Dont use FBO's unless absolutely necessary

Negative z-index images dont need FBO based rendering when the
background is opaque. Finishes up support for background_opacity.

Fixes #31

CHANGELOG.rst

@@ -6,9 +6,14 @@ kitty is a feature full, cross-platform, *fast*, GPU based terminal emulator.
 version 0.6.0 [future]
 ------------------------
 
+- Support background transparency via the background_opacity option. Provided
+  that your OS/window manager supports transparency, you can now have kitty
+  render pixels that have only the default background color as
+  semi-transparent.
+
 - Support multiple top level (OS) windows. These windows all share the sprite
   texture cache on the GPU, further reducing overall resource usage. Use
-  the shortcut `ctrl+shift+n` to open anew top-level window.
+  the shortcut `ctrl+shift+n` to open a new top-level window.
 
 - Add support for a *daemon* mode using the `--single-instance` command line
   option. With this option you can have only a single kitty instance running.

kitty/cell_fragment.glsl

@@ -62,19 +62,36 @@ vec4 calculate_foreground() {
 #endif
 
 void main() {
-#if defined(BACKGROUND) || defined(SPECIAL)
-#if defined(TRANSPARENT) || defined(SPECIAL)
+#ifdef BACKGROUND
+#ifdef TRANSPARENT
     final_color = vec4(background.rgb * bg_alpha, bg_alpha);
 #else
     final_color = vec4(background.rgb, 1.0f);
 #endif 
+#endif
 
-#else 
+#ifdef SPECIAL
+#ifdef TRANSPARENT
+    final_color = vec4(background.rgb * bg_alpha, bg_alpha);
+#else
+    final_color = vec4(background.rgb, bg_alpha);
+#endif 
+#endif
+
+#if defined(FOREGROUND) || defined(SIMPLE) 
+    // FOREGROUND or SIMPLE
     vec4 fg = calculate_foreground();  // pre-multiplied foreground
+
 #ifdef FOREGROUND
+    // FOREGROUND
+#ifdef TRANSPARENT
     final_color = fg;
 #else
+    final_color = vec4(fg.rgb / fg.a, fg.a);
+#endif
 
+#else
+    // SIMPLE
 #ifdef TRANSPARENT
     final_color = alpha_blend_premul(fg.rgb, fg.a, background * bg_alpha, bg_alpha);
     final_color = vec4(final_color.rgb / final_color.a, final_color.a);

kitty/shaders.c

@@ -303,20 +303,43 @@ draw_graphics(int program, ssize_t vao_idx, ssize_t gvao_idx, ImageRenderData *d
     bind_vertex_array(vao_idx);
 }
 
+#define BLEND_ONTO_OPAQUE  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);  // blending onto opaque colors
+#define BLEND_PREMULT glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);  // blending of pre-multiplied colors
+
 static void
-draw_all_cells(ssize_t vao_idx, ssize_t gvao_idx, Screen *screen) {
+draw_cells_simple(ssize_t vao_idx, ssize_t gvao_idx, Screen *screen) {
     bind_program(CELL_PROGRAM); 
     glDrawArraysInstanced(GL_TRIANGLE_FAN, 0, 4, screen->lines * screen->columns); 
     if (screen->grman->count) {
         glEnable(GL_BLEND);
-        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);  // blending onto opaque colors
+        BLEND_ONTO_OPAQUE;
         draw_graphics(GRAPHICS_PROGRAM, vao_idx, gvao_idx, screen->grman->render_data, 0, screen->grman->count);
         glDisable(GL_BLEND);
     }
 }
 
 static void
-draw_cells_interleaved(ssize_t vao_idx, ssize_t gvao_idx, Screen *screen, OSWindow *os_window) {
+draw_cells_interleaved(ssize_t vao_idx, ssize_t gvao_idx, Screen *screen) {
+    bind_program(CELL_BG_PROGRAM); 
+    glDrawArraysInstanced(GL_TRIANGLE_FAN, 0, 4, screen->lines * screen->columns); 
+    glEnable(GL_BLEND);
+    BLEND_ONTO_OPAQUE;
+
+    if (screen->grman->num_of_negative_refs) draw_graphics(GRAPHICS_PROGRAM, vao_idx, gvao_idx, screen->grman->render_data, 0, screen->grman->num_of_negative_refs);
+
+    bind_program(CELL_SPECIAL_PROGRAM); 
+    glDrawArraysInstanced(GL_TRIANGLE_FAN, 0, 4, screen->lines * screen->columns); 
+
+    bind_program(CELL_FG_PROGRAM); 
+    glDrawArraysInstanced(GL_TRIANGLE_FAN, 0, 4, screen->lines * screen->columns); 
+
+    if (screen->grman->num_of_positive_refs) draw_graphics(GRAPHICS_PROGRAM, vao_idx, gvao_idx, screen->grman->render_data, screen->grman->num_of_negative_refs, screen->grman->num_of_positive_refs);
+
+    glDisable(GL_BLEND);
+}
+
+static void
+draw_cells_interleaved_premult(ssize_t vao_idx, ssize_t gvao_idx, Screen *screen, OSWindow *os_window) {
     if (!os_window->offscreen_texture_id) {
         glGenTextures(1, &os_window->offscreen_texture_id);
         glBindTexture(GL_TEXTURE_2D, os_window->offscreen_texture_id);
@@ -334,7 +357,7 @@ draw_cells_interleaved(ssize_t vao_idx, ssize_t gvao_idx, Screen *screen, OSWind
     bind_program(CELL_BG_PROGRAM); 
     glDrawArraysInstanced(GL_TRIANGLE_FAN, 0, 4, screen->lines * screen->columns); 
     glEnable(GL_BLEND);
-    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);  // blending of pre-multiplied colors
+    BLEND_PREMULT;
 
     if (screen->grman->num_of_negative_refs) draw_graphics(GRAPHICS_PREMULT_PROGRAM, vao_idx, gvao_idx, screen->grman->render_data, 0, screen->grman->num_of_negative_refs);
 
@@ -390,9 +413,13 @@ draw_cells(ssize_t vao_idx, ssize_t gvao_idx, GLfloat xstart, GLfloat ystart, GL
         glUniform1i(glGetUniformLocation(program_id(CELL_FG_PROGRAM), "sprites"), SPRITE_MAP_UNIT);  
         cell_constants_set = true; 
     }
-    bool needs_complex_rendering = screen->grman->num_of_negative_refs || (screen->grman->num_of_positive_refs && os_window->is_semi_transparent);
-    if (needs_complex_rendering) draw_cells_interleaved(vao_idx, gvao_idx, screen, os_window);
-    else draw_all_cells(vao_idx, gvao_idx, screen);
+    if (os_window->is_semi_transparent) {
+        if (screen->grman->count) draw_cells_interleaved_premult(vao_idx, gvao_idx, screen, os_window);
+        else draw_cells_simple(vao_idx, gvao_idx, screen);
+    } else {
+        if (screen->grman->num_of_negative_refs) draw_cells_interleaved(vao_idx, gvao_idx, screen);
+        else draw_cells_simple(vao_idx, gvao_idx, screen);
+    }
 }
 // }}}