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diff --git a/freetype/docs/reference/ft2-lcd_filtering.html b/freetype/docs/reference/ft2-lcd_filtering.html index 9965c432d..da50ff5f5 100644 --- a/freetype/docs/reference/ft2-lcd_filtering.html +++ b/freetype/docs/reference/ft2-lcd_filtering.html @@ -3,7 +3,7 @@ <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> -<title>FreeType-2.4.11 API Reference</title> +<title>FreeType-2.5.0 API Reference</title> <style type="text/css"> body { font-family: Verdana, Geneva, Arial, Helvetica, serif; color: #000000; @@ -35,7 +35,7 @@ <table align=center><tr><td><font size=-1>[<a href="ft2-index.html">Index</a>]</font></td> <td width="100%"></td> <td><font size=-1>[<a href="ft2-toc.html">TOC</a>]</font></td></tr></table> -<center><h1>FreeType-2.4.11 API Reference</h1></center> +<center><h1>FreeType-2.5.0 API Reference</h1></center> <center><h1> LCD Filtering @@ -49,6 +49,20 @@ LCD Filtering <table align=center width="87%"><tr><td> <p>The <a href="ft2-lcd_filtering.html#FT_Library_SetLcdFilter">FT_Library_SetLcdFilter</a> API can be used to specify a low-pass filter which is then applied to LCD-optimized bitmaps generated through <a href="ft2-base_interface.html#FT_Render_Glyph">FT_Render_Glyph</a>. This is useful to reduce color fringes which would occur with unfiltered rendering.</p> <p>Note that no filter is active by default, and that this function is <b>not</b> implemented in default builds of the library. You need to #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your ‘ftoption.h’ file in order to activate it.</p> +<p>FreeType generates alpha coverage maps, which are linear by nature. For instance, the value 0x80 in bitmap representation means that (within numerical precision) 0x80/0xff fraction of that pixel is covered by the glyph's outline. The blending function for placing text over a background is</p> +<pre class="colored"> + dst = alpha * src + (1 - alpha) * dst , +</pre> +<p>which is known as OVER. However, when calculating the output of the OVER operator, the source colors should first be transformed to a linear color space, then alpha blended in that space, and transformed back to the output color space.</p> +<p>When linear light blending is used, the default FIR5 filtering weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as they have been designed for black on white rendering while lacking gamma correction. To preserve color neutrality, weights for a FIR5 filter should be chosen according to two free parameters ‘a’ and ‘c’, and the FIR weights should be</p> +<pre class="colored"> + [a - c, a + c, 2 * a, a + c, a - c] . +</pre> +<p>This formula generates equal weights for all the color primaries across the filter kernel, which makes it colorless. One suggested set of weights is</p> +<pre class="colored"> + [0x10, 0x50, 0x60, 0x50, 0x10] , +</pre> +<p>where ‘a’ has value 0x30 and ‘b’ value 0x20. The weights in filter may have a sum larger than 0x100, which increases coloration slightly but also improves contrast.</p> </td></tr></table><br> <table align=center width="75%"><tr><td> <h4><a name="FT_LcdFilter">FT_LcdFilter</a></h4> |