Save Screen Image

Hello, when i save the screen image, the result is a file with the correct name and the correct directory on my device, but the file is empy. This is my code:

void setup() { size(1200, 700); strokeWeight(4); background(255); textSize(30); rect(0,0,1200,700); rect(10,650,180,40); rect(1000,650,180,40); fill(0); text("CLEAR",40,680); text("FATTO",1040,680); }

void draw() { if (mouseY<640){ line(mouseX, mouseY, pmouseX, pmouseY);} if ((mouseX>10)&(mouseX<200)&(mouseY>650)&(mouseY<700)){ fill(255); noStroke(); rect(2,2,1195,645);stroke(0);} if ((mouseX>1000)&(mouseX<1200)&(mouseY>650)&(mouseY<700)){ fill(255); noStroke(); rect(2,645,1195,695); delay(100); saveFrame("/sdcard/infoconterm/firma.png"); delay(200); exit(); }}

I activated WR_EXTERNAL_STORAGE and READ_EXTERNAL_STORAGE.