Start work on background opacity

kitty/border_fragment.glsl

@@ -1,7 +1,8 @@
 #version GLSL_VERSION
+uniform float background_opacity;
 in vec3 color;
 out vec4 final_color;
 
 void main() {
-    final_color = vec4(color, 1);
+    final_color = vec4(color, background_opacity);
 }

kitty/config.py

@@ -214,6 +214,10 @@ def positive_float(x):
     return max(0, float(x))
 
 
+def unit_float(x):
+    return max(0, min(float(x), 1))
+
+
 def adjust_line_height(x):
     if x.endswith('%'):
         return float(x[:-1].strip()) / 100.0
@@ -258,6 +262,7 @@ type_map = {
     'macos_option_as_alt': to_bool,
     'box_drawing_scale': box_drawing_scale,
     'x11_bell_volume': int,
+    'background_opacity': unit_float,
 }
 
 for name in (

kitty/glfw.c

@@ -226,6 +226,8 @@ create_os_window(PyObject UNUSED *self, PyObject *args) {
         return NULL;
     }
     glfwWindowHint(GLFW_VISIBLE, visible ? GLFW_TRUE : GLFW_FALSE);
+    bool want_semi_transparent = (1.0 - OPT(background_opacity) > 0.1) ? true : false;
+    glfwWindowHint(GLFW_TRANSPARENT_FRAMEBUFFER, want_semi_transparent);
     GLFWwindow *glfw_window = glfwCreateWindow(width, height, title, NULL, global_state.num_os_windows ? global_state.os_windows[0].handle : NULL);
     if (glfw_window == NULL) {
         fprintf(stderr, "Failed to create a window at size: %dx%d, using a standard size instead.\n", width, height);
@@ -274,6 +276,14 @@ create_os_window(PyObject UNUSED *self, PyObject *args) {
     w->is_focused = true;
     w->cursor_blink_zero_time = now;
     w->last_mouse_activity_at = now;
+    w->is_semi_transparent = glfwGetWindowAttrib(w->handle, GLFW_TRANSPARENT_FRAMEBUFFER);
+    if (want_semi_transparent && !w->is_semi_transparent) {
+        static bool warned = false;
+        if (!warned) {
+            fprintf(stderr, "Failed to enable transparency. This happens when your desktop environment does not support compositing.\n");
+            warned = true;
+        }
+    }
     return PyLong_FromUnsignedLongLong(w->id);
 }
 

kitty/kitty.conf

@@ -43,6 +43,11 @@ foreground       #dddddd
 # The background color
 background       #000000
 
+# The opacity of the background. A number between 0 and 1, where 1 is opaque and 0 is fully transparent.
+# Note that this will only work if supported by the OS (for instance, when using a compositor under X11).
+# Be aware that using a value less than 1.0 is a performance hit.
+background_opacity 1.0
+
 # The foreground for selections
 selection_foreground #000000
 

kitty/shaders.c

@@ -337,7 +337,9 @@ draw_cells(ssize_t vao_idx, ssize_t gvao_idx, GLfloat xstart, GLfloat ystart, GL
 #undef SCALE
     static bool cell_constants_set = false;
     if (!cell_constants_set) { 
+        bind_program(CELL_PROGRAM);
         glUniform1i(glGetUniformLocation(program_id(CELL_PROGRAM), "sprites"), SPRITE_MAP_UNIT);  
+        bind_program(CELL_FOREGROUND_PROGRAM);
         glUniform1i(glGetUniformLocation(program_id(CELL_FOREGROUND_PROGRAM), "sprites"), SPRITE_MAP_UNIT);  
         cell_constants_set = true; 
     }
@@ -377,7 +379,7 @@ draw_cursor(CursorRenderInfo *cursor, bool is_focused) {
 // }}}
 
 // Borders {{{
-enum BorderUniforms { BORDER_viewport, NUM_BORDER_UNIFORMS };
+enum BorderUniforms { BORDER_viewport, BORDER_background_opacity, NUM_BORDER_UNIFORMS };
 static GLint border_uniform_locations[NUM_BORDER_UNIFORMS] = {0};
 
 static void
@@ -385,9 +387,10 @@ init_borders_program() {
     Program *p = programs + BORDERS_PROGRAM;
     int left = NUM_BORDER_UNIFORMS;
     for (int i = 0; i < p->num_of_uniforms; i++, left--) {
-#define SET_LOC(which) if (strcmp(p->uniforms[i].name, #which) == 0) border_uniform_locations[BORDER_##which] = p->uniforms[i].location
-        SET_LOC(viewport);
-        else { fatal("Unknown uniform in borders program"); return; }
+#define SET_LOC(which) (strcmp(p->uniforms[i].name, #which) == 0) border_uniform_locations[BORDER_##which] = p->uniforms[i].location
+        if SET_LOC(viewport);
+        else if SET_LOC(background_opacity);
+        else { fatal("Unknown uniform in borders program: %s", p->uniforms[i].name); return; }
     }
     if (left) { fatal("Left over uniforms in borders program"); return; }
 #undef SET_LOC
@@ -416,6 +419,11 @@ draw_borders(ssize_t vao_idx, unsigned int num_border_rects, BorderRect *rect_bu
             unmap_vao_buffer(vao_idx, 0);
         }
         bind_program(BORDERS_PROGRAM);
+        static bool constants_set = false;
+        if (!constants_set) {
+            constants_set = true;
+            glUniform1f(border_uniform_locations[BORDER_background_opacity], OPT(background_opacity));
+        }
         glUniform2ui(border_uniform_locations[BORDER_viewport], viewport_width, viewport_height);
         bind_vertex_array(vao_idx);
         glDrawArraysInstanced(GL_TRIANGLE_FAN, 0, 4, num_border_rects);

kitty/state.c

@@ -307,6 +307,7 @@ PYWRAP1(set_options) {
     S(focus_follows_mouse, PyObject_IsTrue);
     S(cursor_blink_interval, PyFloat_AsDouble);
     S(cursor_stop_blinking_after, PyFloat_AsDouble);
+    S(background_opacity, PyFloat_AsDouble);
     S(cursor_shape, PyLong_AsLong);
     S(x11_bell_volume, PyLong_AsLong);
     S(mouse_hide_wait, PyFloat_AsDouble);

kitty/state.h

@@ -23,6 +23,7 @@ typedef struct {
     int adjust_line_height_px;
     int x11_bell_volume;
     float adjust_line_height_frac;
+    float background_opacity;
 } Options;
 
 typedef struct {
@@ -101,6 +102,7 @@ typedef struct {
     bool viewport_size_dirty;
     double last_resize_at;
     bool has_pending_resizes;
+    bool is_semi_transparent;
 } OSWindow;