Re: [Transforms] perspective description


On Oct 10, 2013, at 3:52 AM, Dean Jackson <dino@apple.com> wrote:

> "The perspective and perspective-origin properties can be used to add a feeling of depth to a scene by making elements higher on the Z axis (closer to the viewer) appear larger, and those further away to appear smaller. The scaling is proportional to d/(d − Z) where d, the value of perspective, is the distance from the drawing plane to the the assumed position of the viewer’s eye.”
> 
> This is sort-of true, but misleading and confusing. The key is “assumed position”, because it isn’t as if the viewer is really placed at d along the Z axis. If that were the case then simply adding perspective would make everything smaller. e.g.
> 
> <style>
> .container {
>    perspective: 400;
>    width: 100px;
>    height: 100px;
> }
> .box {
>    width: 100px;
>    height: 100px;
>    background: blue;
>    transform: translateZ(0);
> }
> </style>
> 
> <div class="container">
>    <div class="box"></div>
> </div>
> 
> The above wording would imply that changing perspective to 800 would make the box smaller. It doesn’t. The box is always going to be 100x100 while it is at Z=0.
> 
> I suggest we update the wording to emphasise the bit about how movements along the Z axis have a proportional change in element size. i.e. if you translateZ(perspective/2) your element will scale by a factor of 2. Then we can explain that this is because we place the viewer at the perspective location, but always draw Z=0 at natural size. This is basically just the reverse order of what we have now.
> 
> The other benefit of this is that it explains why infinite perspective means everything looks flat: there is no amount you could move along the Z axis in order to make a proportional change in size. However, the confusing thing is that we say "perspective: none" to mean infinite :)

Not sure if it is confusing that 'perspective: none' means infinite. None is not a number but finite is. After all, you do not have a perspective, therefore 'none' perspective :) We can clarify this better for the property value definition though. I'll do it for 'none'.

If there are no objections to your suggested wording, could you please add it to the specification?

Greetings,
Dirk

> 
> Dean
> 
>

Received on Thursday, 10 October 2013 05:26:32 UTC