CODEWITHSUNDEEP
delphicanvasiconsdelphi-2009stretch
Wodzu
Wodzu

Reputation: 6979

For what design reason TCanvas.StretchDraw is not working as expected for the TIcon?

Recently I've discovered that TCanvas.StretchDraw will not work as expected for object which is a TIcon instance (quick look at TIcon.Draw and DrawIconEx method tells why). Delphi help acknowledges that fact. I know the workaround but I do not know the reason behind such design decision in VCL. Does anyone know why they decided to left TIcon untouched in this matter?

Upvotes: 3

Views: 580

Answers (1)

Arnaud Bouchez
Arnaud Bouchez

Reputation: 43033

Icons are not regular bitmaps. This is mostly due to historical design and technical reasons.

It did make sense at time when icons were small 32x32 pixels large and 16 colors (good old days!) that icons were never to be stretched on screen.

But there is also a "common sense" technical reason. Such small bitmaps are usually very hard to be re-sized by an algorithm (and default GDI strech algoritm is very fast but produces also very bad result in respect to other interpolation modes, e.g. available with GDI+), so it was decided to embed a set of icons within the executable, as resources: one icon per size. The strech process benefits of being done at design time, at pixel level, by an icon designer. And - back those days - it was also much less resource consuming to use dedicated icons, with a reduced color palette.

Since you are supposed to have a set of icons with a pre-defined size for each, you do not need to use StrechDraw, but just select the right icon to display.

So if you want to display an icon with a given size, ensure you pickup the right size, or get the biggest icon and upsize it, using a temporary bitmap - or DrawIconEx(). Or, even better, do not use icons, but a bitmap, or a vectorial drawing if you expect huge picture size.

Upvotes: 1

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