Improve fl_contrast() functionality and documentation
- swap 3rd and 4th parameter of fl_contrast() - change 'Fl_Fontsize fs' to a neutral 'int size' parameter - clarify documentation of fl_contrast_level() and that different contrast algorithms use different level values - clarify that level 39 for the CIELAB mode results in "sufficient" contrast, i.e. /readable/ text which is the only intention of fl_contrast(). Reasoning: swapping the arguments looks more natural, i.e. the context before another argument (size) is what one would usually expect. Since the 'size' argument doesn't have be a font size, making it a simple 'int size' is more flexible and correct for future extensions.
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Albrecht Schlosser
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7410498a7f
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d0922792ae
@ -1161,19 +1161,22 @@ FL_EXPORT Fl_Color fl_inactive(Fl_Color c);
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with fl_contrast_mode(FL_CONTRAST_CUSTOM).
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Example:
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\code
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Fl_Color my_contrast(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int context) {
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Fl_Color my_contrast(Fl_Color fg, Fl_Color bg, int context, int size) {
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// calculate contrast and ...
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return color;
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}
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// call this early in your main() program:
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fl_contrast_function(my_contrast);
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fl_contrast_mode(FL_CONTRAST_CUSTOM);
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fl_contrast_level(50); // optional, must be called after fl_contrast_mode()
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\endcode
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\see fl_contrast(Fl_Color, Fl_Color, Fl_Fontsize, int)
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For parameters and types see fl_contrast(Fl_Color, Fl_Color, int, int).
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\see fl_contrast(Fl_Color, Fl_Color, int, int)
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\see fl_contrast_mode(int)
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*/
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typedef Fl_Color (Fl_Contrast_Function)(Fl_Color, Fl_Color, Fl_Fontsize, int);
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typedef Fl_Color (Fl_Contrast_Function)(Fl_Color, Fl_Color, int, int);
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FL_EXPORT void fl_contrast_function(Fl_Contrast_Function *f);
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@ -1195,7 +1198,7 @@ FL_EXPORT int fl_contrast_level();
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FL_EXPORT void fl_contrast_mode(int mode);
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FL_EXPORT int fl_contrast_mode();
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FL_EXPORT Fl_Color fl_contrast(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs = 0, int context = 0);
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FL_EXPORT Fl_Color fl_contrast(Fl_Color fg, Fl_Color bg, int context = 0, int size = 0);
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FL_EXPORT double fl_lightness(Fl_Color color);
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FL_EXPORT double fl_luminance(Fl_Color color);
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@ -1,7 +1,7 @@
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//
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// Color contrast functions for the Fast Light Tool Kit (FLTK).
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//
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// Copyright 1998-2022 by Bill Spitzak and others.
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// Copyright 1998-2024 by Bill Spitzak and others.
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//
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// This library is free software. Distribution and use rights are outlined in
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// the file "COPYING" which should have been included with this file. If this
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@ -46,8 +46,8 @@ static int fl_contrast_level_[10] = {
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static Fl_Contrast_Function *fl_contrast_function_ = 0;
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// The following function is (and must be!) the same as Fl::get_color()
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// but can be inlined. We need this additional implementation because all contrast
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// The following function is (and must be!) the same as Fl::get_color() but
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// can be inlined. We need this additional implementation because all contrast
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// related functions have been moved from fl_color.cxx to fl_contrast.cxx
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// or have been directly implemented in fl_contrast.cxx (new functions).
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// Inlining will hopefully prevent an extra function call.
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@ -139,12 +139,13 @@ double fl_lightness(Fl_Color color) {
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This can be used to tune the legacy fl_contrast() function to achieve
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slightly better results. The default value is defined per contrast mode
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(see below). Values between 50 and 70 are recommended but you can raise
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it up to 100. Lower values than 50 are probably not useful.
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(see below). Values between 50 and 70 may be useful for the legacy contrast
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mode but you can raise it up to 100. Lower values than 50 are probably not
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useful.
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The contrast \p level affects not only the legacy (1.3.x) fl_contrast()
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function but also the new CIELAB contrast mode which is the default since
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FLTK 1.4.0.
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FLTK 1.4.0. See default value below.
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Other contrast modes are currently not affected by the contrast level.
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@ -157,7 +158,7 @@ double fl_lightness(Fl_Color color) {
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The default contrast level is
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- 50 in mode FL_CONTRAST_LEGACY (compatible with FLTK 1.3.x)
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- 55 in mode FL_CONTRAST_CIELAB
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- 39 in mode FL_CONTRAST_CIELAB (similar threshold as in FLTK 1.3.x)
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- 0 (undefined) for all other modes
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See the description of fl_contrast_mode(int mode) for more information about
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@ -166,7 +167,7 @@ double fl_lightness(Fl_Color color) {
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Example:
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\code
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fl_contrast_mode(FL_CONTRAST_LEGACY);
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fl_contrast_level(65);
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fl_contrast_level(60);
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\endcode
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A \p level greater than 50 (probably best in the range 50 to 70) may achieve
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@ -208,7 +209,7 @@ int fl_contrast_level() {
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- FL_CONTRAST_NONE (not recommended: returns the foreground color)
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- FL_CONTRAST_LEGACY (same as in FLTK 1.3.x)
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- FL_CONTRAST_CIELAB (default since FLTK 1.4.0)
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- FL_CONTRAST_CIELAB (better, this is the default since FLTK 1.4.0)
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- FL_CONTRAST_CUSTOM (you must define your own contrast algorithm)
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If you set FL_CONTRAST_CUSTOM you must also register your custom
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@ -227,12 +228,24 @@ int fl_contrast_level() {
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in higher contrast, i.e. the algorithm switches "earlier" to
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black or white mode.
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- FL_CONTRAST_CIELAB: defaut level is 55 which appears to be a good
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- FL_CONTRAST_CIELAB: defaut level is 39 which appears to be a good
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value. The higher the level is, the more contrast is to be expected.
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Values in the range below 55 accept lower contrast and values above
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55 switch "earlier" to black or white. Values between 45 and 65 may
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Values in the range below 39 accept lower contrast and values above
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39 switch "earlier" to black or white. Values between 36 and 46 may
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yield usable contrast experience.
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\note The goal of fl_contrast() is to achieve a "sufficient" contrast
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between text and background. Level 39 in CIELAB mode means that the
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accepted contrast is about 39% of the lightness difference between
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both colors. This can be perceived as very low contrast in some cases,
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but the text should at least be readable. Note that the highest possible
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contrast value on a medium gray background is 50% (either black or white).
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Bill Spitzak wrote on May 16, 2024 in fltk.general in thread "FLTK 1.4
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Menu Bar Style": <i>"I would certainly aim for a function that does not
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alter color combinations where it is physically possible to read the
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text, even if squinting is needed."</i>\n
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See https://groups.google.com/g/fltkgeneral/c/EkWI4HTHSLA/m/rsZunZ1vAwAJ
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\param[in] mode if invalid, FL_CONTRAST_CIELAB will be selected
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\since 1.4.0
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@ -269,15 +282,16 @@ int fl_contrast_mode() {
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Your custom contrast function must provide the signature
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\code
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Fl_Color my_contrast_function(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int context)
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Fl_Color my_contrast_function(Fl_Color fg, Fl_Color bg, int context, int size)
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\endcode
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The arguments are the same as for the full fl_contrast() function since FLTK 1.4.
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You can use the supplied fontsize \p fs to modify the result. Depending on the
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caller the \p fs parameter can be 0 (default) or a valid fontsize.
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You can use the supplied \p size to modify the result. Depending on the
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caller the \p size parameter can be 0 (default) or a valid size. In the context
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of text, i.e. \p context == 0, the \p size parameter is the fontsize.
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The \p context parameter is not yet used and will always be 0 unless included in
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a call to fl_contrast(). The value 0 should be interpreted as text.
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a call to fl_contrast(). The value 0 must be interpreted as text.
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In the future the \p context argument will be used to supply a different context
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than text (small icons, large icons, etc.). The exact usage is not yet specified.
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@ -309,10 +323,10 @@ void fl_contrast_function(Fl_Contrast_Function *f) {
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\param[in] fs,context fontsize and context (unused)
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\return contrasting color
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*/
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static Fl_Color fl_contrast_legacy(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int context) {
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static Fl_Color fl_contrast_legacy(Fl_Color fg, Fl_Color bg, int context, int size) {
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(void) fs; // currently ignored
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(void) context; // currently ignored
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(void) size; // currently ignored
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// internal static variables, recalculated only if fl_contrast_level() is changed
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@ -378,10 +392,10 @@ static Fl_Color fl_contrast_legacy(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int
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\param[in] fs,context unused: fontsize and context
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\return contrasting color
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*/
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static Fl_Color fl_contrast_cielab(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int context) {
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static Fl_Color fl_contrast_cielab(Fl_Color fg, Fl_Color bg, int context, int size) {
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(void) fs; // currently ignored
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(void) context; // currently ignored
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(void) size; // currently ignored
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double tc = (double)fl_contrast_level(); // sufficient contrast threshold
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double tbw = 50.; // black/white threshold
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@ -421,15 +435,16 @@ static Fl_Color fl_contrast_cielab(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int
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You can change the behavior of fl_contrast() in several ways:
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- Change the "level" (sensitivity) for contrast calculation, see fl_contrast_level().
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Valid levels are 0 - 100, the default "medium" value is 50. If you raise the level
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above 50 the overall contrast will generally be higher, i.e. the required contrast
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to return the foreground color is raised and therefore the calculated color will
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switch "earlier" to either black or white.
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Valid levels are 0 - 100, the default "medium" value depends on the contrast mode.
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If you raise the level above the default value the overall contrast will generally
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be higher, i.e. the required contrast to return the foreground color is raised and
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therefore the calculated color switches "earlier" to either black or white.
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In other words, using the following values:
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- 0 will always use the foreground color
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- 50 will use the default, unmodified algorithm
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- 0 always uses the foreground color
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- the default, unmodified algorithm allows a sufficient contrast such that the text
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is readable
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- 100 will always use black or white
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- values larger than 50 may yield slightly better results.
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Changing the \p level is particularly useful and intended for the "legacy mode"
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to improve the results partially. Values slightly above 50 (50 - 70) will likely
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return the best results (50 is the default, as used in FLTK 1.3.x).
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@ -454,25 +469,33 @@ static Fl_Color fl_contrast_cielab(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int
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to use the new default function:
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- FL_CONTRAST_CIELAB, based on the CIELAB (L*a*b*) color model. This function
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is superior regarding the visual contrast perception but may be slower.
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is superior regarding the human contrast perception but may be slightly
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slower - which should not matter on a modern CPU. The default contrast
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level in this mode is 39 which results in a very similar experience as the
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old contrast function but avoids unreadable border cases.
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<b>This is the default since FLTK 1.4.0.</b>
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- FL_CONTRAST_CUSTOM, your own contrast calculation function.
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In the future we \b may provide even more (and superior) contrast algorithms.
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The new parameters \p fs and \p context (since 1.4.0) are defined for future
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extensions and are currently not used. Default values are 0.
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The new parameters \p context and \p size (since 1.4.0) are defined for
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future extensions and are currently not used. Default values are 0.
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- The \p context is intended to differentiate text and other kinds
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of objects, e.g. radio buttons, check marks, or icon types.
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- The \p size parameter is an unspecified (object) size that may be used to
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calculate the required contrast. In text mode this must be the font size.
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Rule: the larger the object (font), the lower the required contrast.
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\note These new optional parameters must be provided in the custom contrast
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function which is the reason why they are added now. In the future we may use
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the fontsize to adjust the calculated contrast, and users defining their own
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contrast functions may use them in their functions.
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the (font) size to adjust the calculated contrast, and users defining their
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own contrast functions may use them in their functions.
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\param[in] fg foreground (text) color
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\param[in] bg background color
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\param[in] fs font size (optional, default = 0 == undefined)
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\param[in] context graphical context (optional, default = 0 == text)
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\param[in] size unspecified size (optional, default = 0 == undefined)
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\return contrasting color: \p fg, \p FL_BLACK, or \p FL_WHITE
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@ -480,21 +503,21 @@ static Fl_Color fl_contrast_cielab(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int
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\see fl_contrast_mode(int)
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\see fl_contrast_function()
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*/
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Fl_Color fl_contrast(Fl_Color fg, Fl_Color bg, Fl_Fontsize fs, int context) {
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Fl_Color fl_contrast(Fl_Color fg, Fl_Color bg, int context, int size) {
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switch (fl_contrast_mode_) {
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case FL_CONTRAST_LEGACY:
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return fl_contrast_legacy(fg, bg, fs, context);
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return fl_contrast_legacy(fg, bg, context, size);
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case FL_CONTRAST_CUSTOM:
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if (fl_contrast_function_)
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return (fl_contrast_function_)(fg, bg, fs, context);
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return (fl_contrast_function_)(fg, bg, context, size);
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// FALLTHROUGH
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case FL_CONTRAST_CIELAB:
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return fl_contrast_cielab(fg, bg, fs, context);
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return fl_contrast_cielab(fg, bg, context, size);
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default: // unknown (none): return fg
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break;
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