BMP 和 JPEG

int save_screen_to_jpeg(const char* filename, int quality)
{/** 把屏幕内容保存为一个 HBITMAP DDB*/HDC hScrnDC = CreateDC(_T("DISPLAY"), NULL, NULL, NULL);HDC hMemDC = CreateCompatibleDC(hScrnDC);// 获取屏幕分辨率int xScrn = GetDeviceCaps(hScrnDC, HORZRES);int yScrn = GetDeviceCaps(hScrnDC, VERTRES);// 创建位图,并选中HBITMAP hScrnBmp = CreateCompatibleBitmap(hScrnDC, xScrn, yScrn);SelectObject(hMemDC, hScrnBmp);// 复制屏幕内容BitBlt(hMemDC, 0, 0, xScrn, yScrn, hScrnDC, 0, 0, SRCCOPY);// 现在得到了一个 HBITMAP DDB - hScrnBmp/** 通过 hScrnBmp DDB 取得 DIB 数据*/// 获取色深 JPG 只能处理 24 位色,所以不管当前系统设置的色深是多少,我们都要求 GetDIBits 函数返回 24 位的 DIB 数据,同时也不需要调色板//int colorDeepBits = GetDeviceCaps(hScrnBmp, BITSPIXEL);//if(colorDeepBits > 24) colorDeepBits = 24;int colorDeepBits = 24;// 每行像素占用的字节数,每行要对齐4字节.int imageRowSize = (xScrn * colorDeepBits + 31) / 32 * 4; // 分配 DIB 数组unsigned char* dibBuffer = new unsigned char[imageRowSize * yScrn];assert(dibBuffer);memset(dibBuffer, 0, imageRowSize * yScrn); // 清零是个好习惯    // 填充 BMP 信息头BITMAPINFO bmi = {0};bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);bmi.bmiHeader.biWidth = xScrn;bmi.bmiHeader.biHeight = yScrn * -1;  // JPG 压缩需要正序的 DIB 像素流,所以要负数.bmi.bmiHeader.biPlanes = 1;bmi.bmiHeader.biBitCount = colorDeepBits;bmi.bmiHeader.biCompression = BI_RGB;// 获取 DIB 像素数组(DIB_RGB_COLORS 表示获取 RGB 值而不是调色板索引,当然24位位图也没有调色板)int gdiRet = GetDIBits(hMemDC, hScrnBmp, 0, yScrn, dibBuffer, &bmi, DIB_RGB_COLORS);assert(gdiRet == yScrn);assert(bmi.bmiHeader.biSizeImage == imageRowSize * yScrn);// DIB 数据已经获取,所有的 GDI 对象可以释放了.DeleteDC(hScrnDC);DeleteDC(hMemDC);DeleteObject(hScrnBmp);/** 把 DIB 数据压缩为 JPG 数据,用 example.c 中的代码*/// DIB 中颜色的存放顺序是 BGR, 而 JPG 要求的顺序是 RGB, 所以要交换 R 和 B.// 由于有行对齐因素,所以逐行处理for(int row = 0; row < yScrn; ++row){unsigned char* rowData = dibBuffer + imageRowSize * row;for(int col = 0; col < xScrn * 3; col += 3){unsigned char swap = rowData[col];rowData[col] = rowData[col + 2];rowData[col + 2] = swap;}}//把位图数据压缩为 jpegstruct jpeg_compress_struct cinfo;struct jpeg_error_mgr jerr;FILE * outfile;  /* target file */JSAMPROW row_pointer[1]; /* pointer to JSAMPLE row[s] */int row_stride;  /* physical row width in image buffer */int image_width = xScrn;int image_height = yScrn;JSAMPLE* image_buffer = dibBuffer;     // DIB bufferint image_buffer_len = imageRowSize * image_height; // DIB buffer 长度if(fopen_s(&outfile, filename, "wb"))//if ((outfile = fopen_s(filename, "wb")) == NULL) {fprintf(stderr, "can't open %s\n", filename);assert(0);}else{/* Step 1: allocate and initialize JPEG compression object */cinfo.err = jpeg_std_error(&jerr);/* Now we can initialize the JPEG compression object. */jpeg_create_compress(&cinfo);/* Step 2: specify data destination (eg, a file) *//* Note: steps 2 and 3 can be done in either order. */jpeg_stdio_dest(&cinfo, outfile);/* Step 3: set parameters for compression *//* First we supply a description of the input image.* Four fields of the cinfo struct must be filled in:*/cinfo.image_width = image_width;  /* image width and height, in pixels */cinfo.image_height = image_height;cinfo.input_components = 3;  /* # of color components per pixel */ // 因为DIB数据是24位的,所以每个像素占用3个字节cinfo.in_color_space = JCS_RGB;  /* colorspace of input image *//* Now use the library's routine to set default compression parameters.* (You must set at least cinfo.in_color_space before calling this,* since the defaults depend on the source color space.)*/jpeg_set_defaults(&cinfo);/* Now you can set any non-default parameters you wish to.* Here we just illustrate the use of quality (quantization table) scaling:*/jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);/* Step 4: Start compressor *//* TRUE ensures that we will write a complete interchange-JPEG file.* Pass TRUE unless you are very sure of what you're doing.*/jpeg_start_compress(&cinfo, TRUE);/* Step 5: while (scan lines remain to be written) *//*           jpeg_write_scanlines(...); *//* Here we use the library's state variable cinfo.next_scanline as the* loop counter, so that we don't have to keep track ourselves.* To keep things simple, we pass one scanline per call; you can pass* more if you wish, though.*/row_stride = imageRowSize;while (cinfo.next_scanline < cinfo.image_height) {/* jpeg_write_scanlines expects an array of pointers to scanlines.* Here the array is only one element long, but you could pass* more than one scanline at a time if that's more convenient.*/row_pointer[0] = &image_buffer[cinfo.next_scanline * row_stride];//row_pointer[0] = &image_buffer[image_buffer_len - (cinfo.next_scanline + 1) * row_stride];(void)jpeg_write_scanlines(&cinfo, row_pointer, 1);}/* Step 6: Finish compression */jpeg_finish_compress(&cinfo);/* After finish_compress, we can close the output file. */fclose(outfile);/* Step 7: release JPEG compression object *//* This is an important step since it will release a good deal of memory. */jpeg_destroy_compress(&cinfo);}// 释放 DIB 数组delete []dibBuffer;return 0;
}