JPEG图片的解码和压缩流程
目录
目录
JPEG图片解码
一：创建并初始化一个JPEG解码对象（解码对象是一个结构图对象）
/* Step 1: allocate and initialize JPEG decompression object */
/* override error_exit. */
/* Now we can initialize the JPEG decompression object. */
jpeg_create_decompress(&cinfo);
二：创建错误处理对象，并绑定解码对象
/* Step 2: We set up the normal JPEG error routines */
cinfo.err = jpeg_std_error(&jerr);
三：打开指定带解码的JPEG图片
/* Step 3: specify data source (eg, a file) */
jpeg_stdio_src(&cinfo, infile);
四：获取待解码图片的图片信息，解析文件头。JPEG文件中包含一些元数据，如图像尺寸、颜色空间信息等。解析文件头可以获得这些元数据，为后续处理做准备。
/* Step 4: read file parameters with jpeg_read_header() */
(void)jpeg_read_header(&cinfo, TRUE);
/* We can ignore the return value from jpeg_read_header since
* (a) suspension is not possible with the stdio data source, and
* (b) we passed TRUE to reject a tables-only JPEG file as an error.
* See libjpeg.txt for more info.
*/
五：设置解码参数。（可根据自身要求选择对应参数，也可使用默认的参数）
/* Step 5: set parameters for decompression */
/* In this example, we don't need to change any of the defaults set by
* jpeg_read_header(), so we do nothing here.
*/
六：开始解码
/* Step 6: Start decompressor */
(void)jpeg_start_decompress(&cinfo);
/* We can ignore the return value since suspension is not possible
* with the stdio data source.
*/
/* We may need to do some setup of our own at this point before reading
* the data. After jpeg_start_decompress() we have the correct scaled
* output image dimensions available, as well as the output colormap
* if we asked for color quantization.
* In this example, we need to make an output work buffer of the right size.
*/
/* JSAMPLEs per row in output buffer */
row_stride = cinfo.output_width * cinfo.output_components; // 计算一行的大小
/* Make a one-row-high sample array that will go away when done with image */
buffer = calloc(1, row_stride);
七：调用循环从JPEG图片中以行为单位进行解码
/* Step 7: while (scan lines remain to be read) */
/* jpeg_read_scanlines(...); */
/* Here we use the library's state variable cinfo.output_scanline as the
* loop counter, so that we don't have to keep track ourselves.
*/
int data = 0;
while (cinfo.output_scanline < cinfo.output_height)
{
/* jpeg_read_scanlines expects an array of pointers to scanlines.
* Here the array is only one element long, but you could ask for
* more than one scanline at a time if that's more convenient.
*/
(void)jpeg_read_scanlines(&cinfo, &buffer, 1); // 从上到下，从左到右 RGB RGB RGB RGB
for (int i = 0; i < cinfo.output_width; ++i) // 012 345
{
data |= buffer[3 * i] << 16; // R
data |= buffer[3 * i + 1] << 8; // G
data |= buffer[3 * i + 2]; // B
// 把像素点写入到LCD的指定位置
lcd_mp[800 * start_y + start_x + 800 * (cinfo.output_scanline - 1) + i] = data;
data = 0;
}
}
八： 完成解码
/* Step 8: Finish decompression */
(void)jpeg_finish_decompress(&cinfo);
/* We can ignore the return value since suspension is not possible
* with the stdio data source.
*/
九：释放解码对象
/* Step 9: Release JPEG decompression object */
/* This is an important step since it will release a good deal of memory. */
jpeg_destroy_decompress(&cinfo);
/* After finish_decompress, we can close the input file.
* Here we postpone it until after no more JPEG errors are possible,
* so as to simplify the setjmp error logic above. (Actually, I don't
* think that jpeg_destroy can do an error exit, but why assume anything...)
*/
fclose(infile);
/* At this point you may want to check to see whether any corrupt-data
* warnings occurred (test whether jerr.pub.num_warnings is nonzero).
*/
/* And we're done! */
return 1;
