u-boot 顯示logo，並說明其原理part1

jpegtopnm Benson.jpg l ppmquant 31 l ppmtobmp -bpp 8 > Benson-8bit.bmp

紅色l是管線符號，因無法正常顯示所以用小寫L取代

以上這個指令必須先安裝netpbm，其作用是將jpeg檔轉成特定格式的bmp檔。將Benson-8bit.bmp更改檔名為denx.bmp並存到tools\logos，重新compile u-boot，便可完成，如上圖。接著我們來說明其draw logo原理：

在/tools/Makefile：

LOGO_H = $(OBJTREE)/include/bmp_logo.h

及

ifeq ($(LOGO_BMP),)

LOGO_BMP= logos/denx.bmp

endif

ifeq ($(VENDOR),atmel)

LOGO_BMP= logos/atmel.bmp

endif

及

$(LOGO_H)：$(obj)bmp_logo $(LOGO_BMP)

$(obj)./bmp_logo $(LOGO_BMP) >$@　＃這個$@就是bmp_logo.h

也就是說$(LOGO_H)這個目標的必要條件是bmp_logo 及denx.bmp這兩個檔案，才能產生bmp_logo.h這個主角，它就是denx.bmp的位元圖陣列。

所以在uboot主目錄中的makefile中有這麼一行：@rm -f $(obj)include/bmp_logo.h。在clean 時會將其刪除。

以上資訊若使用Source Insight根本無法搜尋得到，終就還是得到linux使用gerp。像這次我的搜尋指令如下：grep -Hwn "bmp_logo.h" -r ./ 。接著我們來了解一下bmp_logo.c：這是一支單一檔案所完成的獨立執行檔；我先把代碼列出；再來加入註解；另外也必須了解點陣圖的格式，請參考點陣圖（Bitmap）檔案格式

#include (stdio.h)
#include (stdlib.h)
#if defined(__linux__)
#include (stdint.h)
#else
#ifdef __CYGWIN__
#include "elf.h"
#else
#include (inttypes.h)
#endif
#endif

typedef struct bitmap_s { /* bitmap description */
uint16_t width;
uint16_t height;
uint8_t palette[256*3]; //調色盤資料
uint8_t *data; //圖的bit_map資料指標
} bitmap_t;
#define DEFAULT_CMAP_SIZE 16 /* size of default color map */

/* Neutralize little endians. */
uint16_t le_short(uint16_t x) //因為檔案格式是依照LITTLE ENDIAN排列，因此要找出實際數值必須作移位。
{
uint16_t val;

uint8_t *p = (uint8_t *)(&x);
val = (*p++ & 0xff) 左移 0;

val = (*p & 0xff) 左移 8;

return val;

}

void skip_bytes (FILE *fp, int n)

{

while (n-- > 0) fgetc (fp);

}

int main (int argc, char *argv[])
{
int i, x;
FILE *fp;
bitmap_t bmp;
bitmap_t *b = &bmp;
uint16_t data_offset, n_colors;

if (argc < color="#cc33cc">//判斷參數少於2便顯示fail

fprintf (stderr, "Usage: %s file\n", argv[0]);

exit (EXIT_FAILURE);

}
if ((fp = fopen (argv[1], "rb")) == NULL) {//開啟檔案，若失敗顯示fail

perror (argv[1]);

exit (EXIT_FAILURE);

}
if (fgetc (fp) != 'B' fgetc (fp) != 'M') { //判斷檔頭前2個byte是否為"BM"，否則顯示錯誤

fprintf (stderr, "%s is not a bitmap file.\n", argv[1]);

exit (EXIT_FAILURE);

}
/*
* read width and height of the image, and the number of colors used;
* ignore the rest
*/
skip_bytes (fp, 8);
fread (&data_offset, sizeof (uint16_t), 1, fp);
skip_bytes (fp, 6);
fread (&b->width, sizeof (uint16_t), 1, fp);
skip_bytes (fp, 2);
fread (&b->height, sizeof (uint16_t), 1, fp);
skip_bytes (fp, 22);
fread (&n_colors, sizeof (uint16_t), 1, fp);
skip_bytes (fp, 6);

/*
* Repair endianess. 求出實際數值，包括data_offset b->width b->height n_colors
*/
data_offset = le_short(data_offset);
b->width = le_short(b->width);
b->height = le_short(b->height);
n_colors = le_short(n_colors);

/* assume we are working with an 8-bit file */
if ((n_colors == 0) or (n_colors > 256 - DEFAULT_CMAP_SIZE)) {
/* reserve DEFAULT_CMAP_SIZE color map entries for default map */
n_colors = 256 - DEFAULT_CMAP_SIZE;
}
//使用printf便可印到bmp_logo.h那是因為$(obj)./bmp_logo $(LOGO_BMP) >$@ 。 這個">"轉向指令。

以下的代碼就不用再加註解，只要比對一下產生出來的bmp_logo.h，就知道了
printf ("/*\n"
" * Automatically generated by \"tools/bmp_logo\"\n"
" *\n"
" * DO NOT EDIT\n"
" *\n"
" */\n\n\n"
"#ifndef __BMP_LOGO_H__\n"
"#define __BMP_LOGO_H__\n\n"
"#define BMP_LOGO_WIDTH\t\t%d\n"
"#define BMP_LOGO_HEIGHT\t\t%d\n"
"#define BMP_LOGO_COLORS\t\t%d\n"
"#define BMP_LOGO_OFFSET\t\t%d\n"
"\n",
b->width, b->height, n_colors,
DEFAULT_CMAP_SIZE);

/* allocate memory */
if ((b->data = (uint8_t *)malloc(b->width * b->height)) == NULL) {
fclose (fp);
printf ("Error allocating memory for file %s.\n", argv[1]);
exit (EXIT_FAILURE);
}

/* read and print the palette information */
printf ("unsigned short bmp_logo_palette[] = {\n");

for (i=0; ipalette[(int)(i*3+2)] = fgetc(fp);
b->palette[(int)(i*3+1)] = fgetc(fp);
b->palette[(int)(i*3+0)] = fgetc(fp);
x=fgetc(fp);

printf ("%s0x0%X%X%X,%s",
((i%8) == 0) ? "\t" : " ",
(b->palette[(int)(i*3+0)] >> 4) & 0x0F,
(b->palette[(int)(i*3+1)] >> 4) & 0x0F,
(b->palette[(int)(i*3+2)] >> 4) & 0x0F,
((i%8) == 7) ? "\n" : ""
);
}

/* seek to offset indicated by file header */
fseek(fp, (long)data_offset, SEEK_SET);

/* read the bitmap; leave room for default color map */
printf ("\n");
printf ("};\n");
printf ("\n");
printf ("unsigned char bmp_logo_bitmap[] = {\n");
for (i=(b->height-1)*b->width; i>=0; i-=b->width) {
for (x = 0; x <>width; x++) {
b->data[(uint16_t) i + x] = (uint8_t) fgetc (fp) + DEFAULT_CMAP_SIZE;
}
}
fclose (fp);

for (i=0; i<(b->height*b->width); ++i) {
if ((i%8) == 0)
putchar ('\t');
printf ("0x%02X,%c", b->data[i], ((i%8) == 7) ? '\n' : ' ');
}
printf ("\n"
"};\n\n"
"#endif /* __BMP_LOGO_H__ */\n"
);

return (0);
}