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Documentation: explain how images are best handled in the context of GUI rescaling.

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git-svn-id: file:///fltk/svn/fltk/branches/branch-1.4@12798 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
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Manolo Gouy 2018-03-25 06:15:50 +00:00
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@ -78,18 +78,19 @@ with ctrl/+/-/0/ keystrokes which enlarge, shrink, and reset, respectively,
all FLTK windows on a screen and their content.
Under Mac OS X, the corresponding GUI scaling shortcuts are cmd/+/-/0/.
When drawing images to a screen whose scale factor value is > 1, the image pixels
are mapped to the larger number of pixels present in
a screen area sized (in FLTK units) as the image. This operation is done internally
by the library using diverse scaling methods that vary with the image type
(Fl_Pixmap, Fl_Bitmap or Fl_RGB_Image) and the platform.
A situation of special interest arises when drawing Fl_Shared_Image's. The
Fl_Shared_Image::scale() member function allows to set the image drawing size
(in FLTK units) independently of its pixel size. Image pixels are mapped to the
pixels of the image drawing area on the screen which is determined by the scale
factor value. Therefore, the image data can fill the drawing area of the screen at
full physical resolution even for high values of the scale factor if the original
image is large enough.
GUI rescaling involves also image drawing: the screen area covered by the drawn image
contains a number of pixels that grows with the scale factor. When FLTK draws images,
it maps the image data (the size of these data is given by Fl_Image::data_w() and
Fl_Image::data_h())) to the screen area whose size (in FLTK units) is given by
Fl_Image::w() and Fl_Image::h(). How exactly such mapping is performed depends on the
image type, the platform and some hardware features. The most common
case for Fl_RGB_Image's is that FLTK uses a scaled drawing system feature that directly
maps image data to screen pixels. An important feature of FLTK for image drawing
is the Fl_Image::scale() member function, new in FLTK version 1.4. This function
controls the image drawing size (in FLTK units) independantly from the size
of the image data. An image with large enough data size can thus be drawn at the
full resolution of the screen even when the screen area covered by the image grows
following the GUI scale factor.
The Fl_Image_Surface class is intended to create an Fl_RGB_Image from a series
of FLTK drawing operations. The Fl_Image_Surface constructor allows to control