update backend to support rotating and resizing about the centerpoint

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/StampController.java

@@ -239,34 +239,54 @@ public class StampController {
 
         PDRectangle pageSize = page.getMediaBox();
         float x, y;
-
-        if (overrideX >= 0 && overrideY >= 0) {
-            // Use override values if provided
-            x = overrideX;
-            y = overrideY;
-        } else {
-            x = calculatePositionX(pageSize, position, fontSize, font, fontSize, stampText, margin);
-            y =
-                    calculatePositionY(
-                            pageSize, position, calculateTextCapHeight(font, fontSize), margin);
-        }
         // Split the stampText into multiple lines
-        String[] lines = stampText.split("\\\\n");
+        String[] lines = stampText.split("\\r?\\n|\\\\n");
 
         // Calculate dynamic line height based on font ascent and descent
         float ascent = font.getFontDescriptor().getAscent();
         float descent = font.getFontDescriptor().getDescent();
         float lineHeight = ((ascent - descent) / 1000) * fontSize;
 
+        // Compute a single pivot for the entire text block to avoid line-by-line wobble
+        float capHeight = calculateTextCapHeight(font, fontSize);
+        float blockHeight = Math.max(lineHeight, lineHeight * Math.max(1, lines.length));
+        float maxWidth = 0f;
+        for (String ln : lines) {
+            maxWidth = Math.max(maxWidth, calculateTextWidth(ln, font, fontSize));
+        }
+
+        if (overrideX >= 0 && overrideY >= 0) {
+            // Use override values if provided
+            x = overrideX;
+            y = overrideY;
+        } else {
+            // Base positioning on the true multi-line block size
+            x = calculatePositionX(pageSize, position, maxWidth, null, 0, null, margin);
+            y = calculatePositionY(pageSize, position, blockHeight, margin);
+        }
+
+        // After anchoring the block, draw from the top line downward
+        float adjustedX = x;
+        float adjustedY = y;
+        float pivotX = adjustedX + maxWidth / 2f;
+        float pivotY = adjustedY + blockHeight / 2f;
+
+        // Apply rotation about the block center at the graphics state level
+        contentStream.saveGraphicsState();
+        contentStream.transform(Matrix.getTranslateInstance(pivotX, pivotY));
+        contentStream.transform(Matrix.getRotateInstance(Math.toRadians(rotation), 0, 0));
+        contentStream.transform(Matrix.getTranslateInstance(-pivotX, -pivotY));
+
         contentStream.beginText();
         for (int i = 0; i < lines.length; i++) {
             String line = lines[i];
-            // Set the text matrix for each line with rotation
-            contentStream.setTextMatrix(
-                    Matrix.getRotateInstance(Math.toRadians(rotation), x, y - (i * lineHeight)));
+            // Start from top line: yTop = adjustedY + blockHeight - capHeight
+            float yLine = adjustedY + blockHeight - capHeight - (i * lineHeight);
+            contentStream.setTextMatrix(Matrix.getTranslateInstance(adjustedX, yLine));
             contentStream.showText(line);
         }
         contentStream.endText();
+        contentStream.restoreGraphicsState();
     }
 
     private void addImageStamp(
@@ -310,9 +330,17 @@ public class StampController {
         }
 
         contentStream.saveGraphicsState();
-        contentStream.transform(Matrix.getTranslateInstance(x, y));
+        // Rotate and scale about the center of the image
+        float centerX = x + (desiredPhysicalWidth / 2f);
+        float centerY = y + (desiredPhysicalHeight / 2f);
+        contentStream.transform(Matrix.getTranslateInstance(centerX, centerY));
         contentStream.transform(Matrix.getRotateInstance(Math.toRadians(rotation), 0, 0));
-        contentStream.drawImage(xobject, 0, 0, desiredPhysicalWidth, desiredPhysicalHeight);
+        contentStream.drawImage(
+                xobject,
+                -desiredPhysicalWidth / 2f,
+                -desiredPhysicalHeight / 2f,
+                desiredPhysicalWidth,
+                desiredPhysicalHeight);
         contentStream.restoreGraphicsState();
     }
 

frontend/src/components/shared/ButtonSelector.tsx

@@ -1,4 +1,5 @@
 import { Button, Group, Stack, Text } from "@mantine/core";
+import FitText from "./FitText";
 
 export interface ButtonOption<T> {
   value: T;
@@ -13,6 +14,8 @@ interface ButtonSelectorProps<T> {
   label?: string;
   disabled?: boolean;
   fullWidth?: boolean;
+  buttonClassName?: string;
+  textClassName?: string;
 }
 
 const ButtonSelector = <T extends string>({
@@ -22,6 +25,8 @@ const ButtonSelector = <T extends string>({
   label = undefined,
   disabled = false,
   fullWidth = true,
+  buttonClassName,
+  textClassName,
 }: ButtonSelectorProps<T>) => {
   return (
     <Stack gap='var(--mantine-spacing-sm)'>
@@ -41,6 +46,7 @@ const ButtonSelector = <T extends string>({
             color={value === option.value ? 'var(--color-primary-500)' : 'var(--text-muted)'}
             onClick={() => onChange(option.value)}
             disabled={disabled || option.disabled}
+            className={buttonClassName}
             style={{
               flex: fullWidth ? 1 : undefined,
               height: 'auto',
@@ -48,7 +54,13 @@ const ButtonSelector = <T extends string>({
               fontSize: 'var(--mantine-font-size-sm)'
             }}
           >
-            {option.label}
+            <FitText
+              text={option.label}
+              lines={1}
+              minimumFontScale={0.5}
+              fontSize={10}
+              className={textClassName}
+            />
           </Button>
         ))}
       </Group>

frontend/src/components/tools/addStamp/StampPreview.module.css

@@ -127,7 +127,9 @@
 /* Information text container */
 .informationContainer {
   background-color: var(--information-text-bg);
-  padding: 8px;
+  padding: 2px;
+  padding-left: 8px;
+  padding-right: 8px;
   border-radius: 10px;
   margin-top: 8px;
   margin-bottom: 8px;
@@ -138,8 +140,8 @@
 }
 
 .informationText {
-  font-size: 0.875rem;
-  font-weight: 500;
+  font-size: 0.75rem;
+  font-weight: 400;
   color: var(--information-text-color);
 }
 

frontend/src/components/tools/addStamp/StampPreview.tsx

@@ -110,6 +110,48 @@ export default function StampPreview({ parameters, onParameterChange, file, show
     )
   ), [containerSize, parameters, imageMeta, pageSize, showQuickGrid, hoverTile, pageThumbnail]);
 
+  // Keep center fixed when scaling via slider (or any fontSize changes)
+  const prevDimsRef = useRef<{ fontSize: number; widthPx: number; heightPx: number; leftPx: number; bottomPx: number } | null>(null);
+  useEffect(() => {
+    const itemStyle = style.item as any;
+    if (!itemStyle || containerSize.width <= 0 || containerSize.height <= 0) return;
+
+    const parse = (v: any) => parseFloat(String(v).replace('px', '')) || 0;
+    const leftPx = parse(itemStyle.left);
+    const bottomPx = parse(itemStyle.bottom);
+    const widthPx = parse(itemStyle.width);
+    const heightPx = parse(itemStyle.height);
+
+    const prev = prevDimsRef.current;
+    const hasOverrides = parameters.overrideX >= 0 && parameters.overrideY >= 0;
+    const canAdjust = hasOverrides && !showQuickGrid;
+    if (
+      prev &&
+      canAdjust &&
+      parameters.fontSize !== prev.fontSize &&
+      prev.widthPx > 0 &&
+      prev.heightPx > 0 &&
+      widthPx > 0 &&
+      heightPx > 0
+    ) {
+      const centerX = prev.leftPx + prev.widthPx / 2;
+      const centerY = prev.bottomPx + prev.heightPx / 2;
+      const newLeftPx = centerX - widthPx / 2;
+      const newBottomPx = centerY - heightPx / 2;
+
+      const widthPts = pageSize?.widthPts ?? 595.28;
+      const heightPts = pageSize?.heightPts ?? 841.89;
+      const scaleX = containerSize.width / widthPts;
+      const scaleY = containerSize.height / heightPts;
+      const newLeftPts = Math.max(0, Math.min(containerSize.width, newLeftPx)) / scaleX;
+      const newBottomPts = Math.max(0, Math.min(containerSize.height, newBottomPx)) / scaleY;
+      onParameterChange('overrideX', newLeftPts as any);
+      onParameterChange('overrideY', newBottomPts as any);
+    }
+
+    prevDimsRef.current = { fontSize: parameters.fontSize, widthPx, heightPx, leftPx, bottomPx };
+  }, [parameters.fontSize, style.item, containerSize, pageSize, showQuickGrid, parameters.overrideX, parameters.overrideY, onParameterChange]);
+
   // Drag/resize/rotate interactions
   const draggingRef = useRef<{ type: 'move' | 'resize' | 'rotate'; startX: number; startY: number; initLeft: number; initBottom: number; initHeight: number; centerX: number; centerY: number } | null>(null);
 

frontend/src/components/tools/addStamp/StampPreviewUtils.ts

@@ -192,7 +192,7 @@ export function computeStampPreviewStyle(
       height: `${heightPx}px`,
       opacity: displayOpacity,
       transform: `rotate(${-parameters.rotation}deg)`,
-      transformOrigin: 'left bottom',
+      transformOrigin: 'center center',
       color: parameters.customColor,
       display: 'flex',
       flexDirection: 'column',

frontend/src/components/tools/addStamp/useAddStampOperation.ts

@@ -9,9 +9,7 @@ export const buildAddStampFormData = (parameters: AddStampParameters, file: File
   formData.append('pageNumbers', parameters.pageNumbers);
   formData.append('customMargin', 'medium'); 
   formData.append('position', String(parameters.position));
-  const effectiveFontSize = parameters.stampType === 'text'
-    ? parameters.fontSize * 2.2 // upscale to match the preview size
-    : parameters.fontSize;
+  const effectiveFontSize = parameters.fontSize;
   formData.append('fontSize', String(effectiveFontSize));
   formData.append('rotation', String(parameters.rotation));
   formData.append('opacity', String(parameters.opacity / 100));

frontend/src/tools/AddStamp.tsx

@@ -11,7 +11,7 @@ import StampPreview from "../components/tools/addStamp/StampPreview";
 import LocalIcon from "../components/shared/LocalIcon";
 import styles from "../components/tools/addStamp/StampPreview.module.css";
 import { Tooltip } from "../components/shared/Tooltip";
-import FitText from "../components/shared/FitText";
+import ButtonSelector from "../components/shared/ButtonSelector";
 
 const AddStamp = ({ onPreviewFile, onComplete, onError }: BaseToolProps) => {
   const { t } = useTranslation();
@@ -99,24 +99,17 @@ const AddStamp = ({ onPreviewFile, onComplete, onError }: BaseToolProps) => {
         <Stack gap="md" justify="space-between" flex={1}>
           <div>
             <Text size="sm" fw={500} mb="xs">{t('AddStampRequest.stampType', 'Stamp Type')}</Text>
-            <Group className={styles.modeToggleGroup} grow>
-              <Button
-                variant={params.parameters.stampType === 'text' ? 'filled' : 'outline'}
-                className={styles.modeToggleButton}
-                onClick={() => params.updateParameter('stampType', 'text')}
-                disabled={endpointLoading}
-              >
-                {t('watermark.type.1', 'Text')}
-              </Button>
-              <Button
-                variant={params.parameters.stampType === 'image' ? 'filled' : 'outline'}
-                className={styles.modeToggleButton}
-                onClick={() => params.updateParameter('stampType', 'image')}
-                disabled={endpointLoading}
-              >
-                {t('watermark.type.2', 'Image')}
-              </Button>
-            </Group>
+            <ButtonSelector
+              value={params.parameters.stampType}
+              onChange={(v: 'text' | 'image') => params.updateParameter('stampType', v)}
+              options={[
+                { value: 'text', label: t('watermark.type.1', 'Text') },
+                { value: 'image', label: t('watermark.type.2', 'Image') },
+              ]}
+              disabled={endpointLoading}
+              buttonClassName={styles.modeToggleButton}
+              textClassName={styles.modeToggleButtonText}
+            />
           </div>
 
           {params.parameters.stampType === 'text' && (
@@ -208,45 +201,31 @@ const AddStamp = ({ onPreviewFile, onComplete, onError }: BaseToolProps) => {
         <Stack gap="md" justify="space-between">
           {/* Mode toggle: Quick grid vs Custom drag - only show for image stamps */}
           {params.parameters.stampType === 'image' && (
-            <Group className={styles.modeToggleGroup} grow>
-              <Button
-                variant={quickPositionModeSelected ? 'filled' : 'outline'}
-                className={styles.modeToggleButton}
-                onClick={() => {
-                  setQuickPositionModeSelected(true);
-                  setCustomPositionModeSelected(false);
-                }}
-              >
-                <FitText
-                  text={t('quickPosition', 'Quick Position')}
-                  lines={1}
-                  minimumFontScale={0.5}
-                  fontSize={10}
-                  className={styles.modeToggleButtonText}
-                />
-              </Button>
-              <Button
-                variant={customPositionModeSelected ? 'filled' : 'outline'}
-                className={styles.modeToggleButton}
-                onClick={() => {
-                  setQuickPositionModeSelected(false);
-                  setCustomPositionModeSelected(true);
-                }}
-              >
-                <FitText
-                  text={t('customPosition', 'Custom Position')}
-                  lines={1}
-                  minimumFontScale={0.5}
-                  fontSize={10}
-                  className={styles.modeToggleButtonText}
-                />
-              </Button>
-            </Group>
+            <ButtonSelector
+              value={quickPositionModeSelected ? 'quick' : 'custom'}
+              onChange={(v: 'quick' | 'custom') => {
+                const isQuick = v === 'quick';
+                setQuickPositionModeSelected(isQuick);
+                setCustomPositionModeSelected(!isQuick);
+              }}
+              options={[
+                { value: 'quick', label: t('quickPosition', 'Quick Position') },
+                { value: 'custom', label: t('customPosition', 'Custom Position') },
+              ]}
+              disabled={endpointLoading}
+              buttonClassName={styles.modeToggleButton}
+              textClassName={styles.modeToggleButtonText}
+            />
           )}
 
           {params.parameters.stampType === 'image' && customPositionModeSelected && (
             <div className={styles.informationContainer}>
-              <Text className={styles.informationText}>{t('customPosition', 'Drag the stamp to the desired location in the preview window.')}</Text>
+              <Text className={styles.informationText}>{t('AddStampRequest.customPosition', 'Drag the stamp to the desired location in the preview window.')}</Text>
+            </div>
+          )}
+          {params.parameters.stampType === 'image' && !customPositionModeSelected && (
+            <div className={styles.informationContainer}>
+              <Text className={styles.informationText}>{t('AddStampRequest.quickPosition', 'Select a position on the page to place the stamp.')}</Text>
             </div>
           )}