render: dim and brighten using linear rgb interpolation

Adds setting tweak.dim-amount, similar to bold-text-in-bright-amount.

Closes #2006

CHANGELOG.md

@@ -64,6 +64,12 @@
 ### Changed
 
 * UTF-8 error recovery now discards fewer bytes.
+* Auto-calculated dimmed and brightened colors (e.g. when custom dim
+  colors has not configured) is now done by linear RGB interpolation,
+  rather than converting to HSL and adjusting the luminance
+  ([#2006][2006]).
+
+[2006]: https://codeberg.org/dnkl/foot/issues/2006
 
 
 ### Deprecated

config.c

@@ -2759,6 +2759,9 @@ parse_section_tweak(struct context *ctx)
     else if (streq(key, "sixel"))
         return value_to_bool(ctx, &conf->tweak.sixel);
 
+    else if (streq(key, "dim-amount"))
+        return value_to_float(ctx, &conf->dim.amount);
+
     else if (streq(key, "bold-text-in-bright-amount"))
         return value_to_float(ctx, &conf->bold_in_bright.amount);
 
@@ -3288,6 +3291,7 @@ config_load(struct config *conf, const char *conf_path,
         .resize_by_cells = true,
         .resize_keep_grid = true,
         .resize_delay_ms = 100,
+        .dim = { .amount = 1.5 },
         .bold_in_bright = {
             .enabled = false,
             .palette_based = false,

config.h

@@ -155,6 +155,10 @@ struct config {
 
     uint16_t resize_delay_ms;
 
+    struct {
+        float amount;
+    } dim;
+
     struct {
         bool enabled;
         bool palette_based;

doc/foot.ini.5.scd

@@ -396,7 +396,7 @@ empty string to be set, but it must be quoted: *KEY=""*)
 *bold-text-in-bright*
 	Semi-boolean. When enabled, bold text is rendered in a brighter
 	color (in addition to using a bold font). The color is brightened
-	by increasing its luminance.
+	by blending it with white.
 	
 	If set to *palette-based*, rather than a simple *yes|true*, colors
 	matching one of the 8 regular palette colors will be brightened
@@ -986,8 +986,8 @@ can configure the background transparency with the _alpha_ option.
 	an entry in the color palette. Applications emit them by combining
 	a color value, and a "dim" attribute.
 	
-	By default, foot implements this by reducing the luminance of the
-	current color. This is a generic approach that applies to both
+	By default, foot implements this by blending the current color
+	with black. This is a generic approach that applies to both
 	colors from the 256-color palette, as well as 24-bit RGB colors.
 	
 	You can change this behavior by setting the *dimN* options. When
@@ -999,7 +999,7 @@ can configure the background transparency with the _alpha_ option.
 	the corresponding *regularN* color will be used.
 	
 	If the current color does not match any known color, it is dimmed
-	by reducing the luminance (i.e. the same behavior as if the *dimN*
+	by blending with black (i.e. the same behavior as if the *dimN*
 	options are unconfigured). 24-bit RGB colors will typically fall
 	into this category.
 	
@@ -1940,6 +1940,9 @@ any of these options.
 	Boolean. When enabled, foot will process sixel images. Default:
 	_yes_
 
+*dim-amount*
+	Amount by which dimmed text is darkened. Default: _1.5_.
+
 *bold-text-in-bright-amount*
 	Amount by which bold fonts are brightened when
 	*bold-text-in-bright* is set to *yes* (the *palette-based* variant

hsl.c

@@ -2,41 +2,6 @@
 
 #include <math.h>
 
-#include "util.h"
-
-void
-rgb_to_hsl(uint32_t rgb, int *hue, int *sat, int *lum)
-{
-    double r = (double)((rgb >> 16) & 0xff) / 255.;
-    double g = (double)((rgb >> 8) & 0xff) / 255.;
-    double b = (double)((rgb >> 0) & 0xff) / 255.;
-
-    double x_max = max(max(r, g), b);
-    double x_min = min(min(r, g), b);
-    double V = x_max;
-
-    double C = x_max - x_min;
-    double L = (x_max + x_min) / 2.;
-
-    *lum = 100 * L;
-
-    if (C == 0.0)
-        *hue = 0;
-    else if (V == r)
-        *hue = 60. * (0. + (g - b) / C);
-    else if (V == g)
-        *hue = 60. * (2. + (b - r) / C);
-    else if (V == b)
-        *hue = 60. * (4. + (r - g) / C);
-    if (*hue < 0)
-        *hue += 360;
-
-    double S = C == 0.0
-        ? 0
-        : C / (1. - fabs(2. * L - 1.));
-    *sat = 100 * S;
-}
-
 uint32_t
 hsl_to_rgb(int hue, int sat, int lum)
 {

hsl.h

@@ -2,5 +2,4 @@
 
 #include <stdint.h>
 
-void rgb_to_hsl(uint32_t rgb, int *hue, int *sat, int *lum);
 uint32_t hsl_to_rgb(int hue, int sat, int lum);

render.c

@@ -34,7 +34,6 @@
 #include "config.h"
 #include "cursor-shape.h"
 #include "grid.h"
-#include "hsl.h"
 #include "ime.h"
 #include "quirks.h"
 #include "search.h"
@@ -271,13 +270,23 @@ color_hex_to_pixman(uint32_t color, bool srgb)
     return color_hex_to_pixman_with_alpha(color, 0xffff, srgb);
 }
 
+static inline int i_lerp(int from, int to, float t) {
+    return from + (to - from) * t;
+}
+
 static inline uint32_t
-color_decrease_luminance(uint32_t color)
+color_blend_towards(uint32_t from, uint32_t to, float amount)
 {
-    uint32_t alpha = color & 0xff000000;
-    int hue, sat, lum;
-    rgb_to_hsl(color, &hue, &sat, &lum);
-    return alpha | hsl_to_rgb(hue, sat, lum / 1.5);
+    if (unlikely(amount == 0))
+        return from;
+    float t = 1 - 1/amount;
+
+    uint32_t alpha = from & 0xff000000;
+    uint8_t r = i_lerp((from>>16)&0xff, (to>>16)&0xff, t);
+    uint8_t g = i_lerp((from>>8)&0xff, (to>>8)&0xff, t);
+    uint8_t b = i_lerp((from>>0)&0xff, (to>>0)&0xff, t);
+
+    return alpha | (r<<16) | (g<<8) | (b<<0);
 }
 
 static inline uint32_t
@@ -286,25 +295,24 @@ color_dim(const struct terminal *term, uint32_t color)
     const struct config *conf = term->conf;
     const uint8_t custom_dim = conf->colors.use_custom.dim;
 
-    if (likely(custom_dim == 0))
-        return color_decrease_luminance(color);
+    if (unlikely(custom_dim != 0)) {
+        for (size_t i = 0; i < 8; i++) {
+            if (((custom_dim >> i) & 1) == 0)
+                continue;
 
-    for (size_t i = 0; i < 8; i++) {
-        if (((custom_dim >> i) & 1) == 0)
-            continue;
+            if (term->colors.table[0 + i] == color) {
+                /* "Regular" color, return the corresponding "dim" */
+                return conf->colors.dim[i];
+            }
 
-        if (term->colors.table[0 + i] == color) {
-            /* "Regular" color, return the corresponding "dim" */
-            return conf->colors.dim[i];
-        }
-
-        else if (term->colors.table[8 + i] == color) {
-            /* "Bright" color, return the corresponding "regular" */
-            return term->colors.table[i];
+            else if (term->colors.table[8 + i] == color) {
+                /* "Bright" color, return the corresponding "regular" */
+                return term->colors.table[i];
+            }
         }
     }
 
-    return color_decrease_luminance(color);
+    return color_blend_towards(color, 0x00000000, conf->dim.amount);
 }
 
 static inline uint32_t
@@ -322,11 +330,7 @@ color_brighten(const struct terminal *term, uint32_t color)
         return color;
     }
 
-    int hue, sat, lum;
-    rgb_to_hsl(color, &hue, &sat, &lum);
-
-    lum = (int)roundf(lum * term->conf->bold_in_bright.amount);
-    return hsl_to_rgb(hue, sat, min(lum, 100));
+    return color_blend_towards(color, 0x00ffffff, term->conf->bold_in_bright.amount);
 }
 
 static void
@@ -798,7 +802,7 @@ render_cell(struct terminal *term, pixman_image_t *pix,
         _fg = color_brighten(term, _fg);
 
     if (cell->attrs.blink && term->blink.state == BLINK_OFF)
-        _fg = color_decrease_luminance(_fg);
+        _fg = color_blend_towards(_fg, 0x00000000, term->conf->dim.amount);
 
     const bool gamma_correct = render_do_linear_blending(term);
     pixman_color_t fg = color_hex_to_pixman(_fg, gamma_correct);