Make visual bell flash much more gentle

Right now visual bell makes background flash sharply with bright white
(when configured with darkish color theme). This causes eye strain,
especially prominent in unlit environments.

This change makes background bounce smoothly between regular bg color
and highlight (selection) bg color for the configured visual bell
duration. Intensity is animated with cubic easing functions. It
currently peaks at 20% of the duration, this is hardcoded.

kitty/cell_vertex.glsl

@@ -12,7 +12,7 @@
 
 // Inputs {{{
 layout(std140) uniform CellRenderData {
-    float xstart, ystart, dx, dy, sprite_dx, sprite_dy, background_opacity, cursor_text_uses_bg;
+    float xstart, ystart, dx, dy, sprite_dx, sprite_dy, background_opacity, cursor_text_uses_bg, visual_bell_intensity;
 
     uint default_fg, default_bg, highlight_fg, highlight_bg, cursor_color, cursor_text_color, url_color, url_style, inverted;
 
@@ -236,6 +236,7 @@ void main() {
 
 #if defined(SPECIAL) || defined(SIMPLE)
     // Selection and cursor
+    bg = max(bg, choose_color(visual_bell_intensity, color_to_vec(highlight_bg), bg));
     bg = choose_color(float(is_selected & ONE), color_to_vec(highlight_bg), bg);
     background = choose_color(cell_has_block_cursor, color_to_vec(cursor_color), bg);
 #if !defined(TRANSPARENT) && defined(SPECIAL)

kitty/screen.c

@@ -1324,12 +1324,43 @@ screen_use_latin1(Screen *self, bool on) {
 bool
 screen_invert_colors(Screen *self) {
     bool inverted = false;
+    if (self->modes.mDECSCNM) inverted = true;
+    return inverted;
+}
+
+float
+ease_out_cubic(float phase) {
+    return 1.0f - powf(1.0f - phase, 3.0f);
+}
+
+float
+ease_in_out_cubic(float phase) {
+    return phase < 0.5f ?
+        4.0f * powf(phase, 3.0f) :
+        1.0f - powf(-2.0f * phase + 2.0f, 3.0f) / 2.0f;
+}
+
+float
+visual_bell_intensity(float phase) {
+    float peak = 0.2f;
+    float fade = 1.0f - peak;
+    if (phase < peak)
+        return ease_out_cubic(phase / peak);
+    else
+        return ease_in_out_cubic((1.0f - phase) / fade);
+}
+
+float
+screen_visual_bell_intensity(Screen *self) {
     if (self->start_visual_bell_at > 0) {
-        if (monotonic() - self->start_visual_bell_at <= OPT(visual_bell_duration)) inverted = true;
+        monotonic_t progress = monotonic() - self->start_visual_bell_at;
+        monotonic_t duration = OPT(visual_bell_duration);
+        if (progress <= duration) {
+            return visual_bell_intensity((float)progress / duration);
+        }
         else self->start_visual_bell_at = 0;
     }
-    if (self->modes.mDECSCNM) inverted = inverted ? false : true;
-    return inverted;
+    return 0.0f;
 }
 
 void

kitty/screen.h

@@ -194,6 +194,7 @@ void screen_apply_selection(Screen *self, void *address, size_t size);
 bool screen_is_selection_dirty(Screen *self);
 bool screen_has_selection(Screen*);
 bool screen_invert_colors(Screen *self);
+float screen_visual_bell_intensity(Screen *self);
 void screen_update_cell_data(Screen *self, void *address, FONTS_DATA_HANDLE, bool cursor_has_moved);
 bool screen_is_cursor_visible(Screen *self);
 bool screen_selection_range_for_line(Screen *self, index_type y, index_type *start, index_type *end);

kitty/shaders.c

@@ -250,9 +250,9 @@ send_graphics_data_to_gpu(size_t image_count, ssize_t gvao_idx, const ImageRende
 }
 
 static inline void
-cell_update_uniform_block(ssize_t vao_idx, Screen *screen, int uniform_buffer, GLfloat xstart, GLfloat ystart, GLfloat dx, GLfloat dy, CursorRenderInfo *cursor, bool inverted, OSWindow *os_window) {
+cell_update_uniform_block(ssize_t vao_idx, Screen *screen, int uniform_buffer, GLfloat xstart, GLfloat ystart, GLfloat dx, GLfloat dy, CursorRenderInfo *cursor, bool inverted, GLfloat visual_bell_intensity, OSWindow *os_window) {
     struct CellRenderData {
-        GLfloat xstart, ystart, dx, dy, sprite_dx, sprite_dy, background_opacity, cursor_text_uses_bg;
+        GLfloat xstart, ystart, dx, dy, sprite_dx, sprite_dy, background_opacity, cursor_text_uses_bg, visual_bell_intensity;
 
         GLuint default_fg, default_bg, highlight_fg, highlight_bg, cursor_color, cursor_text_color, url_color, url_style, inverted;
 
@@ -295,6 +295,7 @@ cell_update_uniform_block(ssize_t vao_idx, Screen *screen, int uniform_buffer, G
     rd->sprite_dx = 1.0f / (float)x; rd->sprite_dy = 1.0f / (float)y;
     rd->inverted = inverted ? 1 : 0;
     rd->background_opacity = os_window->is_semi_transparent ? os_window->background_opacity : 1.0f;
+    rd->visual_bell_intensity = visual_bell_intensity;
 
 #define COLOR(name) colorprofile_to_color(screen->color_profile, screen->color_profile->overridden.name, screen->color_profile->configured.name)
     rd->default_fg = COLOR(default_fg); rd->default_bg = COLOR(default_bg); rd->highlight_fg = COLOR(highlight_fg); rd->highlight_bg = COLOR(highlight_bg);
@@ -617,8 +618,9 @@ void
 draw_cells(ssize_t vao_idx, ssize_t gvao_idx, GLfloat xstart, GLfloat ystart, GLfloat dx, GLfloat dy, Screen *screen, OSWindow *os_window, bool is_active_window, bool can_be_focused) {
     CELL_BUFFERS;
     bool inverted = screen_invert_colors(screen);
+    GLfloat intensity = (GLfloat)screen_visual_bell_intensity(screen);
 
-    cell_update_uniform_block(vao_idx, screen, uniform_buffer, xstart, ystart, dx, dy, &screen->cursor_render_info, inverted, os_window);
+    cell_update_uniform_block(vao_idx, screen, uniform_buffer, xstart, ystart, dx, dy, &screen->cursor_render_info, inverted, intensity, os_window);
 
     bind_vao_uniform_buffer(vao_idx, uniform_buffer, cell_program_layouts[CELL_PROGRAM].render_data.index);
     bind_vertex_array(vao_idx);