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.

Mark functions computing intensity and easing as inline

Do smarter blending of visual bell flash

Blend highlight color with pegtop's softlight mode over both background
and half as much over foreground. This should help with flash visibility
in light themed and inverted colors contexts.

Blend flash in BACKGROUND pass also

Revert "Do smarter blending of visual bell flash"

This reverts commit 9a269d55c7.

Revert "Blend flash in BACKGROUND pass also"

This reverts commit 756332cb9d.

Revert "Make visual bell flash much more gentle"

This reverts commit cbfe5d59ad.

Render flash overlay in a separate drawcall

Trigger frequent repaints to make for smooth animation

Attenuate flash a bit more

kitty/child-monitor.c

@@ -615,7 +615,7 @@ prepare_to_render_os_window(OSWindow *os_window, monotonic_t now, unsigned int *
 }
 
 static void
-render_os_window(OSWindow *os_window, monotonic_t now, unsigned int active_window_id, color_type active_window_bg, unsigned int num_visible_windows, bool all_windows_have_same_bg) {
+render_os_window(OSWindow *os_window, unsigned int active_window_id, color_type active_window_bg, unsigned int num_visible_windows, bool all_windows_have_same_bg) {
     // ensure all pixels are cleared to background color at least once in every buffer
     if (os_window->clear_count++ < 3) blank_os_window(os_window);
     Tab *tab = os_window->tabs + os_window->active_tab;
@@ -637,8 +637,7 @@ render_os_window(OSWindow *os_window, monotonic_t now, unsigned int active_windo
             bool is_active_window = i == tab->active_window;
             draw_cells(WD.vao_idx, WD.gvao_idx, WD.xstart, WD.ystart, WD.dx * x_ratio, WD.dy * y_ratio, WD.screen, os_window, is_active_window, true);
             if (WD.screen->start_visual_bell_at != 0) {
-                monotonic_t bell_left = OPT(visual_bell_duration) - (now - WD.screen->start_visual_bell_at);
-                set_maximum_wait(bell_left);
+                set_maximum_wait(OPT(repaint_delay));
             }
             w->cursor_visible_at_last_render = WD.screen->cursor_render_info.is_visible; w->last_cursor_x = WD.screen->cursor_render_info.x; w->last_cursor_y = WD.screen->cursor_render_info.y; w->last_cursor_shape = WD.screen->cursor_render_info.shape;
         }
@@ -726,7 +725,7 @@ render(monotonic_t now, bool input_read) {
         if (prepare_to_render_os_window(w, now, &active_window_id, &active_window_bg, &num_visible_windows, &all_windows_have_same_bg, scan_for_animated_images)) needs_render = true;
         if (w->last_active_window_id != active_window_id || w->last_active_tab != w->active_tab || w->focused_at_last_render != w->is_focused) needs_render = true;
         if (w->render_calls < 3 && w->bgimage && w->bgimage->texture_id) needs_render = true;
-        if (needs_render) render_os_window(w, now, active_window_id, active_window_bg, num_visible_windows, all_windows_have_same_bg);
+        if (needs_render) render_os_window(w, active_window_id, active_window_bg, num_visible_windows, all_windows_have_same_bg);
         if (w->is_focused) change_menubar_title(w->window_title);
     }
     last_render_at = now;

kitty/screen.c

@@ -1512,12 +1512,43 @@ screen_use_latin1(Screen *self, bool on) {
 bool
 screen_invert_colors(Screen *self) {
     bool inverted = false;
+    if (self->modes.mDECSCNM) inverted = true;
+    return inverted;
+}
+
+static inline float
+ease_out_cubic(float phase) {
+    return 1.0f - powf(1.0f - phase, 3.0f);
+}
+
+static inline 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;
+}
+
+static inline 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

@@ -207,6 +207,7 @@ 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

@@ -458,6 +458,23 @@ draw_tint(bool premult, Screen *screen, GLfloat xstart, GLfloat ystart, GLfloat
     glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
 }
 
+void
+draw_visual_bell_flash(GLfloat intensity, GLfloat xstart, GLfloat ystart, GLfloat w, GLfloat h, Screen *screen) {
+    glEnable(GL_BLEND);
+    // BLEND_PREMULT
+    glBlendFuncSeparate(GL_ONE, GL_ONE_MINUS_SRC_ALPHA, GL_ZERO, GL_ONE);
+    bind_program(TINT_PROGRAM);
+    // TODO config option worth?
+    GLfloat attenuation = 0.4f;
+    color_type flash = colorprofile_to_color(screen->color_profile, screen->color_profile->overridden.highlight_bg, screen->color_profile->configured.highlight_bg);
+#define C(shift) ((((GLfloat)((flash >> shift) & 0xFF)) / 255.0f) * intensity * attenuation)
+    glUniform4f(tint_program_layout.tint_color_location, C(16), C(8), C(0), intensity * attenuation);
+#undef C
+    glUniform4f(tint_program_layout.edges_location, xstart, ystart - h, xstart + w, ystart);
+    glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
+    glDisable(GL_BLEND);
+}
+
 static void
 draw_cells_interleaved(ssize_t vao_idx, ssize_t gvao_idx, Screen *screen, OSWindow *w, GLfloat xstart, GLfloat ystart, GLfloat width, GLfloat height) {
     glEnable(GL_BLEND);
@@ -650,6 +667,11 @@ draw_cells(ssize_t vao_idx, ssize_t gvao_idx, GLfloat xstart, GLfloat ystart, GL
                 vao_idx, gvao_idx, screen, os_window, xstart, ystart, w, h);
         else draw_cells_simple(vao_idx, gvao_idx, screen);
     }
+
+    float intensity = screen_visual_bell_intensity(screen);
+    if (intensity > 0.0f) {
+        draw_visual_bell_flash((GLfloat)intensity, xstart, ystart, w, h, screen);
+    }
 }
 // }}}