簡單取得JPEG長寬

目前用WxWidgets寫一個線上偵測抓圖的軟體，依規則尋網頁中的圖檔，下載，抓取長寬的設定，判斷是否繼續下載。

在Googole CodeSearch搜尋了很久，都是使用LIB或是讀檔的方式比較麻煩，我只是要取得圖檔長寬而已啊.. 在努力搜尋之後，皇天不負苦心人還是讓我找到一篇，並修改成一個函式與大家分享，以下是我改寫的程式：

bool GetJpgWidthHeight(wxChar* buf,int size,int&Width,int& Height)
{
    bool ret=false;
    #define M_SOF0  0xC0    /* Start Of Frame N */
    #define M_SOF1  0xC1    /* N indicates which compression process */
    #define M_SOF2  0xC2    /* Only SOF0-SOF2 are now in common use */
    #define M_SOF3  0xC3
    #define M_SOF5  0xC5    /* NB: codes C4 and CC are NOT SOF markers */
    #define M_SOF6  0xC6
    #define M_SOF7  0xC7
    #define M_SOF9  0xC9
    #define M_SOF10 0xCA
    #define M_SOF11 0xCB
    #define M_SOF13 0xCD
    #define M_SOF14 0xCE
    #define M_SOF15 0xCF
    #define M_SOI   0xD8  /* Start Of Image (beginning of datastream) */
    #define M_EOI   0xD9    /* End Of Image (end of datastream) */
    #define M_SOS   0xDA    /* Start Of Scan (begins compressed data) */
    #define M_APP0  0xE0    /* Application-specific marker, type N */
    #define M_APP12 0xEC    /* (we don't bother to list all 16 APPn's) */
    #define M_COM   0xFE    /* COMment */

    int marker;
    int Index=0;

    int c1, c2;
    c1 = (BYTE)buf[Index++];
    c2 = (BYTE)buf[Index++];
    if (c1 != 0xFF || c2 != M_SOI)
        return false;
    else
    {
        marker = c2;
    }

    unsigned int SkipLen=0;

    for (;;)
    {
        marker=(BYTE)buf[Index++];

        if(Index>=size)
            return false;

        while (marker != 0xFF)
        {
            marker=(BYTE)buf[Index++];
            if(Index>=size)
                return false;
        }

        do
        {
            marker=(BYTE)buf[Index++];
            if(Index>=size)
                return false;

        } while (marker == 0xFF);

        switch (marker)
        {
            case M_SOF0:    /* Baseline */
            case M_SOF1:    /* Extended sequential, Huffman */
            case M_SOF2:    /* Progressive, Huffman */
            case M_SOF3:    /* Lossless, Huffman */
            case M_SOF5:    /* Differential sequential, Huffman */
            case M_SOF6:    /* Differential progressive, Huffman */
            case M_SOF7:    /* Differential lossless, Huffman */
            case M_SOF9:    /* Extended sequential, arithmetic */
            case M_SOF10:   /* Progressive, arithmetic */
            case M_SOF11:   /* Lossless, arithmetic */
            case M_SOF13:   /* Differential sequential, arithmetic */
            case M_SOF14:   /* Differential progressive, arithmetic */
            case M_SOF15:   /* Differential lossless, arithmetic */
                Index+=3;

                if(Index>=size)
                    return false;

                c1=(BYTE)buf[Index++];
                c2=(BYTE)buf[Index++];

                Width=(int)(((unsigned int) c1) << 8) + ((unsigned int) c2);

                c1=(BYTE)buf[Index++];
                c2=(BYTE)buf[Index++];

                Height=(int)(((unsigned int) c1) << 8) + ((unsigned int) c2);

                return true;

                break;
            case M_SOS:     /* stop before hitting compressed data */
                return false;
                break;
            case M_EOI:     /* in case it's a tables-only JPEG stream */
                return false;
                break;
            default:
                c1=(BYTE)buf[Index++];
                c2=(BYTE)buf[Index++];

                SkipLen=(((unsigned int) c1) << 8) + ((unsigned int) c2);

                if (SkipLen < 2)
                    return false;

                Index+=SkipLen-2;
                if(Index>=size)
                     return false;

                break;
        }

    }
    return ret;
}

以上有些地方沒有偵測是否超過範圍，也不知道是不是通用所有的JPEG格式，所以使用以上程式發生問題本人概不負責 XD

另外傳入的buffer 要夠大 才會顯示真的長寬，有的圖檔的Width Height 位置非常的後面，並不是固定的位置

歡迎轉載，但請標明出處，謝謝。
參考網址： Google CodeSearch