[chaoscope] Re: Image Sizes
- From: Chaoscope <chaoscope@xxxxxxxxxxxxxx>
- To: chaoscope@xxxxxxxxxxxxx
- Date: Wed, 12 May 2004 23:55:47 +0100
Hi,
Regarding the previous post about image sizes on the large end, what
about on the small end?
Try creating an image that is 0 x 0 - it works and even allows you to F3
and F4. Saving the image, on the other hand, saves a 0 kb file.
Imagine the amount of 0 x 0 images you could fit in a web gallery!
-1 x -1 works as well, will allow you F3, F4, and save, but when you
save you save an all black, 65532 x 1, 192 kb, file.
Last test this early morning (it's almost five in the morning here - I
woke up with Chaoscope on the brain and had to do these few tests ;) ) -
create a new -10 x -10 image or -100 x -100 image - where's the window
(the smaller one with the image, not the larger one)? Note that the
Width/Height fields display correctly (in that only positive values can
be used) but also incorrectly (in that they are not the actual size of
the window...) - where are the height/width values coming from? Are
they abs() after being passed from the image window?
It's a long story. The drawing interface is a class derived from
TBitmap, which is Borland VCL encapsulation of a Windows Bitmap
resource. I leave the height as it is and pass it on to the base class,
the property is an int so negative values are possible, although you'd
expect a C++ exception when the height is set to 0 or less.
The three lowest bits of the width value are cleared because OpenGL
viewport width must be a multiple of 8 (or had to be on my previous
graphic card implementation of OpenGL). They're cleared using a mask
(0xFFFC or 65532) which also truncate the value to 16 bits, but remains
an int, hence the sign change. A similar kludge caused the crash of the
first Ariane 5.
I admit there's very little testing done on Width/Height values. "None"
would be more accurate. I just make sure there's no memory allocated if
the height or the width are negative.
Nicolas Desprez
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