How to restrict drawing area for Sprites/Nodes?

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How to restrict drawing area for Sprites/Nodes?

(9 posts) (3 voices)

Started 1 year ago by impguard
Latest reply from impguard

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impguard
Member

Looked around the forum, but really couldn't find a definitive answer. I'm trying to overlay a sprite on top of another, using blend modes to create sort of a reflection effect/glow effect on the sprite. Then, as the sprite changes position, I can move the overlay/blended/reflection sprite to simulate dynamic lighting.

However, the one main obstacle atm is that sprites cannot be overlayed or restricted to a certain area to draw. My question is this: Is there a way to restrict the drawing area for a sprite? So if I add a layer to a scene, can I restrict any sprites/labels etc. to a certain invisible box? This way, anything being drawn outside this box will not appear on screen even if its coordinates are actually on the iphone/ipad screen?

Posted 1 year ago #
mobilebros
Moderator

Take a look at glScissor and this post about it, it might be what you want:

http://www.cocos2d-iphone.org/forum/topic/3993#post-23924

Posted 1 year ago #
impguard
Member

Hmm, I'll be testing this out. But does this mean that there's no higher level code that can accomplish this task at the moment? (So I need to dive into a bit of OpenGL coding).

Posted 1 year ago #
impguard
Member

Another small question. After fiddling around with this, I confused myself with how Cocos2D draws itself. What methods are being called when to draw every sprite, every layer etc.? I used to think this was called with a the visit method, but creating a custom CCSprite subclass and overriding the visit, I found out that the visit method was never being called to draw the sprite. So how does the sprite get drawn?

Posted 1 year ago #
mobilebros
Moderator

To first question: there is no higher level alternative that I know of built in to cocos.

The visit method's purpose is to do its transformation, call it's children's visit method, and draw itself (aka it calls [self draw];)

The draw method will do whatever needs to happen to draw to the screen.

Posted 1 year ago #
impguard
Member

Hmm, thanks for the quick reply.

I have some follow up questions then. I would assume then that all the drawing occurs in the draw method? So to add a scissor, I'll have to override the draw method in my CCSprite class?

Also, I made a test project where I created a custom Box CCSprite class and overrode the visit method. On the HelloWorldScene, I removed everything inside init and created my Box CCSprite there/added it to the layer.

However, if I place a breakpoint in the custom visit method I made, the programming never reaches that point, but the box sprite does appear on screen?

Posted 1 year ago #
mobilebros
Moderator
You're right about the drawing being in the draw method, the only reason putting scissor in the visit method instead would be so that all the children of that node/sprite also get clipped.

Your draw method could look like this:
```
-(void) draw
{
        glEnable(GL_SCISSOR_TEST);
	glScissor(x, y, width, height);
	[super draw];
	glDisable(GL_SCISSOR_TEST);
}
```
Just keep in mind that the scissor coordinates "weird", they're always portrait-wise I believe.

Not sure about your test project, you should have hit the visit method for sure, I suppose make sure it looks like this:
```
-(void) visit
{
}
```
Posted 1 year ago #
bradparks
Member

this post talks about the weird coordinates of glScissor in more detail, too (it's actually just another post further on down in the thread above mentioned by @mobilebros):

http://www.cocos2d-iphone.org/forum/topic/3993#post-35014

Posted 1 year ago #
impguard
Member

Thank you. Mobilebros, you were right, something weird was happening with my computer, but visit is being called now.

Yes, that link does help, though doing the math myself is beneficial :D GLScissor behaves as if the screen was always homescreen button down , as mobile bros said. Thanks for all your help, this cutting works now :D

Posted 1 year ago #

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