minigui:增加__mg_save_jpg保存JPEG格式图像

叁歲伎倆 2022-05-14 07:15 192阅读 0赞

`minigui` 库中有`SaveBitmapToFile`和`SaveMyBitmapToFile`可以将一个(BITMAP或MYBITMAP)图像对象存储为文件。然而图像格式只支持bmp（`libminigui-3.2.0`）。  
但嵌入式平台上存储空间有限，我确实需要存储为JPEG啊。  
怎么办？自己撸一个。  
JPEG压缩的流程都是固定的，网上有很多代码，JPEG自己也有例子，这不难，复杂的地方是将一个`MYBITMAP`对象的图像数据转成JPEG压缩所需要的RGB color space(色彩空间)。`minigui`支持RGB,BRG,RGBA,RGB565,index 16 index 256,几种color space，RGB最简单，不需要转，其他几种color space都需要分别进行转换。

以下为JPEG压缩流程主函数代码:

int __fl_save_jpg (MG_RWops* fp, MYBITMAP* mybmp, RGB* pal)
    {
        j_compress_ptr cinfo;
        struct my_error_mgr *jerr;
        JSAMPROW linebuffer = NULL;
        JSAMPROW row_pointer[1];
        MYBITMAP_get_pixel_row get_row;
        int retcode = ERR_BMP_CANT_SAVE;
    
        /* Step 1: 创建 JPEG 压缩对象 */
        cinfo = calloc (1, sizeof(struct jpeg_compress_struct));
    
        if(NULL == cinfo)
        {
            fprintf(stderr, "__fl_save_jpg allocation error!\n");
            return ERR_BMP_MEM;
        }
        jpeg_create_compress(cinfo);
    
        /* Step 2: 创建错误处理对象  */
        jerr = cinfo->mem->alloc_small((j_common_ptr) cinfo, JPOOL_IMAGE,sizeof(struct my_error_mgr));
    
        if(NULL == jerr)
        {
            retcode = ERR_BMP_MEM;
            goto do_finally;
        }
        memset(jerr,0,sizeof(struct my_error_mgr));
    
        /* We set up the normal JPEG error routines first. */
        cinfo->err = jpeg_std_error (&jerr->pub);
        jerr->pub.error_exit = my_error_exit;
    
        /* Establish the setjmp return context for my_error_exit to use. */
        if (setjmp (jerr->setjmp_buffer)) {
            /* 出错退出 */
            fprintf(stderr, "__fl_save_jpg error!\n");
            goto do_finally;
        }
        /* 只支持 RWAREA_TYPE_STDIO 类型的MG_RWops对象，否则报错,
         *  MEM 类型的对象不能动态管理内存，不安全，所以不支持 */
        if(RWAREA_TYPE_STDIO != fp->type )
        {
            fprintf(stderr, "unsupported type of MG_RWops,only support RWAREA_TYPE_STDIO so far\n");
            longjmp (jerr->setjmp_buffer, 1);
        }
        /* Step 3: 将MG_RWops对象的FILE设置为压缩数据输出对象 */
        jpeg_stdio_dest(cinfo, fp->hidden.stdio.fp);
    
        /* Step 4: 初始化JPEG 压缩对象 */
        /* for JPEG compression, supported color space : JCS_GRAYSCALE,JCS_RGB,JCS_YCbCr,JCS_CMYK,JCS_YCCK
         * in this case,MYBITMAP is base on RGB ,
         * we can select JCS_RGB only,so we must convert all color space (eg.RGBA,BGR,RGB565,...) to RGB
         * */
        cinfo->in_color_space = JCS_RGB;
        cinfo->image_width = mybmp->w;
        cinfo->image_height = mybmp->h;
        cinfo->input_components = 3;
        /* set jpeg compression parameters to default */
        jpeg_set_defaults(cinfo);
    
    #if _MGIMAGE_JPG_SAVE_QUALITY > 0 && _MGIMAGE_JPG_SAVE_QUALITY <= 100
        /* 如果定义了_MGIMAGE_JPG_SAVE_QUALITY,就用它来指定压缩质量，否则使用默认值 75
         * see also: libjpeg/jcparam.c jpeg_set_defaults function.
         * */
        jpeg_set_quality(cinfo, _MGIMAGE_JPG_SAVE_QUALITY, TRUE);
    #endif
    
        int mybmp_type = mybmp->flags & MYBMP_TYPE_MASK;
        /* 根据色彩空间类型确定 色彩空间转换函数(本文后面有提供) */
        switch(mybmp->depth)
        {
        case 4:
            get_row = MYBITMAP_get_pixel_row_pal16;
            break;
        case 8:
            get_row = MYBITMAP_get_pixel_row_pal256;
            break;
        case 16:
            get_row = MYBITMAP_get_pixel_row_RGB565;
            break;
        case 24:
            if(MYBMP_TYPE_RGB == mybmp_type)
                get_row = MYBITMAP_get_pixel_row_RGB;
            else
                get_row = MYBITMAP_get_pixel_row_BGR;
            break;
        case 32:
            get_row = MYBITMAP_get_pixel_row_RGBA;
            break;
        default:
            fprintf(stderr, "invalid MYBITMAP.depth = %d\n",mybmp->depth);
            longjmp (jerr->setjmp_buffer, 1);
            break;
    
        }
        if(24 == mybmp->depth && MYBMP_TYPE_RGB == mybmp_type)
        {
            /*
             * do nothing while RGB type,
             * the MYBITMAP_get_pixel_row function will return address in MYBITMAP.bits data directly,
             * without using line buffer
             * */
        }
        else
        {
            /* Allocate one-row buffer for color space conversion(4 byte alignment)  */
            linebuffer = (JSAMPROW)cinfo->mem->alloc_large
                ((j_common_ptr) cinfo, JPOOL_IMAGE,
                (((cinfo->image_width + 3) & ~3) * cinfo->input_components));
            if(NULL == linebuffer)
            {
                retcode = ERR_BMP_MEM;
                fprintf(stderr, "libjpeg allocation error!\n");
                longjmp (jerr->setjmp_buffer, 1);
            }
        }
        /* Step 5: 逐行执行压缩 */
        jpeg_start_compress(cinfo, TRUE);
        while (cinfo->next_scanline < cinfo->image_height) {
            /* 调用色彩空间转函数将一行数据转为RGB */
            row_pointer[0] = get_row(cinfo->next_scanline, mybmp, pal, linebuffer);
            jpeg_write_scanlines(cinfo, row_pointer, 1);
        }
    
        jpeg_finish_compress(cinfo);
        retcode = ERR_BMP_OK;
    do_finally:
        /* clean up the JPEG object, free the objects and return. */
        jpeg_destroy_compress (cinfo);
        free (cinfo);
        return retcode;
    }

色彩空间转代码：

//  索引色(16色) 转换为RGB
    static BYTE* MYBITMAP_get_pixel_row_pal16(unsigned int next_scanline,
                           MYBITMAP* mybmp,RGB* pal,BYTE* linebuffer)
    {
    
        BYTE* bits = mybmp->bits + mybmp->pitch * next_scanline;
        RGB rgb0,rgb1;
        for (int i = 0, j = 0, end_i = (mybmp->w + 1) >> 1; i < end_i; ++i) {
    
            rgb0 = pal[ (bits[ i ] & 0XF0) >> 4 ];
            rgb1 = pal[  bits[ i ] & 0X0F ];
    
            linebuffer[ j ++ ] = rgb0.r;
            linebuffer[ j ++ ] = rgb0.g;
            linebuffer[ j ++ ] = rgb0.b;
            linebuffer[ j ++ ] = rgb1.r;
            linebuffer[ j ++ ] = rgb1.g;
            linebuffer[ j ++ ] = rgb1.b;
        }
        return linebuffer;
    
    }
    // 索引色(256色) 转换为RGB
    static BYTE* MYBITMAP_get_pixel_row_pal256(unsigned int next_scanline,
                           MYBITMAP* mybmp,RGB* pal,BYTE* linebuffer)
    {
    
        BYTE* bits = mybmp->bits + mybmp->pitch * next_scanline;
        RGB rgb;
        for (int i = 0, j = 0; i < mybmp->w; i++) {
    
            rgb = pal [bits[i] ];
    
            linebuffer[ j++ ] = rgb.r;
            linebuffer[ j++ ] = rgb.g;
            linebuffer[ j++ ] = rgb.b;
    
        }
        return linebuffer;
    
    }
    
    #define RGB_FROM_RGB565(pixel, r, g, b)                         \
    {                                                               \
        r = (((pixel&0xF800)>>11)<<3);                              \
        g = (((pixel&0x07E0)>>5)<<2);                               \
        b = ((pixel&0x001F)<<3);                                    \
    }
    
    // RGB565 转换为 RGB
    static BYTE* MYBITMAP_get_pixel_row_RGB565(unsigned int next_scanline,
                           MYBITMAP* mybmp,RGB* pal,BYTE* linebuffer)
    {
    
        Uint16* bits = (Uint16*)(mybmp->bits + mybmp->pitch * next_scanline);
    
        for (int i = 0, j = 0; i < mybmp->w; i++) {
    
            RGB_FROM_RGB565(bits[i], linebuffer[ j ++ ], linebuffer[ j ++ ], linebuffer[ j ++ ])
    
        }
        return linebuffer;
    
    }
    //  RGB to RGB,直接返回MYBITMAP的图像数据地址
    static BYTE* MYBITMAP_get_pixel_row_RGB(unsigned int next_scanline,
            MYBITMAP* mybmp,RGB* pal,BYTE* linebuffer)
    {
        return (JSAMPROW)(mybmp->bits + mybmp->pitch * next_scanline);
    }
    
    // BGR to RGB
    static BYTE* MYBITMAP_get_pixel_row_BGR(unsigned int next_scanline,
            MYBITMAP* mybmp,RGB* pal,BYTE* linebuffer)
    {
        BYTE* bits = mybmp->bits + mybmp->pitch * next_scanline;
    
        for (int i = 0, end_i = mybmp->w * 3; i < end_i; i += 3) {
            linebuffer[ i     ] = bits[ i + 2 ];
            linebuffer[ i + 1 ] = bits[ i + 1 ];
            linebuffer[ i + 2 ] = bits[ i     ];
        }
        return linebuffer;
    }
    
    #ifndef _MGIMAGE_JPG_RGBA_BGCOLOR
    /* background color for RGBA color space conversion,for example: 0xFF0000 is red */
    #define _MGIMAGE_JPG_RGBA_BGCOLOR 0xFFFFFF
    #endif
    
    // RGBA to RGB，转时会将alpha通道值与R,G,B合并计算出新的RGB值
    static BYTE* MYBITMAP_get_pixel_row_RGBA(unsigned int next_scanline,
            MYBITMAP* mybmp,RGB* pal,BYTE* linebuffer)
    {
        RGB* bits = (RGB*)(mybmp->bits + mybmp->pitch * next_scanline);
        RGB pixel;
        RGB bgcolor = {
                (_MGIMAGE_JPG_RGBA_BGCOLOR >> 16) & 0xFF,/* red */
                (_MGIMAGE_JPG_RGBA_BGCOLOR >>  8) & 0xFF,/* green */
                 _MGIMAGE_JPG_RGBA_BGCOLOR        & 0xFF,/* blue */
                0x00                                     /* alpha,no used */
        };
        for (int i = 0,j = 0; i < mybmp->w; i++, j += 3) {
            pixel = bits[i];
            /* alpha composite,
             * C = Cx * ALPHAx + (1 - ALPHAx) * Cbg
             * see also : https://en.wikipedia.org/wiki/Alpha_compositing#Analytical_derivation_of_the_over_operator */
            linebuffer[ j     ]  = (JSAMPLE)((Uint32)pixel.r * pixel.a >> 8); /* red */
            linebuffer[ j + 1 ]  = (JSAMPLE)((Uint32)pixel.g * pixel.a >> 8); /* green */
            linebuffer[ j + 2 ]  = (JSAMPLE)((Uint32)pixel.b * pixel.a >> 8); /* blue */
    
            linebuffer[ j     ] += (JSAMPLE)((Uint32)bgcolor.r * (255 - pixel.a) >> 8); /* red   + background color*/
            linebuffer[ j + 1 ] += (JSAMPLE)((Uint32)bgcolor.g * (255 - pixel.a) >> 8); /* green + background color */
            linebuffer[ j + 2 ] += (JSAMPLE)((Uint32)bgcolor.b * (255 - pixel.a) >> 8); /* blue  + background color */
        }
        return linebuffer;
    }

好了那个童鞋，不要抄笔记了，完整的代码我已经pull request到`MiniGUI` 官方github仓库([https://github.com/VincentWei/minigui][https_github.com_VincentWei_minigui])，MiniGUI作者已经将pull request合并到主版本。你可以直接下载三个更新文件：  
[src/mybmp/jpeg.c][src_mybmp_jpeg.c]，  
[src/include/readbmp.h][src_include_readbmp.h],  
[src/mybmp/mybmp.c][src_mybmp_mybmp.c]  
覆盖自己的`MiniGUI`源码重新编译就可以了。

[https_github.com_VincentWei_minigui]: https://github.com/VincentWei/minigui
[src_mybmp_jpeg.c]: https://github.com/VincentWei/minigui/blob/master/src/mybmp/jpeg.c
[src_include_readbmp.h]: https://github.com/VincentWei/minigui/blob/master/src/include/readbmp.h
[src_mybmp_mybmp.c]: https://github.com/VincentWei/minigui/blob/master/src/mybmp/mybmp.c