Dither default background to avoid color banding.

Telegram/SourceFiles/boxes/background_box.cpp

@@ -239,6 +239,7 @@ void BackgroundBox::Inner::sortPapers() {
 		return std::make_tuple(
 			data.id() == current,
 			night ? data.isDark() : !data.isDark(),
+			Data::IsDefaultWallPaper(data),
 			!data.isDefault() && !Data::IsLegacy1DefaultWallPaper(data),
 			Data::IsLegacy2DefaultWallPaper(data),
 			Data::IsLegacy1DefaultWallPaper(data));

Telegram/SourceFiles/boxes/background_preview_box.cpp

@@ -622,6 +622,7 @@ void BackgroundPreviewBox::paintDate(Painter &p) {
 	if (!date || !_serviceBg) {
 		return;
 	}
+	auto hq = PainterHighQualityEnabler(p);
 	const auto text = date->text;
 	const auto bubbleHeight = st::msgServicePadding.top() + st::msgServiceFont->height + st::msgServicePadding.bottom();
 	const auto bubbleTop = st::msgServiceMargin.top();
@@ -758,7 +759,7 @@ void BackgroundPreviewBox::checkLoadedDocument() {
 	};
 	_generating = Data::ReadImageAsync(
 		_media.get(),
-		Window::Theme::ProcessBackgroundImage,
+		Window::Theme::PreprocessBackgroundImage,
 		generateCallback);
 }
 

Telegram/SourceFiles/mainwidget.cpp

@@ -1265,7 +1265,7 @@ void MainWidget::checkChatBackground() {
 	};
 	_background->generating = Data::ReadImageAsync(
 		media.get(),
-		Window::Theme::ProcessBackgroundImage,
+		Window::Theme::PreprocessBackgroundImage,
 		generateCallback);
 }
 

Telegram/SourceFiles/window/themes/window_theme.cpp

@@ -431,12 +431,89 @@ bool InitializeFromSaved(Saved &&saved) {
 	return true;
 }
 
-QImage validateBackgroundImage(QImage image) {
+[[nodiscard]] QImage DitherImage(QImage image) {
+	Expects(image.bytesPerLine() == image.width() * 4);
+
+	const auto width = image.width();
+	const auto height = image.height();
+
+	if (width < 16 || height < 16) {
+		return image;
+	}
+
+	const auto area = width * height;
+	const auto shifts = std::make_unique<uchar[]>(area);
+	memset_rand(shifts.get(), area);
+
+	// shiftx = int(shift & 0x0F) - 8; shifty = int(shift >> 4) - 8;
+	// Clamp shifts close to edges.
+	for (auto y = 0; y != 8; ++y) {
+		const auto min = 8 - y;
+		const auto shifted = (min << 4);
+		auto shift = shifts.get() + y * width;
+		for (const auto till = shift + width; shift != till; ++shift) {
+			if ((*shift >> 4) < min) {
+				*shift = shifted | (*shift & 0x0F);
+			}
+		}
+	}
+	for (auto y = height - 7; y != height; ++y) {
+		const auto max = 8 + (height - y - 1);
+		const auto shifted = (max << 4);
+		auto shift = shifts.get() + y * width;
+		for (const auto till = shift + width; shift != till; ++shift) {
+			if ((*shift >> 4) > max) {
+				*shift = shifted | (*shift & 0x0F);
+			}
+		}
+	}
+	for (auto shift = shifts.get(), ytill = shift + area
+		; shift != ytill
+		; shift += width - 8) {
+		for (const auto till = shift + 8; shift != till; ++shift) {
+			const auto min = (till - shift);
+			if ((*shift & 0x0F) < min) {
+				*shift = (*shift & 0xF0) | min;
+			}
+		}
+	}
+	for (auto shift = shifts.get(), ytill = shift + area; shift != ytill;) {
+		shift += width - 7;
+		for (const auto till = shift + 7; shift != till; ++shift) {
+			const auto max = 8 + (till - shift - 1);
+			if ((*shift & 0x0F) > max) {
+				*shift = (*shift & 0xF0) | max;
+			}
+		}
+	}
+
+	auto result = image;
+	result.detach();
+
+	const auto src = reinterpret_cast<const uint32*>(image.constBits());
+	const auto dst = reinterpret_cast<uint32*>(result.bits());
+	for (auto index = 0; index != area; ++index) {
+		const auto shift = shifts[index];
+		const auto shiftx = int(shift & 0x0F) - 8;
+		const auto shifty = int(shift >> 4) - 8;
+		dst[index] = src[index + (shifty * width) + shiftx];
+	}
+
+	return result;
+}
+
+[[nodiscard]] QImage PostprocessBackgroundImage(
+		QImage image,
+		const Data::WallPaper &paper) {
 	if (image.format() != QImage::Format_ARGB32_Premultiplied) {
 		image = std::move(image).convertToFormat(
 			QImage::Format_ARGB32_Premultiplied);
 	}
 	image.setDevicePixelRatio(cRetinaFactor());
+	if (Data::IsDefaultWallPaper(paper)
+		|| Data::details::IsTestingDefaultWallPaper(paper)) {
+		return DitherImage(std::move(image));
+	}
 	return image;
 }
 
@@ -530,7 +607,9 @@ ChatBackground::ChatBackground() : _adjustableColors({
 }
 
 void ChatBackground::setThemeData(QImage &&themeImage, bool themeTile) {
-	_themeImage = validateBackgroundImage(std::move(themeImage));
+	_themeImage = PostprocessBackgroundImage(
+		std::move(themeImage),
+		Data::ThemeWallPaper());
 	_themeTile = themeTile;
 }
 
@@ -627,8 +706,12 @@ void ChatBackground::checkUploadWallPaper() {
 	});
 }
 
+QImage ChatBackground::postprocessBackgroundImage(QImage image) {
+	return PostprocessBackgroundImage(std::move(image), _paper);
+}
+
 void ChatBackground::set(const Data::WallPaper &paper, QImage image) {
-	image = ProcessBackgroundImage(std::move(image));
+	image = PreprocessBackgroundImage(std::move(image));
 
 	const auto needResetAdjustable = Data::IsDefaultWallPaper(paper)
 		&& !Data::IsDefaultWallPaper(_paper)
@@ -657,7 +740,7 @@ void ChatBackground::set(const Data::WallPaper &paper, QImage image) {
 			image.load(qsl(":/gui/art/background.jpg"));
 			setPaper(Data::details::TestingDefaultWallPaper());
 		}
-		image = validateBackgroundImage(std::move(image));
+		image = postprocessBackgroundImage(std::move(image));
 		setPreparedImage(image, image);
 	} else {
 		if (Data::IsLegacy1DefaultWallPaper(_paper)) {
@@ -681,7 +764,7 @@ void ChatBackground::set(const Data::WallPaper &paper, QImage image) {
 				: image));
 		if (const auto fill = _paper.backgroundColor()) {
 			if (_paper.isPattern() && !image.isNull()) {
-				auto prepared = validateBackgroundImage(
+				auto prepared = postprocessBackgroundImage(
 					Data::PreparePatternImage(
 						image,
 						*fill,
@@ -697,7 +780,7 @@ void ChatBackground::set(const Data::WallPaper &paper, QImage image) {
 				}
 			}
 		} else {
-			image = validateBackgroundImage(std::move(image));
+			image = postprocessBackgroundImage(std::move(image));
 			setPreparedImage(image, image);
 		}
 	}
@@ -1015,7 +1098,7 @@ void ChatBackground::keepApplied(const Object &object, bool write) {
 		}
 	} else if (Data::details::IsTestingThemeWallPaper(_paper)) {
 		setPaper(Data::ThemeWallPaper());
-		_themeImage = validateBackgroundImage(base::duplicate(_original));
+		_themeImage = postprocessBackgroundImage(base::duplicate(_original));
 		_themeTile = tile();
 		if (write) {
 			writeNewBackgroundSettings();
@@ -1403,7 +1486,7 @@ QColor AdjustedColor(QColor original, QColor background) {
 	).toRgb();
 }
 
-QImage ProcessBackgroundImage(QImage image) {
+QImage PreprocessBackgroundImage(QImage image) {
 	constexpr auto kMaxSize = 2960;
 
 	if (image.format() != QImage::Format_ARGB32_Premultiplied) {

Telegram/SourceFiles/window/themes/window_theme.h

@@ -98,9 +98,9 @@ bool LoadFromContent(
 	const QByteArray &content,
 	not_null<Instance*> out,
 	Cached *outCache);
-QColor CountAverageColor(const QImage &image);
+[[nodiscard]] QColor CountAverageColor(const QImage &image);
-QColor AdjustedColor(QColor original, QColor background);
+[[nodiscard]] QColor AdjustedColor(QColor original, QColor background);
-QImage ProcessBackgroundImage(QImage image);
+[[nodiscard]] QImage PreprocessBackgroundImage(QImage image);
 
 struct BackgroundUpdate {
 	enum class Type {
@@ -215,6 +215,7 @@ private:
 	[[nodiscard]] bool isNonDefaultThemeOrBackground();
 	[[nodiscard]] bool isNonDefaultBackground();
 	void checkUploadWallPaper();
+	[[nodiscard]] QImage postprocessBackgroundImage(QImage image);
 
 	friend bool IsNightMode();
 	friend void SetNightModeValue(bool nightMode);

Telegram/lib_ui

@@ -1 +1 @@
-Subproject commit 12429c198d9176e109af282d28198ef1bd143971
+Subproject commit 1ed242718ee2dd6363ff738acb9151e649500ed7