Question

libjpeg-turbo here的说明因此描述了TurboJPEG API：“此API包装了libjpeg-turbo，并提供了一个易于使用的界面，用于压缩和解压缩内存中的JPEG图像”。很好，但是有一些使用此API的可靠实例吗？只是想在内存中解压一个相当香草的jpeg。

我找到了一些像https://github.com/erlyvideo/jpeg/blob/master/c_src/jpeg.c这样的位，它似乎使用的是TurboJPEG API，但还有更多可靠/多变的例子吗？

libjpeg-turbo的来源已有详细记录，因此确实有帮助。

Answer 1

好的，我知道你已经解决了你的问题，但有些人，就像我一样，可能会搜索一些简单的例子，我将分享我创造的内容。这是一个压缩和解压缩RGB图像的例子。否则我认为TurboJPEG的API文档很容易理解！

<强>压缩：

#include <turbojpeg.h>

const int JPEG_QUALITY = 75;
const int COLOR_COMPONENTS = 3;
int _width = 1920;
int _height = 1080;
long unsigned int _jpegSize = 0;
unsigned char* _compressedImage = NULL; //!< Memory is allocated by tjCompress2 if _jpegSize == 0
unsigned char buffer[_width*_height*COLOR_COMPONENTS]; //!< Contains the uncompressed image

tjhandle _jpegCompressor = tjInitCompress();

tjCompress2(_jpegCompressor, buffer, _width, 0, _height, TJPF_RGB,
          &_compressedImage, &_jpegSize, TJSAMP_444, JPEG_QUALITY,
          TJFLAG_FASTDCT);

tjDestroy(_jpegCompressor);

//to free the memory allocated by TurboJPEG (either by tjAlloc(), 
//or by the Compress/Decompress) after you are done working on it:
tjFree(&_compressedImage);

之后，您将在_compressedImage中获得压缩图像。要解压缩，您必须执行以下操作：

<强>减压

#include <turbojpeg.h>

long unsigned int _jpegSize; //!< _jpegSize from above
unsigned char* _compressedImage; //!< _compressedImage from above

int jpegSubsamp, width, height;
unsigned char buffer[width*height*COLOR_COMPONENTS]; //!< will contain the decompressed image

tjhandle _jpegDecompressor = tjInitDecompress();

tjDecompressHeader2(_jpegDecompressor, _compressedImage, _jpegSize, &width, &height, &jpegSubsamp);

tjDecompress2(_jpegDecompressor, _compressedImage, _jpegSize, buffer, width, 0/*pitch*/, height, TJPF_RGB, TJFLAG_FASTDCT);

tjDestroy(_jpegDecompressor);

一些随意的想法：

当我写论文时，我刚刚回来了，我注意到如果你在一个循环中运行压缩，最好存储JPEG缓冲区的最大大小，而不是每个都分配一个新的转。基本上，而不是做：

long unsigned int _jpegSize = 0;

tjCompress2(_jpegCompressor, buffer, _width, 0, _height, TJPF_RGB,
          &_compressedImage, &_jpegSize, TJSAMP_444, JPEG_QUALITY,
          TJFLAG_FASTDCT);

我们将添加一个对象变量，保持分配的内存long unsigned int _jpegBufferSize = 0;的大小，在每次压缩循环之前，我们将jpegSize设置回该值：

long unsigned int jpegSize = _jpegBufferSize;

tjCompress2(_jpegCompressor, buffer, _width, 0, _height, TJPF_RGB,
          &_compressedImage, &jpegSize, TJSAMP_444, JPEG_QUALITY,
          TJFLAG_FASTDCT);

_jpegBufferSize = _jpegBufferSize >= jpegSize? _jpegBufferSize : jpegSize;

在压缩之后，可以将内存大小与实际的jpegSize进行比较，如果它高于先前的内存大小，则将其设置为jpegSize。

Answer 2

最后，我使用了互联网上的随机代码组合（例如https://github.com/erlyvideo/jpeg/blob/master/c_src/jpeg.c）和libjeg-turbo的.c和头文件，这些文档都有详细记录。 This官方API也是一个很好的信息来源。

Answer 3

这是我用来从内存加载jpeg的代码片段。也许它需要一些修复，因为我从项目中的不同文件中提取它。它将加载 - 灰度和rgb图像（bpp将设置为1或3）。

struct Image
{
    int bpp;
    int width;
    int height;
    unsigned char* data;
};

struct jerror_mgr
{
    jpeg_error_mgr base;
    jmp_buf        jmp;
};

METHODDEF(void) jerror_exit(j_common_ptr jinfo)
{
    jerror_mgr* err = (jerror_mgr*)jinfo->err;
    longjmp(err->jmp, 1);
}

METHODDEF(void) joutput_message(j_common_ptr)
{
}

bool Image_LoadJpeg(Image* image, unsigned char* img_data, unsigned int img_size)
{
    jpeg_decompress_struct jinfo;
    jerror_mgr jerr;

    jinfo.err = jpeg_std_error(&jerr.base);
    jerr.base.error_exit = jerror_exit;
    jerr.base.output_message = joutput_message;
    jpeg_create_decompress(&jinfo);

    image->data = NULL;

    if (setjmp(jerr.jmp)) goto bail;

    jpeg_mem_src(&jinfo, img_data, img_size);

    if (jpeg_read_header(&jinfo, TRUE) != JPEG_HEADER_OK) goto bail;

    jinfo.dct_method = JDCT_FLOAT; // change this to JDCT_ISLOW on Android/iOS

    if (!jpeg_start_decompress(&jinfo)) goto bail;

    if (jinfo.num_components != 1 && jinfo.num_components != 3) goto bail;

    image->data = new (std::nothrow) unsigned char [jinfo.output_width * jinfo.output_height * jinfo.output_components];
    if (!image->data) goto bail;

    {
        JSAMPROW ptr = image->data;
        while (jinfo.output_scanline < jinfo.output_height)
        {
            if (jpeg_read_scanlines(&jinfo, &ptr, 1) != 1) goto bail;

            ptr += jinfo.output_width * jinfo.output_components;
        }
    }

    if (!jpeg_finish_decompress(&jinfo)) goto bail;

    image->bpp = jinfo.output_components;
    image->width = jinfo.output_width;
    image->height = jinfo.output_height;

    jpeg_destroy_decompress(&jinfo);

    return true;

bail:
    jpeg_destroy_decompress(&jinfo);
    if (image->data) delete [] data;

    return false;
}

Answer 4

我最终使用下面的代码作为JPEG编码和解码的工作示例。我能找到的最好的例子，它是自包含的，初始化虚拟图像并将编码图像输出到本地文件。

以下代码是不我自己的，信用转到https://sourceforge.net/p/libjpeg-turbo/discussion/1086868/thread/e402d36f/#8722。再次在此发布，以帮助任何人发现难以让libjpeg turbo工作。

#include "turbojpeg.h"
#include <iostream>
#include <string.h>
#include <errno.h>

using namespace std;

int main(void)
{
    unsigned char *srcBuf; //passed in as a param containing pixel data in RGB pixel interleaved format
    tjhandle handle = tjInitCompress();

    if(handle == NULL)
    {
        const char *err = (const char *) tjGetErrorStr();
        cerr << "TJ Error: " << err << " UNABLE TO INIT TJ Compressor Object\n";
        return -1;
    }
    int jpegQual =92;
    int width = 128;
    int height = 128;
    int nbands = 3;
    int flags = 0;
    unsigned char* jpegBuf = NULL;
    int pitch = width * nbands;
    int pixelFormat = TJPF_GRAY;
    int jpegSubsamp = TJSAMP_GRAY;
    if(nbands == 3)
    {
        pixelFormat = TJPF_RGB;
        jpegSubsamp = TJSAMP_411;
    }
    unsigned long jpegSize = 0;

    srcBuf = new unsigned char[width * height * nbands];
    for(int j = 0; j < height; j++)
    {
        for(int i = 0; i < width; i++)
        {
            srcBuf[(j * width + i) * nbands + 0] = (i) % 256;
            srcBuf[(j * width + i) * nbands + 1] = (j) % 256;
            srcBuf[(j * width + i) * nbands + 2] = (j + i) % 256;
        }
    }

    int tj_stat = tjCompress2( handle, srcBuf, width, pitch, height,
        pixelFormat, &(jpegBuf), &jpegSize, jpegSubsamp, jpegQual, flags);
    if(tj_stat != 0)
    {
        const char *err = (const char *) tjGetErrorStr();
        cerr << "TurboJPEG Error: " << err << " UNABLE TO COMPRESS JPEG IMAGE\n";
        tjDestroy(handle);
        handle = NULL;
        return -1;
    }

    FILE *file = fopen("out.jpg", "wb");
    if (!file) {
        cerr << "Could not open JPEG file: " << strerror(errno);
        return -1;
    }
    if (fwrite(jpegBuf, jpegSize, 1, file) < 1) {
        cerr << "Could not write JPEG file: " << strerror(errno);
        return -1;
    }
    fclose(file);

    //write out the compress date to the image file
    //cleanup
    int tjstat = tjDestroy(handle); //should deallocate data buffer
    handle = 0;
}

使用libjpeg-turbo的TurboJPEG的示例或教程

4 个答案: