我在 Visual Studio 2015 中用 C 语言编写了一个测试程序，用于测试 libjpeg-turbo TurboJPEG-API。
在这个测试程序中，我生成一个 800x800 RGB 像素的图像并将其写入磁盘。当我使用TJSAMP_GRAY该tjCompress2功能时，磁盘上生成的图像看起来不错：
当我改用图像时TJSAMP_444，图像看起来很奇怪：
但是，这两个图像都无法在 Adobe Photoshop 中打开，而是在 MS-Paint、Chrome 或 Internet Explorer 中打开。TJSAMP_GRAY除了产生奇怪的图像之外的所有其他选项，质量设置为 100TJSAMP_GRAY会产生奇怪的输出。（这篇文章中的图像是通过屏幕截图复制的，并保存为 png 以用于较小的数据。）turbojpeg.dll我还通过 P-Invoke使用该库编写了一个 C#-Program 。该程序正在产生有效的输出。
问：我的错在哪里？
有问题的源代码：
#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#include <sys/stat.h>
#include "turbojpeg.h"
unsigned char buffer[800 * 800 * 3];
int main(int argc, char** argv)
{
unsigned long size = 0;
unsigned char* compressedImage = NULL;
for (int x = 0; x < 800; x++)
for (int y = 0; y < 800; y++)
{
switch ((x / 80) % 3)
{
case 0:
buffer[(y * 800 + x) * 3] = 255;
buffer[(y * 800 + x) * 3 + 1] = 0;
buffer[(y * 800 + x) * 3 + 2] = 0;
break;
case 1:
buffer[(y * 800 + x) * 3] = 0;
buffer[(y * 800 + x) * 3 + 1] = 255;
buffer[(y * 800 + x) * 3 + 2] = 0;
break;
case 2:
buffer[(y * 800 + x) * 3] = 0;
buffer[(y * 800 + x) * 3 + 1] = 0;
buffer[(y * 800 + x) * 3 + 2] = 255;
break;
}
}
tjhandle compressor = tjInitCompress();
compressedImage = tjAlloc(1024 * 1024 * 4);
size = 1024 * 1024 * 4;
tjCompress2(compressor, buffer, 800, 0, 800, TJPF_RGB, &compressedImage, &size, TJSAMP_444, 80, TJFLAG_NOREALLOC | TJFLAG_ACCURATEDCT);
tjDestroy(compressor);
int handle = _wopen(L"D:\\file.jpeg", _O_CREAT | _O_WRONLY | _O_TRUNC, _S_IREAD | _S_IWRITE);
printf("LENGTH=%ld\n", size);
if (_write(handle, compressedImage, size) != size)
printf("Write Error.\n");
_close(handle);
tjFree(compressedImage);
return 0;
}
为了完整起见，测试 C# 源代码可以正常工作：
class Program
{
[DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr tjInitCompress();
[DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr tjAlloc(int bytes);
[DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
private static extern int tjCompress2(IntPtr handle, IntPtr srcBuf, int width, int pitch, int height, int pixelFormat, ref IntPtr jpegBuf, ref ulong jpegSize, int jpegSubsamp, int jpegQual, int flags);
[DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
private static extern int tjDestroy(IntPtr handle);
[DllImport("turbojpeg", CallingConvention = CallingConvention.Cdecl)]
private static extern void tjFree(IntPtr buffer);
static void Main(string[] args)
{
ulong size = 1024 * 1024 * 4;
byte[] buffer = new byte[800 * 800 * 3];
for (int x = 0; x < 800; x++)
for (int y = 0; y < 800; y++)
{
switch ((x / 80) % 3)
{
case 0:
buffer[(y * 800 + x) * 3] = 255;
buffer[(y * 800 + x) * 3 + 1] = 0;
buffer[(y * 800 + x) * 3 + 2] = 0;
break;
case 1:
buffer[(y * 800 + x) * 3] = 0;
buffer[(y * 800 + x) * 3 + 1] = 255;
buffer[(y * 800 + x) * 3 + 2] = 0;
break;
case 2:
buffer[(y * 800 + x) * 3] = 0;
buffer[(y * 800 + x) * 3 + 1] = 0;
buffer[(y * 800 + x) * 3 + 2] = 255;
break;
}
}
IntPtr umBuffer = Marshal.AllocHGlobal(800 * 800 * 3);
Marshal.Copy(buffer, 0, umBuffer, 800 * 800 * 3);
IntPtr compressor = tjInitCompress();
IntPtr compressedImage = tjAlloc(1024 * 1024 * 4);
tjCompress2(compressor, umBuffer, 800, 0, 800, 0, ref compressedImage, ref size, 0, 80, 5 * 1024);
byte[] mCPData = new byte[size];
Marshal.Copy(compressedImage, mCPData, 0, (int)size);
System.IO.File.WriteAllBytes("D:\\managed.jpeg", mCPData);
tjFree(compressedImage);
Marshal.FreeHGlobal(umBuffer);
}
}