Processing 1.0 - Processing Discourse - PGraphics...am I doing something wrong here?

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Index › Programming Questions & Help › Syntax Questions › PGraphics...am I doing something wrong here

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PGraphics...am I doing something wrong here? (Read 544 times)

milesmeow

PGraphics...am I doing something wrong here?
Oct 22^nd, 2006, 12:17am

Using 0118 Beta.
I'm trying out the PGraphics class for the first time. I'm trying something very simple here. I'm drawing to PGraphics every frame and then drawing PGraphics to the sketch window using image().

Quote:

PGraphics pg;
int x=0;

void setup() {
size(200, 200);

pg = createGraphics(width, height, P2D);
frameRate(1);
}

void draw() {
image(pg, 0, 0);

pg.beginDraw();
pg.stroke(255);
pg.rect(x++, 25, 10, 10);
pg.endDraw();

}

I get the following error, and the line with the 'rect' command is highlighted by Processing:

java.lang.NullPointerException

at processing.core.PGraphics2D.thin_pointAtIndex(PGraphics2D.java:1173)

at processing.core.PGraphics2D.thin_flat_line(PGraphics2D.java:1284)

at processing.core.PGraphics2D.rectImpl(PGraphics2D.java:716)

at processing.core.PGraphics.rect(PGraphics.java:1266)

at Temporary_7691_9576.draw(Temporary_7691_9576.java:16)

at processing.core.PApplet.handleDisplay(PApplet.java:1311)

at processing.core.PGraphics.requestDisplay(PGraphics.java:568)

at processing.core.PApplet.run(PApplet.java:1406)

at java.lang.Thread.run(Thread.java:552)

java.lang.NullPointerException

at processing.core.PGraphics2D.thin_pointAtIndex(PGraphics2D.java:1173)

at processing.core.PGraphics2D.thin_flat_line(PGraphics2D.java:1284)

at processing.core.PGraphics2D.rectImpl(PGraphics2D.java:716)

at processing.core.PGraphics.rect(PGraphics.java:1266)

at Temporary_7691_9576.draw(Temporary_7691_9576.java:16)

at processing.core.PApplet.handleDisplay(PApplet.java:1311)

at processing.core.PGraphics.requestDisplay(PGraphics.java:568)

at processing.core.PApplet.run(PApplet.java:1406)

at java.lang.Thread.run(Thread.java:552)

Ira

Re: PGraphics...am I doing something wrong here?
Reply #1 - Oct 22^nd, 2006, 6:55am

I think there are still some bugs to work out with createGraphics()/2D renderer. However, P3D will get the job done. You also needed a background call.

Code:


PGraphics pg;
int x=0;
 
void setup() {
  size(200, 200);
  pg = createGraphics(width, height, P3D);
  frameRate(1);
}
 
void draw() {
  pg.background(0);
  pg.beginDraw();
  pg.stroke(255);
  pg.noFill();
  pg.rect(x++, 90, 10, 10);
  pg.endDraw();
  image(pg, 0, 0);
 }

-ira

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milesmeow

Re: PGraphics...am I doing something wrong here?
Reply #2 - Oct 22^nd, 2006, 8:04am

Cool Thanks! How come I need to use background()? I tried it without using background() and it seems like nothing draws on the PGraphics object. What if I wanted to draw trails?
ex:
pg.line(pmouseX, pmouseY, mouseX, mouseY);

(the workaround is that I guess I can save all the points and render all the lines for the trail on each frame)

Ira

Re: PGraphics...am I doing something wrong here?
Reply #3 - Oct 22^nd, 2006, 7:56pm

I believe (alright I'm guessing–no time to go source diving today) the reason you need the background() call is to make the (PGraphics) graphics context opaque. Once you call
Code:

 
pg = createGraphics(width, height, P3D);

the PGraphics object has dimension, but it may still be transparent, prior to calling background(). Maybe Ben or someone else can confirm/correct this.

Regarding your question about trails, this should get the job done:
Code:


PGraphics pg;
float x=0;

void setup() {
  size(200, 200);
  pg = createGraphics(width, height, P3D);
  // Any color arg in background() 
  // will actually get the job done here
  pg.background(0);
}

void draw() {
  background(0);
  pg.beginDraw();
  pg.stroke(255);
  if (mousePressed){
    pg.line(pmouseX, pmouseY, mouseX, mouseY); 
  }
  pg.endDraw();
  image(pg, 0, 0);
}

-ira

milesmeow YaBB Newbies Offline Posts: 3	Re: PGraphics...am I doing something wrong here? Reply #4 - Oct 23^rd, 2006, 1:03am Thanks Ira, that totally worked! I was missing the magical pg.background() call.

fry

Re: PGraphics...am I doing something wrong here?
Reply #5 - Oct 23^rd, 2006, 3:45pm

the background() call should go *after* beginDraw().

the black background is because createGraphics with P3D is defaulting to an all black background (this should actually be transparent, i'll fix that in a future release).

we also need to fix that example in the reference because P2D doesn't exist yet, but we already have a bug filed for that. sorry for the confusion.

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