UI: Rounded rectangles from precomputed atlas corners

UIAtlas generates an antialiased white corner for each radius in
ROUND_RECT_RADII (8, 16, 24, 32 UI units) and each style (filled, and
thin/medium/thick outlines from ROUND_RECT_THICKNESS) at the atlas DPI.
DrawBuffer::DrawRoundRect mirrors the corner into all four corners and
stretches the corner's last texel row/column along the edges, so a
round rect is 9 quads (8 for outlines).

Other radii use the nearest corner, scaled. The LogoScreen temporarily
draws some for testing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

UI/UIAtlas.cpp

@@ -9,6 +9,7 @@
 #include "Common/UI/Context.h"
 #include "Common/Data/Format/PNGLoad.h"
 #include "Common/Render/AtlasGen.h"
+#include "Common/Render/DrawBuffer.h"
 #include "Common/Render/ManagedTexture.h"
 #include "Common/Common.h"
 #include "Common/Thread/ParallelLoop.h"
@@ -40,6 +41,9 @@ Atlas *GetUIAtlas() {
 struct ImageMeta {
 	std::string_view id;
 	bool addShadow = false;
+	// If set, the image is a generated round rect corner (see DrawBuffer::DrawRoundRect). In UI units.
+	float cornerRadius = 0.0f;
+	float cornerThickness = 0.0f;  // 0 = filled
 };
 
 // We add shadows to all line-art images that are used for buttons, to improve visibility.
@@ -375,6 +379,28 @@ static bool RasterizeSVG(std::string_view filename, float dpiScale, int maxTextu
 	return true;
 }
 
+// White, premultiplied, antialiased by distance to the edge.
+static void GenerateRoundRectCorner(Image &img, float radius, float thickness, float dpiScale) {
+	const float r = radius * dpiScale;
+	const float t = thickness * dpiScale;
+	const int center = RoundRectCornerCenter(r);
+	img.resize(center + 1, center + 1);
+	img.scale = dpiScale;
+	for (int y = 0; y <= center; y++) {
+		const float dy = std::max((float)center - (y + 0.5f), 0.0f);
+		for (int x = 0; x <= center; x++) {
+			const float dx = std::max((float)center - (x + 0.5f), 0.0f);
+			const float d = sqrtf(dx * dx + dy * dy);
+			float coverage = std::clamp(r - d + 0.5f, 0.0f, 1.0f);
+			if (t > 0.0f) {
+				coverage -= std::clamp(r - t - d + 0.5f, 0.0f, 1.0f);
+			}
+			const u32 a = (u32)(coverage * 255.0f + 0.5f);
+			img.set1(x, y, a * 0x01010101);
+		}
+	}
+}
+
 static bool GenerateUIAtlasImage(Atlas *atlas, float dpiScale, Image *dest, int maxTextureSize, const ImageMeta *imageIDs, size_t imageCount) {
 	Bucket bucket;
 
@@ -445,6 +471,8 @@ static bool GenerateUIAtlasImage(Atlas *atlas, float dpiScale, Image *dest, int
 		} else if (equals(imageIDs[i].id, "I_EMPTY")) {
 			img.resize(16, 16);
 			img.fill(0);
+		} else if (imageIDs[i].cornerRadius > 0.0f) {
+			GenerateRoundRectCorner(img, imageIDs[i].cornerRadius, imageIDs[i].cornerThickness, dpiScale);
 		} else {
 			std::string name = "ui_images/";
 			std::string pngName = PNGNameFromID(imageIDs[i].id);
@@ -491,6 +519,7 @@ static bool GenerateUIAtlasImage(Atlas *atlas, float dpiScale, Image *dest, int
 	atlas->images = new AtlasImage[genAtlasImages.size()];
 	std::copy(genAtlasImages.begin(), genAtlasImages.end(), atlas->images);
 	atlas->num_images = (int)genAtlasImages.size();
+	atlas->dpiScale = dpiScale;
 
 	// For debug, write out the atlas.
 	if (SAVE_DEBUG_ATLAS) {
@@ -510,8 +539,15 @@ Draw::Texture *GenerateUIAtlas(Draw::DrawContext *draw, Atlas *atlas, float dpiS
 		INFO_LOG(Log::G3D, "Regenerating atlas (empty: %s). Dpi scale (changed: %s): %0.2f (invalidate=%d)",
 			g_cachedUIAtlasImage.IsEmpty() ? "true" : "false", dpiScale != g_cachedDpiScale ? "true" : "false", dpiScale, invalidate);
 
+		std::vector<ImageMeta> imageIDs(std::begin(g_uiImageIDs), std::end(g_uiImageIDs));
+		for (int r = 0; r < ROUND_RECT_RADIUS_COUNT; r++) {
+			for (int style = 0; style < (int)RoundRectStyle::COUNT; style++) {
+				imageIDs.push_back({ RoundRectCornerImageName(r, (RoundRectStyle)style), false, ROUND_RECT_RADII[r], ROUND_RECT_THICKNESS[style] });
+			}
+		}
+
 		g_cachedUIAtlasImage.clear();
-		if (!GenerateUIAtlasImage(atlas, dpiScale, &g_cachedUIAtlasImage, draw->GetDeviceCaps().maxTextureSize, g_uiImageIDs, ARRAY_SIZE(g_uiImageIDs))) {
+		if (!GenerateUIAtlasImage(atlas, dpiScale, &g_cachedUIAtlasImage, draw->GetDeviceCaps().maxTextureSize, imageIDs.data(), imageIDs.size())) {
 			ERROR_LOG(Log::G3D, "Failed to generate UI atlas!");
 			return nullptr;
 		}