RGBA – PHP & More

I remember that little prank from the days I was a student. You work on an X terminal, and out of the blue, all the display contents gradually disappear’ Pixel after pixel turns black. But don’t worry – you’ll regain control over your display shortly. shortly.
Everyone can access other X terminal display, and mess with it.

How Does It Work?

This program is a simple one using the GDK library, Gnome’s window management package. Including ‘gdk.h’ will also include:

Cairo – used for Graphics.
GLib used for data structures and main event loop handling.

The Program’s Flow

The main function of the program performs the following steps:
1. Initialize GDK.
2. Create a window whose dimensions are the same as those of the root window.
3. Make the window’s background transparent.
4. Make the window a full-screen window.
5. Add an event handler. to handle Expose events.
The event handler will perform the following steps:
1. Create a list of columns and lengths (number of blackened pixels).
2. Create the Graphics Context for the window.
3. Blacken pixels until all pixels are black.
4. Quit the main loop.

Includes And Structures:

#include <stdio.h>
#include <stdlib.h>  
#include <gdk/gdk.h>

GMainLoop *mainloop;
GList *list;

typedef struct col_and_length_t{
  short col;  // Column number
  short len;  // Number of blackened pixels.
} col_and_length;`

The main function:

int main(int argc, char *argv[]){
  gdk_init(NULL, NULL);
  GdkDisplay *disp=gdk_display_get_default();
  GdkScreen *scr = gdk_display_get_default_screen (disp);
  GdkWindow *root = gdk_screen_get_root_window(scr);
  int rootWidth = gdk_window_get_width(root);
  int rootHeight = gdk_window_get_height(root);
  GdkWindowAttr attr;
  attr.width=rootWidth;
  attr.height=rootHeight;
  attr.x=0;
  attr.y=0;
  attr.window_type = GDK_WINDOW_TOPLEVEL;
  attr.wclass=GDK_INPUT_OUTPUT;

  GdkWindow *newWin=gdk_window_new(root,&attr, GDK_WA_X | GDK_WA_Y);
  gdk_event_handler_set (eventFunc, newWin, NULL);
  GdkRGBA color;
  color.alpha=0;

  gdk_window_set_background_rgba(newWin, &color);
  gdk_window_fullscreen(newWin);
  gdk_event_handler_set (eventFunc, newWin, NULL);
  gdk_window_show(newWin);
  mainloop = g_main_new (TRUE);
  g_main_loop_run (mainloop);
  gdk_display_close(disp);

return 0;
}

The event handler

void start_meltdown(GdkWindow *newWin, int height){
  cairo_t *gc=gdk_cairo_create(newWin);
  cairo_set_line_width(gc,2);
  cairo_set_source_rgb (gc, 0, 0, 0);
  int cell_no,size;
  GList *link;
  col_and_length *link_data;
  size=g_list_length(list);

  while(size>0){
    cell_no=random() % size;
    link = g_list_nth(list,cell_no);
    link_data = (col_and_length *)link->data;
    cairo_move_to(gc, link_data->col, link_data->len);
    cairo_rel_line_to(gc, 0, 1);
    cairo_stroke(gc);
    link_data->len++;
    if (link_data->len >= height){
      list=g_list_remove_link(list, link);
      --size;
    }
  }
  g_main_loop_quit(mainloop);
}

void eventFunc(GdkEvent *evt, gpointer data){
  GdkWindow *newWin = (GdkWindow *)data;
  if (gdk_event_get_event_type(evt) == GDK_EXPOSE && gdk_event_get_window (evt) == newWin){
    int width=gdk_window_get_width(newWin);
    int height=gdk_window_get_height(newWin);
    int i;
    for (i=0; i<width;i++){
      col_and_length *cell=(col_and_length *)calloc(sizeof(col_and_length), 1);
      cell->col=i;
      cell->len=0;
      list = g_list_append(list, cell);
    }
    start_meltdown(newWin,height);
  }

}

Compiling

In linux, compiling a program is easy thanks to the pkg-config command.
Run the following from the command line:

gcc meltdown.c `pkg-config --cflags --libs gdk-3.0` -o meltdown

Now, to run the program type:

./meltdown

Written with StackEdit.

void createCursor(){ double arcEnd=2*M_PI; GdkPixbuf *pixbuf; int stride=cairo_format_stride_for_width(CAIRO_FORMAT_ARGB32, 10); // Will be used later, when we'll create the pixbuf. cairo_surface_t *surf=cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 10, 10); // A canvas of width 10 and height 10. cairo_t *cairo_ctx = cairo_create(surf); cairo_set_source_rgba(cairo_ctx, 0., 1., 1., 1.); // Double precision values for Red, Green, Blue and Alpha channels between 0 and 1. cairo_arc(cairo_ctx,5,5,4,0,arcEnd); // Draw a circle or an arc of a circle whose center is at (5,5) and whose radius is 4 // The arc begins at angle 0 and ends in angle 2Π cairo_fill(cairo_ctx); // Fill the circle with cyan, though we've intended to create a yellow one. // Create an example image for the blog cairo_surface_write_to_png(surf, "/tmp/fortheblog.png"); guchar *data = cairo_image_surface_get_data(surf); // The data is in BGR, reverse order of RGB !!! pixbuf = gdk_pixbuf_new_from_data(data, GDK_COLORSPACE_RGB, true, 8, 10, 10, stride, NULL, NULL); // Stride is the difference in bytes between two // consecutive rows. cursor=gdk_cursor_new_from_pixbuf(gdk_display_get_default(), pixbuf, 5, 5); }

The “gtk_pixbuf_new_from_data” Function.

This function uses the data returned from the function “cairo_image_surface_get_data”. The data is a pointer to a character. The function receiving the data doesn’t know how many characters are referred to, and how to split it into rows and columns. Following is its prototype:

GdkPixbuf *gdk_pixbuf_new_from_data (const guchar *data, GdkColorspace colorspace, gboolean has_alpha, int bits_per_sample, int width, int height, int rowstride, GdkPixbufDestroyNotify destroy_fn, gpointer destroy_fn_data);

Tag: RGBA

Meltdown – The Computer Lab Prank