backend: gl: add dither

man/picom.1.asciidoc

@@ -268,6 +268,9 @@ May also be one of the predefined kernels: `3x3box` (default), `5x5box`, `7x7box
 *--window-shader-fg-rule* 'SHADER':'CONDITION'::
 	Specify GLSL fragment shader path for rendering window contents using patterns. Similar to *--opacity-rule*, arguments should be in the format of 'SHADER:CONDITION', e.g. "shader.frag:name = \'window\'". Leading and trailing whitespaces in 'SHADER' will be trimmed. If 'SHADER' is "default", then the default shader will be used for the matching windows. (This also unfortunately means you can't use a shader file named "default"). Does not work when *--legacy-backends* is enabled.
 
+*--dithered-present*
+	Use higher precision during rendering, and apply dither when presenting the rendered screen. Reduces banding artifacts, but might cause performance degradation. Only works with OpenGL.
+
 FORMAT OF CONDITIONS
 --------------------
 Some options accept a condition string to match certain windows. A condition string is formed by one or more conditions, joined by logical operators.

picom.sample.conf

@@ -215,6 +215,11 @@ blur-background-exclude = [
 # backend = "glx"
 backend = "xrender";
 
+# Use higher precision during rendering, and apply dither when presenting the
+# rendered screen. Reduces banding artifacts, but might cause performance
+# degradation. Only works with OpenGL.
+dithered-present = false;
+
 # Enable/disable VSync.
 # vsync = false
 vsync = true;

src/backend/gl/blur.c

@@ -259,10 +259,10 @@ bool gl_dual_kawase_blur(double opacity, struct gl_blur_context *bctx, const rec
 	return true;
 }
 
-bool gl_blur_impl(double opacity, struct gl_blur_context *bctx, void *mask,
-                  coord_t mask_dst, const region_t *reg_blur,
-                  const region_t *reg_visible attr_unused, GLuint source_texture,
-                  geometry_t source_size, GLuint target_fbo, GLuint default_mask) {
+bool gl_blur_impl(double opacity, struct gl_blur_context *bctx, void *mask, coord_t mask_dst,
+                  const region_t *reg_blur, const region_t *reg_visible attr_unused,
+                  GLuint source_texture, geometry_t source_size, GLuint target_fbo,
+                  GLuint default_mask, bool high_precision) {
 	bool ret = false;
 
 	if (source_size.width != bctx->fb_width || source_size.height != bctx->fb_height) {
@@ -284,7 +284,11 @@ bool gl_blur_impl(double opacity, struct gl_blur_context *bctx, void *mask,
 			}
 
 			glBindTexture(GL_TEXTURE_2D, bctx->blur_textures[i]);
-			glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, tex_size->width,
+			GLint format = GL_RGBA8;
+			if (high_precision) {
+				format = GL_RGBA16;
+			}
+			glTexImage2D(GL_TEXTURE_2D, 0, format, tex_size->width,
 			             tex_size->height, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
 
 			if (bctx->method == BLUR_METHOD_DUAL_KAWASE) {
@@ -406,7 +410,7 @@ bool gl_blur(backend_t *base, double opacity, void *ctx, void *mask, coord_t mas
 	return gl_blur_impl(opacity, bctx, mask, mask_dst, reg_blur, reg_visible,
 	                    gd->back_texture,
 	                    (geometry_t){.width = gd->width, .height = gd->height},
-	                    gd->back_fbo, gd->default_mask_texture);
+	                    gd->back_fbo, gd->default_mask_texture, gd->dithered_present);
 }
 
 static inline void gl_free_blur_shader(gl_blur_shader_t *shader) {

src/backend/gl/gl_common.c

@@ -627,7 +627,7 @@ void gl_resize(struct gl_data *gd, int width, int height) {
 	assert(viewport_dimensions[1] >= gd->height);
 
 	glBindTexture(GL_TEXTURE_2D, gd->back_texture);
-	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB8, width, height, 0, GL_BGR,
+	glTexImage2D(GL_TEXTURE_2D, 0, gd->back_format, width, height, 0, GL_BGR,
 	             GL_UNSIGNED_BYTE, NULL);
 
 	gl_check_err();
@@ -873,7 +873,16 @@ bool gl_init(struct gl_data *gd, session_t *ps) {
 	glUniformMatrix4fv(pml, 1, false, projection_matrix[0]);
 	glUseProgram(0);
 
-	gd->present_prog = gl_create_program_from_str(present_vertex_shader, dummy_frag);
+	gd->dithered_present = ps->o.dithered_present;
+	if (gd->dithered_present) {
+		gd->present_prog = gl_create_program_from_strv(
+		    (const char *[]){present_vertex_shader, NULL},
+		    (const char *[]){present_frag, dither_glsl, NULL});
+	} else {
+		gd->present_prog = gl_create_program_from_strv(
+		    (const char *[]){present_vertex_shader, NULL},
+		    (const char *[]){dummy_frag, NULL});
+	}
 	if (!gd->present_prog) {
 		log_error("Failed to create the present shader");
 		return false;
@@ -907,14 +916,23 @@ bool gl_init(struct gl_data *gd, session_t *ps) {
 	glUniformMatrix4fv(pml, 1, false, projection_matrix[0]);
 	glUseProgram(0);
 
-	// Set up the size of the back texture
-	gl_resize(gd, ps->root_width, ps->root_height);
-
+	// Set up the size and format of the back texture
 	glBindFramebuffer(GL_DRAW_FRAMEBUFFER, gd->back_fbo);
-	glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
-	                       gd->back_texture, 0);
 	glDrawBuffer(GL_COLOR_ATTACHMENT0);
-	if (!gl_check_fb_complete(GL_FRAMEBUFFER)) {
+	const GLint *format = gd->dithered_present ? (const GLint[]){GL_RGB16, GL_RGBA16}
+	                                           : (const GLint[]){GL_RGB8, GL_RGBA8};
+	for (int i = 0; i < 2; i++) {
+		gd->back_format = format[i];
+		gl_resize(gd, ps->root_width, ps->root_height);
+
+		glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+		                       GL_TEXTURE_2D, gd->back_texture, 0);
+		if (glCheckFramebufferStatus(GL_DRAW_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE) {
+			log_info("Using back buffer format %#x", gd->back_format);
+			break;
+		}
+	}
+	if (!gl_check_fb_complete(GL_DRAW_FRAMEBUFFER)) {
 		return false;
 	}
 	glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
@@ -1279,7 +1297,7 @@ void *gl_shadow_from_mask(backend_t *base, void *mask,
 		    1.0, gsctx->blur_context, NULL, (coord_t){0}, &reg_blur, NULL,
 		    source_texture,
 		    (geometry_t){.width = new_inner->width, .height = new_inner->height},
-		    fbo, gd->default_mask_texture);
+		    fbo, gd->default_mask_texture, gd->dithered_present);
 		pixman_region32_fini(&reg_blur);
 	}
 

src/backend/gl/gl_common.h

@@ -107,8 +107,11 @@ struct gl_data {
 	gl_fill_shader_t fill_shader;
 	gl_shadow_shader_t shadow_shader;
 	GLuint back_texture, back_fbo;
+	GLint back_format;
 	GLuint present_prog;
 
+	bool dithered_present;
+
 	GLuint default_mask_texture;
 
 	/// Called when an gl_texture is decoupled from the texture it refers. Returns
@@ -163,10 +166,10 @@ void *gl_clone(backend_t *base, const void *image_data, const region_t *reg_visi
 
 bool gl_blur(backend_t *base, double opacity, void *ctx, void *mask, coord_t mask_dst,
              const region_t *reg_blur, const region_t *reg_visible);
-bool gl_blur_impl(double opacity, struct gl_blur_context *bctx, void *mask,
-                  coord_t mask_dst, const region_t *reg_blur,
-                  const region_t *reg_visible attr_unused, GLuint source_texture,
-                  geometry_t source_size, GLuint target_fbo, GLuint default_mask);
+bool gl_blur_impl(double opacity, struct gl_blur_context *bctx, void *mask, coord_t mask_dst,
+                  const region_t *reg_blur, const region_t *reg_visible attr_unused,
+                  GLuint source_texture, geometry_t source_size, GLuint target_fbo,
+                  GLuint default_mask, bool high_precision);
 void *gl_create_blur_context(backend_t *base, enum blur_method, void *args);
 void gl_destroy_blur_context(backend_t *base, void *ctx);
 struct backend_shadow_context *gl_create_shadow_context(backend_t *base, double radius);
@@ -288,5 +291,6 @@ static const GLuint vert_in_texcoord_loc = 1;
 #define QUOTE(...) #__VA_ARGS__
 
 extern const char vertex_shader[], copy_with_mask_frag[], masking_glsl[], dummy_frag[],
-    fill_frag[], fill_vert[], interpolating_frag[], interpolating_vert[], win_shader_glsl[],
-    win_shader_default[], present_vertex_shader[], shadow_colorization_frag[];
+    present_frag[], fill_frag[], fill_vert[], interpolating_frag[], interpolating_vert[],
+    win_shader_glsl[], win_shader_default[], present_vertex_shader[], dither_glsl[],
+    shadow_colorization_frag[];

src/backend/gl/shaders.c

@@ -9,6 +9,15 @@ const char dummy_frag[] = GLSL(330,
 	}
 );
 
+const char present_frag[] = GLSL(330,
+	uniform sampler2D tex;
+	in vec2 texcoord;
+	vec4 dither(vec4, vec2);
+	void main() {
+		gl_FragColor = dither(texelFetch(tex, ivec2(texcoord.xy), 0), texcoord);
+	}
+);
+
 const char copy_with_mask_frag[] = GLSL(330,
 	uniform sampler2D tex;
 	in vec2 texcoord;
@@ -174,6 +183,29 @@ const char vertex_shader[] = GLSL(330,
 		texcoord = in_texcoord + texorig;
 	}
 );
+const char dither_glsl[] = GLSL(330,
+	// Stolen from: https://www.shadertoy.com/view/7sfXDn
+	float bayer2(vec2 a) {
+		a = floor(a);
+		return fract(a.x / 2. + a.y * a.y * .75);
+	}
+	// 16 * 16 is 2^8, so in total we have equivalent of 16-bit
+	// color depth, should be enough?
+	float bayer(vec2 a16) {
+		vec2  a8 = a16 * .5;
+		vec2  a4 =  a8 * .5;
+		vec2  a2 =  a4 * .5;
+		float bayer32 = ((bayer2(a2) * .25 + bayer2( a4))
+		                             * .25 + bayer2( a8))
+		                             * .25 + bayer2(a16);
+		return bayer32;
+	}
+	vec4 dither(vec4 c, vec2 coord) {
+		vec4 residual = mod(c, 1.0 / 255.0);
+		vec4 dithered = vec4(greaterThan(residual, vec4(1e-4)));
+		return vec4(c + dithered * bayer(coord) / 255.0);
+	}
+);
 const char shadow_colorization_frag[] = GLSL(330,
 	uniform vec4 color;
 	uniform sampler2D tex;

src/backend/xrender/xrender.c

@@ -858,6 +858,10 @@ static void get_blur_size(void *blur_context, int *width, int *height) {
 }
 
 static backend_t *backend_xrender_init(session_t *ps) {
+	if (ps->o.dithered_present) {
+		log_warn("\"dithered-present\" is not supported by the xrender backend.");
+	}
+
 	auto xd = ccalloc(1, struct _xrender_data);
 	init_backend_base(&xd->base, ps);
 

src/config.h

@@ -256,6 +256,8 @@ typedef struct options {
 	/// A list of conditions of windows to which transparent clipping
 	/// should not apply
 	c2_lptr_t *transparent_clipping_blacklist;
+
+	bool dithered_present;
 } options_t;
 
 extern const char *const BACKEND_STRS[NUM_BKEND + 1];

src/config_libconfig.c

@@ -452,6 +452,8 @@ char *parse_config_libconfig(options_t *opt, const char *config_file, bool *shad
 	lcfg_lookup_bool(&cfg, "no-ewmh-fullscreen", &opt->no_ewmh_fullscreen);
 	// --transparent-clipping
 	lcfg_lookup_bool(&cfg, "transparent-clipping", &opt->transparent_clipping);
+	// --dithered_present
+	lcfg_lookup_bool(&cfg, "dithered-present", &opt->dithered_present);
 	// --transparent-clipping-exclude
 	parse_cfg_condlst(&cfg, &opt->transparent_clipping_blacklist,
 	                  "transparent-clipping-exclude");

src/options.c

@@ -168,6 +168,10 @@ static const struct picom_option picom_options[] = {
                                                                              "similar to --opacity-rule. SHADER_PATH can be \"default\", in which case "
                                                                              "the default shader will be used. Does not work when --legacy-backends is "
                                                                              "enabled. See man page for more details"},
+    // 338 is transparent-clipping-exclude
+    {"dithered-present"            , no_argument      , 339, NULL          , "Use higher precision during rendering, and apply dither when presenting the "
+                                                                             "rendered screen. Reduces banding artifacts, but might cause performance "
+                                                                             "degradation. Only works with OpenGL."},
     {"legacy-backends"             , no_argument      , 733, NULL          , "Use deprecated version of the backends."},
     {"monitor-repaint"             , no_argument      , 800, NULL          , "Highlight the updated area of the screen. For debugging."},
     {"diagnostics"                 , no_argument      , 801, NULL          , "Print diagnostic information"},
@@ -716,6 +720,10 @@ bool get_cfg(options_t *opt, int argc, char *const *argv, bool shadow_enable,
 			// --clip-shadow-above
 			condlst_add(&opt->shadow_clip_list, optarg);
 			break;
+		case 339:
+			// --dithered-present
+			opt->dithered_present = true;
+			break;
 		P_CASEBOOL(733, legacy_backends);
 		P_CASEBOOL(800, monitor_repaint);
 		case 801: