From 3774cc54c7aee737ba770039ba99e9bd4314a46f Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Sun, 2 Feb 2020 11:45:48 +0530
Subject: [PATCH] Fix rendering of semi-transparent bg images in
 semi-transparent windows

Output colors must be premultiplied
---
 kitty/bgimage_fragment.glsl | 6 +++++-
 kitty/shaders.c             | 4 +++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/kitty/bgimage_fragment.glsl b/kitty/bgimage_fragment.glsl
index 405784be5..bd15691a6 100644
--- a/kitty/bgimage_fragment.glsl
+++ b/kitty/bgimage_fragment.glsl
@@ -2,10 +2,14 @@
 
 uniform sampler2D image;
 uniform float opacity;
+uniform float premult;
 in vec2 texcoord;
 out vec4 color;
 
 void main() {
     color = texture(image, texcoord);
-    color = vec4(color.rgb, color.a * opacity);
+    float alpha = color.a * opacity;
+    vec4 premult_color = vec4(color.rgb * alpha, alpha);
+    color = vec4(color.rgb, alpha);
+    color = premult * premult_color + (1 - premult) * color;
 }
diff --git a/kitty/shaders.c b/kitty/shaders.c
index fc569aa54..8bfa15c8d 100644
--- a/kitty/shaders.c
+++ b/kitty/shaders.c
@@ -165,7 +165,7 @@ typedef struct {
 static CellProgramLayout cell_program_layouts[NUM_PROGRAMS];
 static ssize_t blit_vertex_array;
 typedef struct {
-    GLint image_location, tiled_location, sizes_location, opacity_location;
+    GLint image_location, tiled_location, sizes_location, opacity_location, premult_location;
 } BGImageProgramLayout;
 static BGImageProgramLayout bgimage_program_layout = {0};
 typedef struct {
@@ -194,6 +194,7 @@ init_cell_program(void) {
     bgimage_program_layout.opacity_location = get_uniform_location(BGIMAGE_PROGRAM, "opacity");
     bgimage_program_layout.sizes_location = get_uniform_location(BGIMAGE_PROGRAM, "sizes");
     bgimage_program_layout.tiled_location = get_uniform_location(BGIMAGE_PROGRAM, "tiled");
+    bgimage_program_layout.premult_location = get_uniform_location(BGIMAGE_PROGRAM, "premult");
     tint_program_layout.tint_color_location = get_uniform_location(TINT_PROGRAM, "tint_color");
     tint_program_layout.edges_location = get_uniform_location(TINT_PROGRAM, "edges");
 }
@@ -362,6 +363,7 @@ draw_bg(OSWindow *w) {
     }
     glUniform4f(bgimage_program_layout.sizes_location,
         (GLfloat)w->window_width, (GLfloat)w->window_height, (GLfloat)w->bgimage->width, (GLfloat)w->bgimage->height);
+    glUniform1f(bgimage_program_layout.premult_location, w->is_semi_transparent ? 1.f : 0.f);
     glActiveTexture(GL_TEXTURE0 + BGIMAGE_UNIT);
     glBindTexture(GL_TEXTURE_2D, w->bgimage->texture_id);
     glDrawArrays(GL_TRIANGLE_FAN, 0, 4);