intersect . . . invisible images

[Mark Waddingham]

On 2017-07-14 12:01, Richmond Mathewson via use-livecode wrote:
> It does seem illogical that while one can set levels of transparency
> with interset
> an image can continue intersecting with another when it is,
> supposedly, invisible . . .

Originally intersect only worked on the rects of the objects - I don't 
think it ever took visibility into account (nor blendLevel, for that 
matter).

When we (I think it was me, actually) added the support for pixel-level 
intersection we just extended the rect approach - so, again, visibility 
(nor blendLevel) were considered.

The threshold parameter isn't really setting levels of transparency - it 
is telling intersect what level of transparency and above should be 
considered as being 'actually there' - which means you can have some 
control over what is considered 'the actual thing you want to intersect' 
and what is considered 'just background'.

Basically intersect works by generating a 'sharp' mask (i.e. which 
pixels are 'on' and which are 'off') by rendering the object offscreen 
and then taking any pixel with alpha > threshold as being 'on' and any 
pixel with alpha < threshold being 'off'. Two objects then intersect if 
when their masks are overlapped there is at least one pixel which is 
'on' in both.

As Lagi pointed out one can imagine that there are use-cases where you 
want to know whether two things intersect regardless of whether they can 
be seen (i.e. visible false, or blendLevel 100). Indeed, I don't recall 
anyone ever commenting on this - so either it was too long ago and I 
have since forgotten, or it is useful the way it currently is.

Indeed, one could consider the current 'intersects' as a 'non-effective' 
form. i.e. It works with the pixels the object is composed of regardless 
of how they are rendered visually. This does suggest that an 'effective' 
form is missing - one which takes into account blendLevel and visible.

(There's even another point of variance here - you might not actually 
want blendLevel taken into account - imagine ghost sprites - you might 
want them very transparent when rendered, but to still intersect as if 
they had 0 blendLevel).

Warmest Regards,

Mark.

-- 
Mark Waddingham ~ mark at livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps