Simple Modes

Some have expressed the interest for simple modes for the Bitbox. An recent commit is providing such mode , on top of the standard kernel.

Introduction

The so-called simple modes are called like this because they provide a frame buffer interface, so the application programmer will only have to write bits and bytes to the video memory - much like it's done on PC. So the interface is a vram buch of memory and a color palette (because the tradeoff is a reduced palette or resolution)

There are two examples in the bitbox SDK (one of them below), but generally you'll need to define a simple makefile variable VGA_SIMPLE_MODE=X with the given mode and include the bitbox.mk standard makefile.

(Yes it's currently defined at compile time - if your program needs to switch between two modes at runtime, it might be time to switch to a standard kernel / engine - it's not difficult !)

The Modes

The modes are the following:

mode	type	colors	resolution	VRAM	kernel mode
0	text	2	80x25	2k+2k	640x480
1	text	2*	132x75	10k+10k	800x600
2	graphical	2	800x600	59k	800x600
3	graphical	4	640x400	64k	640x480
4	graphical	16	400x300	64k	800x600
5	graphical	256	320x200	64k	640x480
10	text	256*	80x30	<10k	640x480
11	text	256*	120x75	<10k	800x600
12	text	256*	80x60	<10k	640x480

*text modes 0 and 1 are only B&W (well, whatever you define as palette 0 and 1). Modes 10 and 11 use a 256 entry palette of u16 fg / u16 bg attributes. Beware that using mode 11 (really stretching the harwdare) with too much color changes per line might create dark lines.

Palettes

All modes use a palette of u16. Set the color by setting a 0rrrrrgggggbbbbb 15- bit word entry in the palette array. By example, to assign a bright red color to color index 4 (zero-based), just set

extern uint16_t palette[16];
palette[4]=0b0111110000000000; //  binary literal or use RGB(x,y,z)

Default palettes

2-color palettes : 0 : black, 1:white
4-color palette: 0 : black, 1 : cyan, 2 : red, 3:light grey
16-color palette : standard CGA 16 color palette (http://en.wikipedia.org/wiki/Color_Graphics_Adapter)
256-color palette : unsurprisingly, this default palette is VGA mode 0x13 standard palette. (see http://fr.wikipedia.org/wiki/Mode_13h )

Text modes : 0,1

Text modes 0 and 1 define text modes with color attributes.

Mode 0 is 80 columns by 25 lines, Mode 1 is 132 columns by 75 lines.

Video Memory has one array of characters (vram)

Examples

Example to draw the ASCII table :

#define SCREEN_W 132
for (int i=0;i<256;i++)
    vram[(i/16)*SCREEN_W + i%16]=i;

Feel free to use memcpy / memset also (strcpy writes a zero character after the string)

char *my_str = "Bitbox rules";
memcpy(&vram[4*132+13], my_str,strlen(my_str));

Graphical modes : 2-5

Graphical mode map linearly pixels (color indexes) to bits in memory, ie to set a pixel as color 1 (defined in the palette) on coordinates 100,50 with mode 2, use

extern uint32_t vram;
// set a bit on the bit array of pixels, made of 32bit words
vram[(50*800+100)/32] |= 1<<(100%32); 

or just use the draw_pixel or draw_line functions (of course to fill/copy a bunch of pixels, you'd better use memory access)

draw_pixel(100,100,1);

Example program

The program can be found in the SDK examples.

Here is it in its entirety:

• Makefile

    NO_USB=1
    NO_SDCARD=1
    # See lib/simple.h for modes
    VGA_SIMPLE_MODE=4
    NAME = sgraph
    GAME_C_FILES = $(NAME).c
    # see this file for options
    include $(BITBOX)/lib/bitbox.mk

• test_simple_graph.c

    #include <stdlib.h> // rand
    #include "simple.h"
 
    void game_init(void) {}
 
    void game_frame(void)
    {
        if (vga_frame%180==0) clear();
        for (int i=0;i<10;i++)
            draw_line( rand()%SCREEN_W, rand()%SCREEN_H,
                rand()%SCREEN_W, rand()%SCREEN_H,
                rand()%(1<<BPP));
    }

• Result

Simple, no ?

Simple Scrolling tilemap + sprites mode

Well, I cheated there is no such mode :)

However, there is a simple example project using the engine that you can use very simply, just take the test_engine project in the SDK and tweak it !