Improve transparency documentation

core/base/src/TAttFill.cxx

@@ -258,7 +258,7 @@ void TAttFill::SetFillAttributes()
 /// Set a transparent fill color.
 /// \param fcolor defines the fill color
 /// \param falpha defines the percentage of opacity from 0. (fully transparent) to 1. (fully opaque).
-/// \note falpha is ignored (treated as 1) if the TCanvas has no GL support activated.
+/// \note falpha is used only on the [supported platforms](\ref col_trans_available).
 
 void TAttFill::SetFillColorAlpha(Color_t fcolor, Float_t falpha)
 {

core/base/src/TAttLine.cxx

@@ -312,7 +312,7 @@ void TAttLine::SetLineAttributes()
 /// Set a transparent line color.
 /// \param lcolor defines the line color
 /// \param lalpha defines the percentage of opacity from 0. (fully transparent) to 1. (fully opaque).
-/// \note lalpha is ignored (treated as 1) if the TCanvas has no GL support activated.
+/// \note lalpha is used only on the [supported platforms](\ref col_trans_available).
 
 void TAttLine::SetLineColorAlpha(Color_t lcolor, Float_t lalpha)
 {

core/base/src/TAttMarker.cxx

@@ -359,7 +359,7 @@ void TAttMarker::SetMarkerAttributes()
 /// Set a transparent marker color.
 /// \param mcolor defines the marker color
 /// \param malpha defines the percentage of opacity from 0. (fully transparent) to 1. (fully opaque).
-/// \note malpha is ignored (treated as 1) if the TCanvas has no GL support activated.
+/// \note malpha is used only on the [supported platforms](\ref col_trans_available).
 
 void TAttMarker::SetMarkerColorAlpha(Color_t mcolor, Float_t malpha)
 {

core/base/src/TAttText.cxx

@@ -426,7 +426,7 @@ void TAttText::SetTextAttributes()
 /// Set a transparent text color.
 /// \param tcolor defines the text color
 /// \param talpha defines the percentage of opacity from 0. (fully transparent) to 1. (fully opaque).
-/// \note talpha is ignored (treated as 1) if the TCanvas has no GL support activated.
+/// \note talpha is used only on the [supported platforms](\ref col_trans_available).
 
 void TAttText::SetTextColorAlpha(Color_t tcolor, Float_t talpha)
 {

core/base/src/TColor.cxx

@@ -1046,9 +1046,22 @@ is set to blue with an opacity of 35% (i.e. a transparency of 65%).
    histo->SetFillColorAlpha(kBlue, 0.35);
 ~~~
 
-The transparency is available on all platforms when the flag `OpenGL.CanvasPreferGL` is set to `1`
-in `$ROOTSYS/etc/system.rootrc`, or on Mac with the Cocoa backend. On the file output
-it is visible with PDF, PNG, Gif, JPEG, SVG, TeX ... but not PostScript.
+\anchor col_trans_available
+
+#### Note:
+
+Transparency is supported by the following graphics backends:
+
+- OpenGL
+- Cocoa/Quartz (macOS)
+- SVG
+- PDF
+- TeX
+- PNG, GIF, and JPEG
+- TWebCanvas
+- Future `TCanvasImp` implementations, such as `qt6canvas`
+
+Transparency is **not** supported by the X11 and PostScript backends.
 
 Alternatively, you can call at the top of your script `gStyle->SetCanvasPreferGL();`.
 Or if you prefer to activate GL for a single canvas `c`, then use `c->SetSupportGL(true);`.