diff --git a/toonz/sources/tnztools/mypainttoonzbrush.cpp b/toonz/sources/tnztools/mypainttoonzbrush.cpp
index c3670dc..deb9b04 100644
--- a/toonz/sources/tnztools/mypainttoonzbrush.cpp
+++ b/toonz/sources/tnztools/mypainttoonzbrush.cpp
@@ -9,7 +9,8 @@
 #include <QColor>
 
 namespace {
-void putOnRasterCM(const TRasterCM32P &out, const TRaster32P &in, int styleId) {
+void putOnRasterCM(const TRasterCM32P &out, const TRaster32P &in, int styleId,
+                   bool lockAlpha) {
   if (!out.getPointer() || !in.getPointer()) return;
   assert(out->getSize() == in->getSize());
   int x, y;
@@ -24,7 +25,13 @@ void putOnRasterCM(const TRasterCM32P &out, const TRaster32P &in, int styleId) {
       TPixel32 *inPix = &in->pixels(y)[x];
       if (inPix->m == 0) continue;
       TPixelCM32 *outPix = &out->pixels(y)[x];
-      bool sameStyleId   = styleId == outPix->getInk();
+      if (lockAlpha && !outPix->isPureInk() && outPix->getPaint() == 0 &&
+          outPix->getTone() == 255) {
+        *outPix =
+            TPixelCM32(outPix->getInk(), outPix->getPaint(), outPix->getTone());
+        continue;
+      }
+      bool sameStyleId = styleId == outPix->getInk();
       // line with the same style : multiply tones
       // line with different style : pick darker tone
       int tone = sameStyleId ? outPix->getTone() * (255 - inPix->m) / 255
@@ -196,7 +203,8 @@ void MyPaintToonzBrush::strokeTo(const TPointD &point, double pressure,
 
 void MyPaintToonzBrush::updateDrawing(const TRasterCM32P rasCM,
                                       const TRasterCM32P rasBackupCM,
-                                      const TRect &bbox, int styleId) const {
+                                      const TRect &bbox, int styleId,
+                                      bool lockAlpha) const {
   if (!rasCM) return;
 
   TRect rasRect    = rasCM->getBounds();
@@ -204,6 +212,6 @@ void MyPaintToonzBrush::updateDrawing(const TRasterCM32P rasCM,
   if (targetRect.isEmpty()) return;
 
   rasCM->copy(rasBackupCM->extract(targetRect), targetRect.getP00());
-  putOnRasterCM(rasCM->extract(targetRect), m_ras->extract(targetRect),
-                styleId);
+  putOnRasterCM(rasCM->extract(targetRect), m_ras->extract(targetRect), styleId,
+                lockAlpha);
 }
\ No newline at end of file
diff --git a/toonz/sources/tnztools/mypainttoonzbrush.h b/toonz/sources/tnztools/mypainttoonzbrush.h
index 352afa5..f26aa60 100644
--- a/toonz/sources/tnztools/mypainttoonzbrush.h
+++ b/toonz/sources/tnztools/mypainttoonzbrush.h
@@ -130,7 +130,7 @@ public:
 
   // colormapped
   void updateDrawing(const TRasterCM32P rasCM, const TRasterCM32P rasBackupCM,
-                     const TRect &bbox, int styleId) const;
+                     const TRect &bbox, int styleId, bool lockAlpha) const;
 };
 
 #endif  // T_BLUREDBRUSH
diff --git a/toonz/sources/tnztools/tooloptions.cpp b/toonz/sources/tnztools/tooloptions.cpp
index 0f64f64..d4b05d2 100644
--- a/toonz/sources/tnztools/tooloptions.cpp
+++ b/toonz/sources/tnztools/tooloptions.cpp
@@ -1784,16 +1784,18 @@ void BrushToolOptionsBox::filterControls() {
   for (QMap<std::string, QLabel *>::iterator it = m_labels.begin();
        it != m_labels.end(); it++) {
     bool isModifier = (it.key().substr(0, 8) == "Modifier");
-    bool isCommon   = (it.key() == "Pressure" || it.key() == "Preset:");
-    bool visible    = isCommon || (isModifier == showModifiers);
+    bool isCommon   = (it.key() == "Lock Alpha" || it.key() == "Pressure" ||
+                     it.key() == "Preset:");
+    bool visible = isCommon || (isModifier == showModifiers);
     it.value()->setVisible(visible);
   }
 
   for (QMap<std::string, ToolOptionControl *>::iterator it = m_controls.begin();
        it != m_controls.end(); it++) {
     bool isModifier = (it.key().substr(0, 8) == "Modifier");
-    bool isCommon   = (it.key() == "Pressure" || it.key() == "Preset:");
-    bool visible    = isCommon || (isModifier == showModifiers);
+    bool isCommon   = (it.key() == "Lock Alpha" || it.key() == "Pressure" ||
+                     it.key() == "Preset:");
+    bool visible = isCommon || (isModifier == showModifiers);
     if (QWidget *widget = dynamic_cast<QWidget *>(it.value()))
       widget->setVisible(visible);
   }
diff --git a/toonz/sources/tnztools/toonzrasterbrushtool.cpp b/toonz/sources/tnztools/toonzrasterbrushtool.cpp
index 08d0621..db1db72 100644
--- a/toonz/sources/tnztools/toonzrasterbrushtool.cpp
+++ b/toonz/sources/tnztools/toonzrasterbrushtool.cpp
@@ -1335,7 +1335,7 @@ void ToonzRasterBrushTool::leftButtonDown(const TPointD &pos,
       if (!updateRect.isEmpty()) {
         // ras->extract(updateRect)->copy(m_workRas->extract(updateRect));
         m_toonz_brush->updateDrawing(ri->getRaster(), m_backupRas, m_strokeRect,
-                                     m_styleId);
+                                     m_styleId, m_modifierLockAlpha.getValue());
       }
       m_lastRect = m_strokeRect;
 
@@ -1439,7 +1439,7 @@ void ToonzRasterBrushTool::leftButtonDrag(const TPointD &pos,
     if (!updateRect.isEmpty()) {
       // ras->extract(updateRect)->copy(m_workRaster->extract(updateRect));
       m_toonz_brush->updateDrawing(ras, m_backupRas, m_strokeSegmentRect,
-                                   m_styleId);
+                                   m_styleId, m_modifierLockAlpha.getValue());
     }
     m_lastRect = m_strokeRect;
 
@@ -1594,7 +1594,7 @@ void ToonzRasterBrushTool::finishRasterBrush(const TPointD &pos,
     if (!updateRect.isEmpty()) {
       // ras->extract(updateRect)->copy(m_workRaster->extract(updateRect));
       m_toonz_brush->updateDrawing(ras, m_backupRas, m_strokeSegmentRect,
-                                   m_styleId);
+                                   m_styleId, m_modifierLockAlpha.getValue());
     }
     TPointD thickOffset(m_maxCursorThick * 0.5,
                         m_maxCursorThick * 0.5);  // TODO