为什么使用 XShmGetImage 捕获时 XImage 的数据指针为空?
Why is an XImage's data pointer null when capturing using XShmGetImage?
我正在编写一个程序,使用带有 C++ 的 Xlib 从一个 window 快速捕获图像、修改它们并将它们输出到另一个 window。我通过 XGetImage 工作:
#include <iostream>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
const int TEST_SIZE = 512;
int main()
{
// Open default display
Display *display = XOpenDisplay(nullptr);
int screen = DefaultScreen(display);
Window rootWin = RootWindow(display, screen);
GC graphicsContext = DefaultGC(display, screen);
// Create new window and subscribe to events
long blackPixel = BlackPixel(display, screen);
long whitePixel = WhitePixel(display, screen);
Window newWin = XCreateSimpleWindow(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, 1, blackPixel, whitePixel);
XMapWindow(display, newWin);
XSelectInput(display, newWin, ExposureMask | KeyPressMask);
// Main event loop for new window
XImage *image;
XEvent event;
bool exposed = false;
bool killWindow = false;
while (!killWindow)
{
// Handle pending events
if (XPending(display) > 0)
{
XNextEvent(display, &event);
if (event.type == Expose)
{
exposed = true;
} else if (event.type == NoExpose)
{
exposed = false;
} else if (event.type == KeyPress)
{
killWindow = true;
}
}
// Capture the original image
image = XGetImage(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, AllPlanes, ZPixmap);
// Modify the image
if (image->data != nullptr)
{
long pixel = 0;
for (int x = 0; x < image->width; x++)
{
for (int y = 0; y < image->height; y++)
{
// Invert the color of each pixel
pixel = XGetPixel(image, x, y);
XPutPixel(image, x, y, ~pixel);
}
}
}
// Output the modified image
if (exposed && killWindow == false)
{
XPutImage(display, newWin, graphicsContext, image, 0, 0, 0, 0, TEST_SIZE, TEST_SIZE);
}
XDestroyImage(image);
}
// Goodbye
XCloseDisplay(display);
}
它生成这样的输出:https://streamable.com/hovg9
我很高兴能走到这一步,但据我所知,性能不会很好地扩展,因为它必须在每一帧为新图像分配 space。事实上,如果没有在循环结束时调用 XDestroyImage,这个程序会在几秒钟内填满我机器上的所有 16GB 内存!
这里推荐的方法似乎是设置一个共享内存space X 可以写入每个帧的内容,我的程序可以随后读取和修改它们而无需任何额外分配。由于对 XShmGetImage 的调用会阻塞并等待 IPC,我相信这意味着我不必担心共享 space.
中的任何并发问题
我尝试使用以下代码实现 XShmGetImage 方法:
#include <iostream>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <X11/Xlib.h>
#include <X11/extensions/XShm.h>
#include <X11/Xutil.h>
const int TEST_SIZE = 512;
int main()
{
// Open default display
Display *display = XOpenDisplay(nullptr);
int screen = DefaultScreen(display);
Window rootWin = RootWindow(display, screen);
GC graphicsContext = DefaultGC(display, screen);
// Create new window and subscribe to events
long blackPixel = BlackPixel(display, screen);
long whitePixel = WhitePixel(display, screen);
Window newWin = XCreateSimpleWindow(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, 1, blackPixel, whitePixel);
XMapWindow(display, newWin);
XSelectInput(display, newWin, ExposureMask | KeyPressMask);
// Allocate shared memory for image capturing
Visual *visual = DefaultVisual(display, 0);
XShmSegmentInfo shminfo;
int depth = DefaultDepth(display, screen);
XImage *image = XShmCreateImage(display, visual, depth, ZPixmap, nullptr, &shminfo, TEST_SIZE, TEST_SIZE);
shminfo.shmid = shmget(IPC_PRIVATE, image->bytes_per_line * image->height, IPC_CREAT | 0666);
shmat(shminfo.shmid, nullptr, 0);
shminfo.shmaddr = image->data;
shminfo.readOnly = False;
XShmAttach(display, &shminfo);
// Main event loop for new window
XEvent event;
bool exposed = false;
bool killWindow = false;
while (!killWindow)
{
// Handle pending events
if (XPending(display) > 0)
{
XNextEvent(display, &event);
if (event.type == Expose)
{
exposed = true;
} else if (event.type == NoExpose)
{
exposed = false;
} else if (event.type == KeyPress)
{
killWindow = true;
}
}
// Capture the original image
XShmGetImage(display, rootWin, image, 0, 0, AllPlanes);
// Modify the image
if (image->data != nullptr) // NEVER TRUE. DATA IS ALWAYS NULL!
{
long pixel = 0;
for (int x = 0; x < image->width; x++)
{
for (int y = 0; y < image->height; y++)
{
// Invert the color of each pixel
pixel = XGetPixel(image, x, y);
XPutPixel(image, x, y, ~pixel);
}
}
}
// Output the modified image
if (exposed && killWindow == false)
{
XShmPutImage(display, newWin, graphicsContext, image, 0, 0, 0, 0, 512, 512, false);
}
}
// Goodbye
XFree(image);
XCloseDisplay(display);
}
不知何故,图像仍在被捕获并发送到新的 window,但 XImage 中的数据指针始终为空。我无法修改任何内容,并且 XGetPixel 和 XPutPixel 宏的任何使用都将失败。请注意,在此视频中,none 的颜色像以前一样反转:https://streamable.com/dckyv
这对我来说没有任何意义。很明显,数据仍在 windows 之间传输,但它在 XImage 结构中的什么位置?如何通过代码访问它?
原来我没有正确阅读 shmat 的签名。它没有 void return 类型,它 return 是一个空指针。这需要直接分配给 XImage 的数据指针,以便数据指针执行任何操作。
在对 XShmPutImage 的调用中,共享内存位置是在内部引用的,而不是 XImage 的数据指针,这就是为什么即使不使用来自 shmat 的 return 值它仍然有效的原因。
这是工作代码,以及我为错误处理和拆卸所做的其他一些补充:
#include <iostream>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <X11/Xlib.h>
#include <X11/extensions/XShm.h>
#include <X11/Xutil.h>
const int TEST_SIZE = 512;
static int handleXError(Display *display, XErrorEvent *event)
{
printf("XErrorEvent triggered!\n");
printf("error_code: %d", event->error_code);
printf("minor_code: %d", event->minor_code);
printf("request_code: %d", event->request_code);
printf("resourceid: %lu", event->resourceid);
printf("serial: %d", event->error_code);
printf("type: %d", event->type);
return 0;
}
int main()
{
// Open default display
XSetErrorHandler(handleXError);
Display *display = XOpenDisplay(nullptr);
int screen = DefaultScreen(display);
Window rootWin = RootWindow(display, screen);
GC graphicsContext = DefaultGC(display, screen);
// Create new window and subscribe to events
long blackPixel = BlackPixel(display, screen);
long whitePixel = WhitePixel(display, screen);
Window newWin = XCreateSimpleWindow(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, 1, blackPixel, whitePixel);
XMapWindow(display, newWin);
XSelectInput(display, newWin, ExposureMask | KeyPressMask);
// Allocate shared memory for image capturing
XShmSegmentInfo shminfo;
Visual *visual = DefaultVisual(display, screen);
int depth = DefaultDepth(display, screen);
XImage *image = XShmCreateImage(display, visual, depth, ZPixmap, nullptr, &shminfo, TEST_SIZE, TEST_SIZE);
shminfo.shmid = shmget(IPC_PRIVATE, image->bytes_per_line * image->height, IPC_CREAT | 0777);
image->data = (char*)shmat(shminfo.shmid, 0, 0);
shminfo.shmaddr = image->data;
shminfo.readOnly = False;
XShmAttach(display, &shminfo);
XSync(display, false);
shmctl(shminfo.shmid, IPC_RMID, 0);
// Main event loop for new window
XEvent event;
bool exposed = false;
bool killWindow = false;
while (!killWindow)
{
// Handle pending events
if (XPending(display) > 0)
{
XNextEvent(display, &event);
if (event.type == Expose)
{
exposed = true;
} else if (event.type == NoExpose)
{
exposed = false;
} else if (event.type == KeyPress)
{
killWindow = true;
}
}
// Capture the original image
XShmGetImage(display, rootWin, image, 0, 0, AllPlanes);
// Modify the image
if(image->data != nullptr) // NEVER TRUE. DATA IS ALWAYS NULL!
{
long pixel = 0;
for (int x = 0; x < image->width; x++)
{
for (int y = 0; y < image->height; y++)
{
// Invert the color of each pixel
pixel = XGetPixel(image, x, y);
XPutPixel(image, x, y, ~pixel);
}
}
}
// Output the modified image
if (exposed && killWindow == false)
{
XShmPutImage(display, newWin, graphicsContext, image, 0, 0, 0, 0, 512, 512, false);
}
}
// Goodbye
XShmDetach(display, &shminfo);
XDestroyImage(image);
shmdt(shminfo.shmaddr);
XCloseDisplay(display);
}
我正在编写一个程序,使用带有 C++ 的 Xlib 从一个 window 快速捕获图像、修改它们并将它们输出到另一个 window。我通过 XGetImage 工作:
#include <iostream>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
const int TEST_SIZE = 512;
int main()
{
// Open default display
Display *display = XOpenDisplay(nullptr);
int screen = DefaultScreen(display);
Window rootWin = RootWindow(display, screen);
GC graphicsContext = DefaultGC(display, screen);
// Create new window and subscribe to events
long blackPixel = BlackPixel(display, screen);
long whitePixel = WhitePixel(display, screen);
Window newWin = XCreateSimpleWindow(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, 1, blackPixel, whitePixel);
XMapWindow(display, newWin);
XSelectInput(display, newWin, ExposureMask | KeyPressMask);
// Main event loop for new window
XImage *image;
XEvent event;
bool exposed = false;
bool killWindow = false;
while (!killWindow)
{
// Handle pending events
if (XPending(display) > 0)
{
XNextEvent(display, &event);
if (event.type == Expose)
{
exposed = true;
} else if (event.type == NoExpose)
{
exposed = false;
} else if (event.type == KeyPress)
{
killWindow = true;
}
}
// Capture the original image
image = XGetImage(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, AllPlanes, ZPixmap);
// Modify the image
if (image->data != nullptr)
{
long pixel = 0;
for (int x = 0; x < image->width; x++)
{
for (int y = 0; y < image->height; y++)
{
// Invert the color of each pixel
pixel = XGetPixel(image, x, y);
XPutPixel(image, x, y, ~pixel);
}
}
}
// Output the modified image
if (exposed && killWindow == false)
{
XPutImage(display, newWin, graphicsContext, image, 0, 0, 0, 0, TEST_SIZE, TEST_SIZE);
}
XDestroyImage(image);
}
// Goodbye
XCloseDisplay(display);
}
它生成这样的输出:https://streamable.com/hovg9
我很高兴能走到这一步,但据我所知,性能不会很好地扩展,因为它必须在每一帧为新图像分配 space。事实上,如果没有在循环结束时调用 XDestroyImage,这个程序会在几秒钟内填满我机器上的所有 16GB 内存!
这里推荐的方法似乎是设置一个共享内存space X 可以写入每个帧的内容,我的程序可以随后读取和修改它们而无需任何额外分配。由于对 XShmGetImage 的调用会阻塞并等待 IPC,我相信这意味着我不必担心共享 space.
中的任何并发问题我尝试使用以下代码实现 XShmGetImage 方法:
#include <iostream>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <X11/Xlib.h>
#include <X11/extensions/XShm.h>
#include <X11/Xutil.h>
const int TEST_SIZE = 512;
int main()
{
// Open default display
Display *display = XOpenDisplay(nullptr);
int screen = DefaultScreen(display);
Window rootWin = RootWindow(display, screen);
GC graphicsContext = DefaultGC(display, screen);
// Create new window and subscribe to events
long blackPixel = BlackPixel(display, screen);
long whitePixel = WhitePixel(display, screen);
Window newWin = XCreateSimpleWindow(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, 1, blackPixel, whitePixel);
XMapWindow(display, newWin);
XSelectInput(display, newWin, ExposureMask | KeyPressMask);
// Allocate shared memory for image capturing
Visual *visual = DefaultVisual(display, 0);
XShmSegmentInfo shminfo;
int depth = DefaultDepth(display, screen);
XImage *image = XShmCreateImage(display, visual, depth, ZPixmap, nullptr, &shminfo, TEST_SIZE, TEST_SIZE);
shminfo.shmid = shmget(IPC_PRIVATE, image->bytes_per_line * image->height, IPC_CREAT | 0666);
shmat(shminfo.shmid, nullptr, 0);
shminfo.shmaddr = image->data;
shminfo.readOnly = False;
XShmAttach(display, &shminfo);
// Main event loop for new window
XEvent event;
bool exposed = false;
bool killWindow = false;
while (!killWindow)
{
// Handle pending events
if (XPending(display) > 0)
{
XNextEvent(display, &event);
if (event.type == Expose)
{
exposed = true;
} else if (event.type == NoExpose)
{
exposed = false;
} else if (event.type == KeyPress)
{
killWindow = true;
}
}
// Capture the original image
XShmGetImage(display, rootWin, image, 0, 0, AllPlanes);
// Modify the image
if (image->data != nullptr) // NEVER TRUE. DATA IS ALWAYS NULL!
{
long pixel = 0;
for (int x = 0; x < image->width; x++)
{
for (int y = 0; y < image->height; y++)
{
// Invert the color of each pixel
pixel = XGetPixel(image, x, y);
XPutPixel(image, x, y, ~pixel);
}
}
}
// Output the modified image
if (exposed && killWindow == false)
{
XShmPutImage(display, newWin, graphicsContext, image, 0, 0, 0, 0, 512, 512, false);
}
}
// Goodbye
XFree(image);
XCloseDisplay(display);
}
不知何故,图像仍在被捕获并发送到新的 window,但 XImage 中的数据指针始终为空。我无法修改任何内容,并且 XGetPixel 和 XPutPixel 宏的任何使用都将失败。请注意,在此视频中,none 的颜色像以前一样反转:https://streamable.com/dckyv
这对我来说没有任何意义。很明显,数据仍在 windows 之间传输,但它在 XImage 结构中的什么位置?如何通过代码访问它?
原来我没有正确阅读 shmat 的签名。它没有 void return 类型,它 return 是一个空指针。这需要直接分配给 XImage 的数据指针,以便数据指针执行任何操作。
在对 XShmPutImage 的调用中,共享内存位置是在内部引用的,而不是 XImage 的数据指针,这就是为什么即使不使用来自 shmat 的 return 值它仍然有效的原因。
这是工作代码,以及我为错误处理和拆卸所做的其他一些补充:
#include <iostream>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <X11/Xlib.h>
#include <X11/extensions/XShm.h>
#include <X11/Xutil.h>
const int TEST_SIZE = 512;
static int handleXError(Display *display, XErrorEvent *event)
{
printf("XErrorEvent triggered!\n");
printf("error_code: %d", event->error_code);
printf("minor_code: %d", event->minor_code);
printf("request_code: %d", event->request_code);
printf("resourceid: %lu", event->resourceid);
printf("serial: %d", event->error_code);
printf("type: %d", event->type);
return 0;
}
int main()
{
// Open default display
XSetErrorHandler(handleXError);
Display *display = XOpenDisplay(nullptr);
int screen = DefaultScreen(display);
Window rootWin = RootWindow(display, screen);
GC graphicsContext = DefaultGC(display, screen);
// Create new window and subscribe to events
long blackPixel = BlackPixel(display, screen);
long whitePixel = WhitePixel(display, screen);
Window newWin = XCreateSimpleWindow(display, rootWin, 0, 0, TEST_SIZE, TEST_SIZE, 1, blackPixel, whitePixel);
XMapWindow(display, newWin);
XSelectInput(display, newWin, ExposureMask | KeyPressMask);
// Allocate shared memory for image capturing
XShmSegmentInfo shminfo;
Visual *visual = DefaultVisual(display, screen);
int depth = DefaultDepth(display, screen);
XImage *image = XShmCreateImage(display, visual, depth, ZPixmap, nullptr, &shminfo, TEST_SIZE, TEST_SIZE);
shminfo.shmid = shmget(IPC_PRIVATE, image->bytes_per_line * image->height, IPC_CREAT | 0777);
image->data = (char*)shmat(shminfo.shmid, 0, 0);
shminfo.shmaddr = image->data;
shminfo.readOnly = False;
XShmAttach(display, &shminfo);
XSync(display, false);
shmctl(shminfo.shmid, IPC_RMID, 0);
// Main event loop for new window
XEvent event;
bool exposed = false;
bool killWindow = false;
while (!killWindow)
{
// Handle pending events
if (XPending(display) > 0)
{
XNextEvent(display, &event);
if (event.type == Expose)
{
exposed = true;
} else if (event.type == NoExpose)
{
exposed = false;
} else if (event.type == KeyPress)
{
killWindow = true;
}
}
// Capture the original image
XShmGetImage(display, rootWin, image, 0, 0, AllPlanes);
// Modify the image
if(image->data != nullptr) // NEVER TRUE. DATA IS ALWAYS NULL!
{
long pixel = 0;
for (int x = 0; x < image->width; x++)
{
for (int y = 0; y < image->height; y++)
{
// Invert the color of each pixel
pixel = XGetPixel(image, x, y);
XPutPixel(image, x, y, ~pixel);
}
}
}
// Output the modified image
if (exposed && killWindow == false)
{
XShmPutImage(display, newWin, graphicsContext, image, 0, 0, 0, 0, 512, 512, false);
}
}
// Goodbye
XShmDetach(display, &shminfo);
XDestroyImage(image);
shmdt(shminfo.shmaddr);
XCloseDisplay(display);
}