained later in the chapter. For an idea of the relationship between the antialiasing factor and the corresponding result, take a look at the image pictured. The first line is drawn without antialiasing (factor 1). The second line is drawn antialiased using factor 2. This means that the number of shades from foreground to background is 2 x 2 = 4. The next line is drawn with an antialiasing factor of 3, so there are 3 x 3 = 9 shades, and so on. Factor 4 should be sufficient for most applications. Increasing the antialiasing factor further does not improve the result dramatically, but increases the calculation time.

Antialiased fonts

Two types of antialiased fonts, low-quality (2bpp) and high-quality (4bpp), are supported. The routines needed to display these fonts are automatically linked when using them. The following table shows the effect on drawing the character "C" without antialiasing and with both types of antialiased fonts:

Font type	Black on white	White on black
Standard (no antialiasing) 1 bpp 2 shades
Low-quality (antialiased) 2 bpp 4 shades
High-quality (antialiased) 4 bpp 16 shades

Antialiased fonts can be created with the emWin font converter. The general purpose of using antialiased fonts is to improve the appearance of text. While the effect of using high-quality antialiasing will be more visually pleasing than low-quality, computation time and memory consumption will increase proportionally. Low-quality (2bpp) fonts require twice the memory of non-antialiased (1bpp) fonts; high-quality (4bpp) fonts require four times the memory.

High-resolution coordinates

When drawing items using antialiasing, the same coordinates are used as for regular (non-antialiasing) drawing routines. This is the default mode. You do not need to consider the antialiasing factor in the function arguments. For example, to draw an antialiased line from (50, 100) to (100, 50) you would write:

GUI_AA_DrawLine(50, 100, 100, 50);

The high-resolution feature of emWin lets you use the virtual space determined by the antialiasing factor and your display size. High-resolution coordinates must be enabled with the routine GUI_AA_EnableHiRes , and may be disabled with GUI_AA_DisableHiRes . Both functions are explained later in the chapter. The advantage of using high-resolution coordinates is that items can be placed not only at physical positions of your display but also "between" them. The virtual space of a high-resolution pixel is illustrated below based on an antialiasing factor of 3:

To draw a line from pixel (50, 100) to (100, 50) in high-resolution mode with antialiasing factor 3, you would write:

GUI_AA_DrawLine(150, 300, 300, 150);

API reference: antialiasing

The table below lists the available routines in the antialiasing package, in alphabetical order within their respective categories. Detailed descriptions of the routines can be found in the sections that follow:

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