What is the fastest way do downscale an image and save it to a stream
or a file?
lazarus / 39-Graphics

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CM630 · 2017-02-14 08:48

I wonder what is the fastest way do downscale an image and save it to
a stream or a file without using external executables (image magic,
etc)?
I will also need some cropping.
All free LPKs are fine.

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Eugene Loza · 2017-02-14 10:05

Well... the fastest down-scale algorithm is "nearest neighbours" which
can be additionally optimized in case downscale is fixed (e.g. scale
is integer). Cropping can also be done by directly processing the
bitmap memory (start here http://wiki.freepascal.org/Developing_with_G
raphics#Direct_pixel_access ). If you aren't too crazy about
optimizations you might just want to Canvas.CopyRect...
The whole thing might look as make a destination bitmap. Scale source
bitmap to destination. Save destination to stream. However, such
approach doesn't look optimal.
If you really want it to be efficient, then I'd advice you to use
OpenGL features, however, I use them through Castle Game Engine, so I
don't know how to make it without installing additional libraries...

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Handoko · 2017-02-14 10:08

Maybe you can use BGRABitmap, it have scale feature:
http://wiki.lazarus.freepascal.org/BGRABitmap_tutorial_11

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CM630 · 2017-02-22 07:10

Thanks for the hints, I was hoping for some first- hand information.
I doubt that there will be any use of OpenGl, since we use Intel VGAs.
I will try when possible and feedback.

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Thaddy · 2017-02-22 07:35

Quote from: CM630 on February 14, 2017, 09:48:11 am
I wonder what is the fastest way do downscale an image and save it to
a stream or a file without using external executables (image magic,
etc)?
I will also need some cropping.
All free LPKs are fine.

FPC has bindings for the imagemagic LIBRARIES, no need to call the
executable to have the speed of imagemagic.
The Imagemagic lib can also be statically linked into your executable
and is (at the moment...) still MUCH (4-5 times) faster than any pure
pascal solution I know of, including BGRABitmap.

Also note that hardware accelerated OpenGL is fully supported by Intel
VGA's on ANY platform that supports OpenGL (That includes Windows,
Linux and Android)