From 10ca32c260c945a5393b6a0a2816d7fa6a982e71 Mon Sep 17 00:00:00 2001
From: Olivier Couet <olivier.couet@gmail.com>
Date: Mon, 11 May 2026 14:00:51 +0200
Subject: [PATCH] TCanvas drawing area size

---
 core/base/inc/TStyle.h      |  5 ++++-
 core/base/src/TStyle.cxx    |  3 +++
 graf2d/gpad/src/TCanvas.cxx | 25 +++++++++++++++++++++++++
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/core/base/inc/TStyle.h b/core/base/inc/TStyle.h
index 4dc3dff2f4fd4..ee515c93cd45d 100644
--- a/core/base/inc/TStyle.h
+++ b/core/base/inc/TStyle.h
@@ -89,6 +89,7 @@ class TStyle : public TNamed, public TAttLine, public TAttFill, public TAttMarke
    Int_t         fCanvasDefW;        ///< Default canvas width
    Int_t         fCanvasDefX;        ///< Default canvas top X position
    Int_t         fCanvasDefY;        ///< Default canvas top Y position
+   Bool_t        fCanvasSizeIsDrawingArea; ///< If true, TCanvas ctor sizes define the drawing area instead of the window size. Default is false.
    Color_t       fPadColor;          ///< Pad color
    Width_t       fPadBorderSize;     ///< Pad border size
    Int_t         fPadBorderMode;     ///< Pad border mode
@@ -194,6 +195,7 @@ class TStyle : public TNamed, public TAttLine, public TAttFill, public TAttMarke
    Int_t            GetCanvasDefW() const      {return fCanvasDefW;}
    Int_t            GetCanvasDefX() const      {return fCanvasDefX;}
    Int_t            GetCanvasDefY() const      {return fCanvasDefY;}
+   Bool_t           GetCanvasSizeIsDrawingArea() const   {return fCanvasSizeIsDrawingArea;}
    Int_t            GetColorPalette(Int_t i) const;
    Int_t            GetColorModelPS() const    {return fColorModelPS;}
    Float_t          GetDateX() const           {return fDateX;}
@@ -351,6 +353,7 @@ class TStyle : public TNamed, public TAttLine, public TAttFill, public TAttMarke
    void             SetCanvasDefW(Int_t w=700) {fCanvasDefW = w;}
    void             SetCanvasDefX(Int_t topx=10) {fCanvasDefX = topx;}
    void             SetCanvasDefY(Int_t topy=10) {fCanvasDefY = topy;}
+   void             SetCanvasSizeIsDrawingArea(Int_t da =kFALSE) {fCanvasSizeIsDrawingArea = da;}
    void             SetLegendBorderSize(Width_t size=4) {fLegendBorderSize = size;}
    void             SetLegendFillColor(Color_t color=0) {fLegendFillColor = color;}
    void             SetLegendFillStyle(Style_t style=1001) {fLegendFillStyle = style;}
@@ -435,7 +438,7 @@ class TStyle : public TNamed, public TAttLine, public TAttFill, public TAttMarke
    void             SavePrimitive(std::ostream &out, Option_t * = "") override;
    void             SaveSource(const char *filename, Option_t *option = nullptr);
 
-   ClassDefOverride(TStyle, 24);  //A collection of all graphics attributes
+   ClassDefOverride(TStyle, 25);  //A collection of all graphics attributes
 };
 
 
diff --git a/core/base/src/TStyle.cxx b/core/base/src/TStyle.cxx
index 9b9d3b32cf2b4..c63ef394f3919 100644
--- a/core/base/src/TStyle.cxx
+++ b/core/base/src/TStyle.cxx
@@ -567,6 +567,7 @@ void TStyle::Copy(TObject &obj) const
    ((TStyle&)obj).fCanvasDefW       = fCanvasDefW;
    ((TStyle&)obj).fCanvasDefX       = fCanvasDefX;
    ((TStyle&)obj).fCanvasDefY       = fCanvasDefY;
+   ((TStyle&)obj).fCanvasSizeIsDrawingArea = fCanvasSizeIsDrawingArea;
    ((TStyle&)obj).fPadColor         = fPadColor;
    ((TStyle&)obj).fPadBorderSize    = fPadBorderSize;
    ((TStyle&)obj).fPadBorderMode    = fPadBorderMode;
@@ -713,6 +714,7 @@ void TStyle::Reset(Option_t *opt)
    fCanvasDefW     = 700;
    fCanvasDefX     = 10;
    fCanvasDefY     = 10;
+   fCanvasSizeIsDrawingArea = kFALSE;
    fPadColor       = fCanvasColor;
    fPadBorderSize  = fCanvasBorderSize;
    fPadBorderMode  = fCanvasBorderMode;
@@ -2149,6 +2151,7 @@ void TStyle::SavePrimitive(std::ostream &out, Option_t * /*= ""*/)
    out << prefix << "SetCanvasDefW(" << GetCanvasDefW() << ");\n";
    out << prefix << "SetCanvasDefX(" << GetCanvasDefX() << ");\n";
    out << prefix << "SetCanvasDefY(" << GetCanvasDefY() << ");\n";
+   out << prefix << "SetCanvasSizeIsDrawingArea(" << GetCanvasSizeIsDrawingArea() << ");\n";
    out << prefix << "SetPadColor(" << TColor::SavePrimitiveColor(GetPadColor()) << ");\n";
    out << prefix << "SetPadBorderSize(" << GetPadBorderSize() << ");\n";
    out << prefix << "SetPadBorderMode(" << GetPadBorderMode() << ");\n";
diff --git a/graf2d/gpad/src/TCanvas.cxx b/graf2d/gpad/src/TCanvas.cxx
index be3908b3b363e..1b668c5abbfd8 100644
--- a/graf2d/gpad/src/TCanvas.cxx
+++ b/graf2d/gpad/src/TCanvas.cxx
@@ -138,6 +138,27 @@ Following example shows how to proceed.
       c->SetWindowSize(500, 500);
    }
 ~~~
+
+\since **ROOT version 6.41/02:**
+
+When SetCanvasSizeIsDrawingArea(kTRUE) is enabled, the width and height passed to the TCanvas
+constructor define the size of the drawable area, rather than the full window size.
+
+Enabling this option makes the canvas behave consistently with typical image/output use cases,
+where the requested size should match the final rendered content.
+
+Example:
+~~~ {.cpp}
+{
+   gStyle->SetCanvasSizeIsDrawingArea(kTRUE);
+   Double_t w = 600;
+   Double_t h = 600;
+   auto c = new TCanvas("c", "c", w, h);
+   c->SaveAs("c.png");
+}
+~~~
+
+In this example, the saved PNG will have exactly 600×600 pixels of drawable content.
 */
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -666,6 +687,10 @@ void TCanvas::Build()
       if (TestBit(kShowEditor))      fCanvasImp->ShowEditor(kTRUE);
       if (TestBit(kShowToolTips))    fCanvasImp->ShowToolTips(kTRUE);
    }
+   if (gStyle->GetCanvasSizeIsDrawingArea()) {
+      SetWindowSize(fWindowWidth  + (fWindowWidth  - GetWw()),
+                    fWindowHeight + (fWindowHeight - GetWh()));
+   }
 }
 
 ////////////////////////////////////////////////////////////////////////////////