Writing directly to screen pixels / per-pixel operations?

beginners
#1

Hi,

Can anyone suggest why the following code, inside the draw() function, does nothing? The program compiles fine (under Linux), it just doesn’t write anything to the screen.

  
  
  
ofImage theScreen;   
unsigned char* myPixels;  
int i;  
  
theScreen.grabScreen(0,0,1024,768);  
  
myPixels = theScreen.getPixels();  
   
i = (400*(theScreen.width*3)) + 500*3;  
   
   
  myPixels[i]   = (unsigned char)255;  
  myPixels[i+1] = (unsigned char)0;  
  myPixels[i+2] = (unsigned char)0;  
     
theScreen.draw(0,0);

As I understand it (from reading wiki examples, though none state explicity how to write tp pixels) this should write a red pixel to somewhere near the center of the screen, but it just shows a blank gray screen. Note that thescren.bpp == 3 in this example, and the gfx setup line is just ofSetupOpenGL(&window, 1024,768, OF_WINDOW);

Thanks for any help.

#2

you need

  
  
theScreen.update();

before

  
  
theScreen.draw(0,0);