On 2017-03-10 11:01, Kostas Michalopoulos via Lazarus wrote:
You can check yourself easily: Ctrl+Click on the p.Width to go to the definition of the property, then Ctrl+Click on the GetWidth getter, Ctrl+Shift+Down arrow to see the code for the getter and see that it simply gets FGraphic's property.
You can check the current sources but that does not mean it will stay that way, or that the current sources do not have a bug in them... the documentation would tell you how it is supposed to act, and, the source is one particular implementation, which could not match up with what the doc says, in that case it is buggy ;-)
But thanks very much for explaining it: indeed I figured any call to .jpeg.someproperty would immediately automagically do the work for you
So what is FGraphic? Ctrl+Home to
move at the top of the source code (you'd be in picture.inc) and search for "FGraphic :=" to see where it is assigned (the source code style in that file seems to put a space before := in assignments). You can see that it is used in a bunch of places, so you can see where those uses are. ForceType is the first one and if you search for that one, you'll find that it is used wherever a type-specific property (p.Jpeg, p.Bitmap, etc) is used - presumably to convert the underlying graphic to the requested type. So the FGraphic object is actually an instance of the TBitmap, TPortableNetworkGraphic, TJpegImage, etc (depending what you loaded and/or accessed last).
This is very useful - I thought maybe it would be in a wiki or docs somewhere.. It may be...
So what p.Width gives you is the FGraphic's width or 0 if there is no FGraphic loaded (because a picture may have nothing loaded until you either load it directly or try to access any of the Bitmap, Jpeg, PNG, etc properties). So if you already have a picture loaded from a PNG file, p.Width, p.Graphic.Width and p.PNG.Width will give you the same number. If your picture doesn't have a graphic (e.g. it is a newly constructed object) then p.Width gives you 0, p.Graphic.Width will crash your program (FGraphic is nil) and p.PNG.Width will give you zero (because accessing the PNG property will call the ForceType function which will create a TPortableNetworkGraphic object that by default is undefined and gives you zero width). Question for the readers: what happens to the image if you load a PNG file with alpha values using TPicture's LoadFromFile, access the TPicture.Jpeg.Width property and then save the picture as a PNG file using TPicture's SaveToFile? Is the alpha component preserved? Do you get jpeg artifacts? Why? Bonus question: why is R := Rect(0, 0, p.Jpeg.Width, p.PNG.Height) worse than R := Rect(0, 0, p.Jpeg.Width, p.PNG.Height)? Why is R := Rect(0, 0, p.Width, p.Height) better than either of them? You can find the answers to those (and many more) using Ctrl+Click, Ctrl+Shift+Up/Down Arrow, Search and back and forward buttons (helps to have a mouse with those). Considering how big and complex LCL is, it helps to be able to navigate and familiarize yourself with its source code :-).
IMO the docs should always describe the theory of an object/procedure, and therefore if any time the source code is buggy, the docs are the official sources to what a function does.. :-)
Thanks much for documenting it in this mailing list ! Some of your comments, IMO should be uploaded to wiki/docs somewhere But I'm unfamiliar with how the docs updating process works -- _______________________________________________ Lazarus mailing list Lazarus@lists.lazarus-ide.org http://lists.lazarus-ide.org/listinfo/lazarus
