Drawing In a Linear Framebuffer: Difference between revisions

 

Pitch and pixel width are usually announced by VESA mode info. Once you know them, you can calculate the place where you plot your pixel as:

 

=== Color ===

 

So a full "putpixel" function for VGA would be

/* example for 320x200 VGA */

void putpixel(int pos_x, int pos_y, unsigned char VGA_COLOR)

*location = VGA_COLOR;

}

 

Finally, in VESA and GOP modes, you usually have truecolor or hicolor, and in both of them, you have to give independent red, green and blue values for each pixel. modeinfo will (again) instruct you of how the RGB components are organized in the pixel bits. E.g. you will have <tt>0xRRRRRGGGGGBBBBB</tt> for 15-bits mode, meaning that #ff0000 red is there <tt>0x7800</tt>, and #808080 grey is <tt>0x4210</tt> (pickup pencil, draw the bits and see by yourself)

static void putpixel(unsigned char* screen, int x,int y, int color) {

unsigned where = x*pixelwidth + y*pitch;

screen[where + 2] = (color >> 16) & 255; // RED

}

 

=== Optimizations ===

 

It can be tempting from here to write other drawing functions from calls to putpixel... Don't. Drawing a filled rectangle means you access successive pixels and then advance by "pitch - rect_width" to fill the next line. If you do a "for(y=100;y<200;y++) for(x=100;x<200;x++) putpixel (screen,x,y,RED);" loop, you'll recompute 'where' about 10,000 times, as well as make an excessive amount of function calls. Even if the compiler has optimized y*pitch into adds and shifts instead of multiplication, it's silly to waste CPU time while you could do

static void fillrect(unsigned char *vram, unsigned char r, unsigned char g, unsigned char b, unsigned char w, unsigned char h) {

unsigned char *where = vram;

}

}

That should be enough to get you started coding (or googling for) a decent video library.

 

 

Once in graphic mode, you no longer have the BIOS or the hardware to draw fonts for you. The basic idea is to have font data for each character and use it to plot (or not to plot) pixels. There are plenty of ways to store those fonts depending on whether they have multiple colors or not, alpha channel or not etc. What you will basically have, however, is:

// holding what you need for every character of the set

font_char* font_data[CHARS];

}

}

 

== Drawing Icons ==

 

For a GUI, you'll probably want to display icons. It's as simple as

void draw_icon(x, y, w, h, pixels) {

for (l = j = 0; l < h; l++) {

}

}

The difficulty is, icons aren't stored as width, height and pixels array on disk. First you have to [[Loading Icons|decode an image file]] to get that information.