diff --git a/core/fxge/dib/cfx_dibbase.cpp b/core/fxge/dib/cfx_dibbase.cpp
index e882d07..0d5ccc5 100644
--- a/core/fxge/dib/cfx_dibbase.cpp
+++ b/core/fxge/dib/cfx_dibbase.cpp
@@ -854,7 +854,6 @@
 }
 
 void CFX_DIBBase::SetPalette(pdfium::span<const uint32_t> src_palette) {
-  static const uint32_t kPaletteSize = 256;
   if (src_palette.empty() || GetBPP() > 8) {
     m_palette.clear();
     return;
diff --git a/core/fxge/dib/cfx_dibbase.h b/core/fxge/dib/cfx_dibbase.h
index 5ad1774..c23a985 100644
--- a/core/fxge/dib/cfx_dibbase.h
+++ b/core/fxge/dib/cfx_dibbase.h
@@ -30,6 +30,8 @@
   static constexpr FXDIB_Format kPlatformRGBFormat = FXDIB_Format::kRgb;
 #endif  // BUILDFLAG(IS_APPLE)
 
+  static constexpr uint32_t kPaletteSize = 256;
+
   ~CFX_DIBBase() override;
 
   virtual uint8_t* GetBuffer() const;
diff --git a/core/fxge/dib/cfx_imagestretcher.cpp b/core/fxge/dib/cfx_imagestretcher.cpp
index 4636d8f..94f7dcc 100644
--- a/core/fxge/dib/cfx_imagestretcher.cpp
+++ b/core/fxge/dib/cfx_imagestretcher.cpp
@@ -12,6 +12,8 @@
 #include "core/fxge/dib/cstretchengine.h"
 #include "core/fxge/dib/fx_dib.h"
 #include "third_party/base/check.h"
+#include "third_party/base/check_op.h"
+#include "third_party/base/span.h"
 
 namespace {
 
@@ -53,30 +55,43 @@
 
 CFX_ImageStretcher::~CFX_ImageStretcher() = default;
 
+// static
+void CFX_ImageStretcher::BuildPaletteFrom1BppSource(
+    const RetainPtr<const CFX_DIBBase>& source,
+    pdfium::span<FX_ARGB> palette_span) {
+  DCHECK_EQ(FXDIB_Format::k1bppRgb, source->GetFormat());
+  DCHECK(source->HasPalette());
+  DCHECK_EQ(CFX_DIBBase::kPaletteSize, palette_span.size());
+
+  int a0;
+  int r0;
+  int g0;
+  int b0;
+  std::tie(a0, r0, g0, b0) = ArgbDecode(source->GetPaletteArgb(0));
+  int a1;
+  int r1;
+  int g1;
+  int b1;
+  std::tie(a1, r1, g1, b1) = ArgbDecode(source->GetPaletteArgb(1));
+  DCHECK_EQ(255, a0);
+  DCHECK_EQ(255, a1);
+
+  for (int i = 0; i < static_cast<int>(CFX_DIBBase::kPaletteSize); ++i) {
+    int r = r0 + (r1 - r0) * i / 255;
+    int g = g0 + (g1 - g0) * i / 255;
+    int b = b0 + (b1 - b0) * i / 255;
+    palette_span[i] = ArgbEncode(255, r, g, b);
+  }
+}
+
 bool CFX_ImageStretcher::Start() {
   if (m_DestWidth == 0 || m_DestHeight == 0)
     return false;
 
   if (m_pSource->GetFormat() == FXDIB_Format::k1bppRgb &&
       m_pSource->HasPalette()) {
-    FX_ARGB pal[256];
-    int a0;
-    int r0;
-    int g0;
-    int b0;
-    std::tie(a0, r0, g0, b0) = ArgbDecode(m_pSource->GetPaletteArgb(0));
-    int a1;
-    int r1;
-    int g1;
-    int b1;
-    std::tie(a1, r1, g1, b1) = ArgbDecode(m_pSource->GetPaletteArgb(1));
-    for (int i = 0; i < 256; ++i) {
-      int a = a0 + (a1 - a0) * i / 255;
-      int r = r0 + (r1 - r0) * i / 255;
-      int g = g0 + (g1 - g0) * i / 255;
-      int b = b0 + (b1 - b0) * i / 255;
-      pal[i] = ArgbEncode(a, r, g, b);
-    }
+    FX_ARGB pal[CFX_DIBBase::kPaletteSize];
+    BuildPaletteFrom1BppSource(m_pSource, pal);
     if (!m_pDest->SetInfo(m_ClipRect.Width(), m_ClipRect.Height(), m_DestFormat,
                           pal)) {
       return false;
diff --git a/core/fxge/dib/cfx_imagestretcher.h b/core/fxge/dib/cfx_imagestretcher.h
index 8a43f7a..a6b2b7e 100644
--- a/core/fxge/dib/cfx_imagestretcher.h
+++ b/core/fxge/dib/cfx_imagestretcher.h
@@ -14,6 +14,7 @@
 #include "core/fxcrt/unowned_ptr.h"
 #include "core/fxge/dib/fx_dib.h"
 #include "core/fxge/dib/scanlinecomposer_iface.h"
+#include "third_party/base/span.h"
 
 class CFX_DIBBase;
 class CStretchEngine;
@@ -29,6 +30,17 @@
                      const FXDIB_ResampleOptions& options);
   ~CFX_ImageStretcher();
 
+  // Builds a new palette with a size of `CFX_DIBBase::kPaletteSize` from the
+  // existing palette in `source`. Note: The caller must make sure that the
+  // parameters meet the following conditions:
+  //   source       - The format must be `FXDIB_Format::k1bppRgb` and it must
+  //                  have a palette.
+  //   palette_span - The size must be `CFX_DIBBase::kPaletteSize` to be able
+  //                  to hold the new palette.
+  static void BuildPaletteFrom1BppSource(
+      const RetainPtr<const CFX_DIBBase>& source,
+      pdfium::span<FX_ARGB> palette_span);
+
   bool Start();
   bool Continue(PauseIndicatorIface* pPause);