Clarify documentation of Fl_Image::copy() and more (#431)

- emphasize that Fl_Image::copy(W, H) creates an image with
  w() == data_w() == W and h() == data_h() == H
- clarify some more docs of Fl_Image methods (ensure that data_w()
  and data_h() are used where appropriate rather than w() and h()
- improve wording, examples, and formatting of related docs.

Closes #431

FL/Fl_Image.H

@@ -40,23 +40,22 @@ enum Fl_RGB_Scaling {
 
 
 /**
- \brief Base class for image caching, scaling and drawing.
+  Base class for image caching, scaling and drawing.
 
- Fl_Image is the base class used for caching, scaling and drawing all kinds of images
+  Fl_Image is the base class used for caching, scaling and drawing all kinds of
- in FLTK. This class keeps track of common image data such as the pixels,
+  images in FLTK. This class keeps track of common image data such as the pixels,
- colormap, width, height, and depth. Virtual methods are used to provide
+  colormap, width, height, and depth. Virtual methods are used to provide
- type-specific image handling.
+  type-specific image handling.
 
- Each image possesses two (width, height) pairs. 1) The width and height of the
+  Each image possesses two (width, height) pairs:
- image raw data are returned by data_w() and data_h(). These values are set
+   -# The width and height of the raw image data are returned by data_w() and
- when the image is created and remain unchanged. 2) The width and height
+      data_h(). These values are set when the image is created and remain unchanged.
- of the area filled by the image when it gets drawn are returned by w() and h().
+   -# The width and height of the area filled by the image when it gets drawn are
- The values are equal to data_w() and data_h() when the image is created,
+      returned by w() and h(). These values are equal to data_w() and data_h() when
- and can be changed by the scale() member function.
+      the image is created and can be changed by the scale() member function.
 
- Since the Fl_Image class does not support image
+  Since the Fl_Image class does not support image drawing by itself, calling
- drawing by itself, calling the draw() method results in
+  the draw() method results in a box with an X in it being drawn instead.
- a box with an X in it being drawn instead.
 */
 class FL_EXPORT Fl_Image {
   friend class Fl_Graphics_Driver;
@@ -96,15 +95,15 @@ protected:
    */
   void d(int D) {d_ = D;}
   /**
-   Sets the current line data size in bytes.
+    Sets the current line data size in bytes.
 
-   Color images may contain extra data that is included after every
+    Color images may contain extra data (padding) that is included after every
-   line of color image data and is normally not present.
+    line of color image data and is normally not present.
 
-   If \p LD is zero, then line data size is assumed to be w() * d() bytes.
+    If \p LD is zero, then line data size is assumed to be data_w() * d() bytes.
 
-   If \p LD is non-zero, then it must be positive and larger than w() * d()
+    If \p LD is non-zero, then it must be positive and larger than data_w() * d()
-   to account for the extra data per line.
+    to account for the extra data per line.
    */
   void ld(int LD) {ld_ = LD;}
   /**
@@ -177,15 +176,17 @@ public:
 
     Fl_RGB_Image has exactly one pointer which points at the R, G, B [, A]
     data array of the image. The total size of this array depends on
-    several attributes like w(), h(), data_w(), data_h(), d() and ld()
+    several attributes like data_w(), data_h(), d() and ld() and is basically
-    and is basically data_w() * data_h() * d() but there are exceptions
+    data_w() * data_h() * d() but there are exceptions if ld() is non-zero:
-    if ld() is non-zero: see description of ld().
+    see description of ld(). Since FLTK 1.4.0 w() and h() are no longer
+    significant for the image data size if scale() has been called on the
+    image to set a different display size.
 
     Other image types have different numbers and types of data pointers
     which are implementation details and not documented here.
 
     \see count(), w(), h(), data_w(), data_h(), d(), ld()
-   */
+  */
   const char * const *data() const {return data_;}
   int fail() const;
   /**
@@ -197,18 +198,18 @@ public:
     \endcode
     where image is an \p Fl_Image \p * pointer.
 
-    However, for subclass Fl_Shared_Image this virtual method is
+    However, for subclass Fl_Shared_Image and its subclasses this virtual
-    reimplemented and maintains shared images.
+    method is reimplemented and maintains shared images.
 
-    This virtual method makes it possible to \p delete all image types in
+    This virtual method makes it possible to \p destroy all image types in
     the same way by calling
     \code
       image->release();
     \endcode
 
-    Reasoning: If you have an 'Fl_Image *' pointer and don't know if the
+    Reasoning: If you have an 'Fl_Image *' base class pointer and don't know
-    object is one of the class Fl_Shared_Image or any other subclass of
+    if the object is one of the class Fl_Shared_Image or any other subclass
-    Fl_Image (for instance Fl_RGB_Image) then you can't just use operator
+    of Fl_Image (for instance Fl_RGB_Image) then you can't just use operator
     delete since this is not appropriate for Fl_Shared_Image objects.
 
     The virtual method release() handles this properly.
@@ -236,16 +237,30 @@ public:
   virtual ~Fl_Image();
   virtual Fl_Image *copy(int W, int H) const;
   /**
-    Creates a copy of the specified image.
+    Creates a copy of the image in the same size.
-    The image should be released when you are done with it.
 
-    Note: since FLTK 1.4.0 you can use Fl_Image::release() for
+    The copied image should be released when you are done with it.
-    all types of images (i.e. all subclasses of Fl_Image) instead
-    of operator \em delete for Fl_Image's and release() for
-    Fl_Shared_Image's.
 
+    This does exactly the same as 'Fl_Image::copy(int W, int H) const' where
+    \p W and \p H are the width and height of the source image, respectively.
+    This applies also to all subclasses of Fl_Image in the FLTK library.
+
+    The following two copy() calls are equivalent:
+    \code
+      Fl_Image *img1 = new Fl_Image(...);
+      // ...
+      Fl_Image *img2 = img1->copy();
+      Fl_Image *img3 = img1->copy(img1->w(), img1->h())
+    \endcode
+
+    For details see 'Fl_Image::copy(int w, int h) const'.
     \see Fl_Image::release()
-    \see Fl_Image::copy(int w, int h)
+
+    \note Since FLTK 1.4.0 this method is 'const'. If you derive your own class
+      from Fl_Image or any subclass your overridden methods of 'Fl_Image::copy() const'
+      and Fl_Image::copy(int, int) const' \b must also be 'const' for inheritage
+      to work properly. This is different than in FLTK 1.3.x and earlier where these
+      methods have not been 'const'.
   */
   Fl_Image *copy() const { return copy(w(), h()); }
   virtual void color_average(Fl_Color c, float i);

src/Fl_Image.cxx

@@ -76,17 +76,28 @@ void Fl_Image::draw_empty(int X, int Y) {
 }
 
 /**
-  Creates a resized copy of the specified image.
+  Creates a resized copy of the image.
-  The image should be released when you are done with it.
 
-    Note: since FLTK 1.4.0 you can use Fl_Image::release() for
+  The copied image should be released when you are done with it.
-    all types of images (i.e. all subclasses of Fl_Image) instead
-    of operator \em delete for Fl_Image's and release() for
-    Fl_Shared_Image's.
 
-  \see Fl_Image::release()
+  Note: since FLTK 1.4.0 you can use Fl_Image::release() for all types
+  of images (i.e. all subclasses of Fl_Image) instead of operator \em delete
+  for Fl_Image's and Fl_Image::release() for Fl_Shared_Image's.
+
+  The copied image data will be converted to the requested size. RGB images
+  are resized using the algorithm set by Fl_Image::RGB_scaling().
+
+  For the copied image the following equations are true:
+  - w() == data_w() == \p W
+  - h() == data_h() == \p H
 
   \param[in] W,H  Requested width and height of the copied image
+
+  \note Since FLTK 1.4.0 this method is 'const'. If you derive your own class
+    from Fl_Image or any subclass your overridden methods of 'Fl_Image::copy() const'
+    and Fl_Image::copy(int, int) const' \b must also be 'const' for inheritage
+    to work properly. This is different than in FLTK 1.3.x and earlier where these
+    methods have not been 'const'.
 */
 Fl_Image *Fl_Image::copy(int W, int H) const {
   return new Fl_Image(W, H, d());
@@ -107,12 +118,13 @@ void Fl_Image::color_average(Fl_Color, float) {
 }
 
 /**
-  The desaturate() method converts an image to
+  The desaturate() method converts an image to grayscale.
-  grayscale. If the image contains an alpha channel (depth = 4),
+
+  If the image contains an alpha channel (depth = 4),
   the alpha channel is preserved.
 
-  An internal copy is made of the original image before
+  An internal copy is made of the original image data before
-  changes are applied, to avoid modifying the original image.
+  changes are applied, to avoid modifying the original image data.
 */
 void Fl_Image::desaturate() {
 }
@@ -128,63 +140,62 @@ Fl_Labeltype Fl_Image::define_FL_IMAGE_LABEL() {
 }
 
 /**
-  The label() methods are an obsolete way to set the
+  This method is an obsolete way to set the image attribute of a widget
-  image attribute of a widget or menu item. Use the
+  or menu item.
-  image() or deimage() methods of the
+
-  Fl_Widget and Fl_Menu_Item classes
+  Use the image() or deimage() methods of the Fl_Widget and Fl_Menu_Item
-  instead.
+  classes instead.
 */
 void Fl_Image::label(Fl_Widget* widget) {
   widget->image(this);
 }
 
 /**
-  The label() methods are an obsolete way to set the
+  This method is an obsolete way to set the image attribute of a widget
-  image attribute of a widget or menu item. Use the
+  or menu item.
-  image() or deimage() methods of the
+
-  Fl_Widget and Fl_Menu_Item classes
+  Use the image() or deimage() methods of the Fl_Widget and Fl_Menu_Item
-  instead.
+  classes instead.
 */
 void Fl_Image::label(Fl_Menu_Item* m) {
   m->label(FL_IMAGE_LABEL, (const char*)this);
 }
 
 /**
- Returns a value that is not 0 if there is currently no image
+  Returns a value that is not 0 if there is currently no image available.
- available.
 
- Example use:
+  Example use:
- \code
+  \code
-    [..]
+    // [..]
-    Fl_Box box(X,Y,W,H);
+      Fl_Box box(X, Y, W, H);
-    Fl_JPEG_Image jpg("/tmp/foo.jpg");
+      Fl_JPEG_Image jpg("/tmp/foo.jpg");
-    switch ( jpg.fail() ) {
+      switch (jpg.fail()) {
         case Fl_Image::ERR_NO_IMAGE:
         case Fl_Image::ERR_FILE_ACCESS:
-            fl_alert("/tmp/foo.jpg: %s", strerror(errno));    // shows actual os error to user
+          fl_alert("/tmp/foo.jpg: %s", strerror(errno));    // shows actual os error to user
-            exit(1);
+          exit(1);
         case Fl_Image::ERR_FORMAT:
-            fl_alert("/tmp/foo.jpg: couldn't decode image");
+          fl_alert("/tmp/foo.jpg: couldn't decode image");
-            exit(1);
+          exit(1);
-    }
+      }
-    box.image(jpg);
+      box.image(jpg);
-    [..]
+  \endcode
- \endcode
 
- \return ERR_NO_IMAGE if no image was found
+  \returns                Image load failure if non-zero
- \return ERR_FILE_ACCESS if there was a file access related error (errno should be set)
+  \retval 0               the image was loaded successfully
- \return ERR_FORMAT if image decoding failed.
+  \retval ERR_NO_IMAGE    no image was found
- */
+  \retval ERR_FILE_ACCESS there was a file access related error (errno should be set)
-int Fl_Image::fail() const
+  \retval ERR_FORMAT      image decoding failed
-{
+*/
-    // if no image exists, ld_ may contain a simple error code
+int Fl_Image::fail() const {
-    if ( (w_<=0) || (h_<=0) || (d_<=0) ) {
+  // if no image exists, ld_ may contain a simple error code
-        if (ld_==0)
+  if ((w_ <= 0) || (h_ <= 0) || (d_ <= 0)) {
-            return ERR_NO_IMAGE;
+    if (ld_ == 0)
-        else
+      return ERR_NO_IMAGE;
-            return ld_;
+    else
-    }
+      return ld_;
-    return 0;
+  }
+  return 0;
 }
 
 void