Add block pixel mode support for image banners

Add support for a `spring.banner.image.pixelmode` property which can
be set to `block` to use unicode block characters when rendering image
banners.

Closes gh-18301

Author: philwebb

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/ImageBanner.java

@@ -62,12 +62,6 @@ public class ImageBanner implements Banner {
 
 	private static final double[] RGB_WEIGHT = { 0.2126d, 0.7152d, 0.0722d };
 
-	private static final char[] PIXEL = { ' ', '.', '*', ':', 'o', '&', '8', '#', '@' };
-
-	private static final int LUMINANCE_INCREMENT = 10;
-
-	private static final int LUMINANCE_START = LUMINANCE_INCREMENT * PIXEL.length;
-
 	private final Resource image;
 
 	public ImageBanner(Resource image) {
@@ -104,12 +98,13 @@ private void printBanner(Environment environment, PrintStream out) throws IOExce
 		int margin = getProperty(environment, "margin", Integer.class, 2);
 		boolean invert = getProperty(environment, "invert", Boolean.class, false);
 		BitDepth bitDepth = getBitDepthProperty(environment);
+		PixelMode pixelMode = getPixelModeProperty(environment);
 		Frame[] frames = readFrames(width, height);
 		for (int i = 0; i < frames.length; i++) {
 			if (i > 0) {
 				resetCursor(frames[i - 1].getImage(), out);
 			}
-			printBanner(frames[i].getImage(), margin, invert, bitDepth, out);
+			printBanner(frames[i].getImage(), margin, invert, bitDepth, pixelMode, out);
 			sleep(frames[i].getDelayTime());
 		}
 	}
@@ -119,6 +114,11 @@ private BitDepth getBitDepthProperty(Environment environment) {
 		return (bitDepth != null) ? BitDepth.of(bitDepth) : BitDepth.FOUR;
 	}
 
+	private PixelMode getPixelModeProperty(Environment environment) {
+		String pixelMode = getProperty(environment, "pixelmode", String.class, null);
+		return (pixelMode != null) ? PixelMode.valueOf(pixelMode.trim().toUpperCase()) : PixelMode.TEXT;
+	}
+
 	private <T> T getProperty(Environment environment, String name, Class<T> targetType, T defaultValue) {
 		return environment.getProperty(PROPERTY_PREFIX + name, targetType, defaultValue);
 	}
@@ -197,7 +197,8 @@ private void resetCursor(BufferedImage image, PrintStream out) {
 		out.print("\033[" + lines + "A\r");
 	}
 
-	private void printBanner(BufferedImage image, int margin, boolean invert, BitDepth bitDepth, PrintStream out) {
+	private void printBanner(BufferedImage image, int margin, boolean invert, BitDepth bitDepth, PixelMode pixelMode,
+			PrintStream out) {
 		AnsiElement background = invert ? AnsiBackground.BLACK : AnsiBackground.DEFAULT;
 		out.print(AnsiOutput.encode(AnsiColor.DEFAULT));
 		out.print(AnsiOutput.encode(background));
@@ -216,7 +217,7 @@ private void printBanner(BufferedImage image, int margin, boolean invert, BitDep
 					out.print(AnsiOutput.encode(ansiColor));
 					lastColor = ansiColor;
 				}
-				out.print(getAsciiPixel(color, invert));
+				out.print(getAsciiPixel(color, invert, pixelMode));
 			}
 			out.println();
 		}
@@ -225,14 +226,17 @@ private void printBanner(BufferedImage image, int margin, boolean invert, BitDep
 		out.println();
 	}
 
-	private char getAsciiPixel(Color color, boolean dark) {
+	private char getAsciiPixel(Color color, boolean dark, PixelMode pixelMode) {
+		char[] pixels = pixelMode.getPixels();
+		int increment = (10 / pixels.length) * 10;
+		int start = increment * pixels.length;
 		double luminance = getLuminance(color, dark);
-		for (int i = 0; i < PIXEL.length; i++) {
-			if (luminance >= (LUMINANCE_START - (i * LUMINANCE_INCREMENT))) {
-				return PIXEL[i];
+		for (int i = 0; i < pixels.length; i++) {
+			if (luminance >= (start - (i * increment))) {
+				return pixels[i];
 			}
 		}
-		return PIXEL[PIXEL.length - 1];
+		return pixels[pixels.length - 1];
 	}
 
 	private int getLuminance(Color color, boolean inverse) {
@@ -277,4 +281,31 @@ int getDelayTime() {
 
 	}
 
+	/**
+	 * Pixel modes supported by the image banner.
+	 */
+	public enum PixelMode {
+
+		/**
+		 * Use text chars for pixels.
+		 */
+		TEXT(' ', '.', '*', ':', 'o', '&', '8', '#', '@'),
+
+		/**
+		 * Use unicode block chars for pixels.
+		 */
+		BLOCK(' ', '\u2591', '\u2592', '\u2593', '\u2588');
+
+		private char[] pixels;
+
+		PixelMode(char... pixels) {
+			this.pixels = pixels;
+		}
+
+		char[] getPixels() {
+			return this.pixels;
+		}
+
+	}
+
 }

spring-boot-project/spring-boot/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -53,6 +53,12 @@
       "description": "The bit depth to use for ANSI colors. Supported values are 4 (16 color) or 8 (256 color).",
       "defaultValue": 4
     },
+    {
+      "name": "spring.banner.image.pixelmode",
+      "type": "org.springframework.boot.ImageBanner$PixelMode",
+      "description": "The pixel mode to use when rendering the image.",
+      "defaultValue": "TEXT"
+    },
     {
       "name": "debug",
       "type": "java.lang.Boolean",

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/ImageBannerTests.java

@@ -179,6 +179,14 @@ void printBannerWhenBitDepthIs8ShouldUseColors() {
 		assertThat(banner.contains(AnsiOutput.encode(Ansi8BitColor.foreground(37))));
 	}
 
+	@Test
+	void printBannerWhenPixelModeIsBlockShouldRenderBlocks() {
+		AnsiOutput.setEnabled(AnsiOutput.Enabled.NEVER);
+		String banner = printBanner("gradient.gif", "spring.banner.image.width=6", "spring.banner.image.margin=0",
+				"spring.banner.image.pixelmode=block");
+		assertThat(banner).contains("\u2588\u2593\u2592\u2591 ");
+	}
+
 	private int getBannerHeight(String banner) {
 		return banner.split(System.lineSeparator()).length - 3;
 	}

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-animated-banner/src/main/resources/application.properties

@@ -0,0 +1,2 @@
+spring.banner.image.bitdepth=8
+spring.banner.image.pixelmode=block