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linux – 如果“纯XCB”OpenGL是不可能的,那么在xcb / glx.h中找到的XCB / GLX API的用途是什么?

作者:互联网

官方的XCB文档告诉我们using OpenGL purely with XCB is impossible:一个人也必须使用Xlib.

巴特梅西(XCB的创始人)This post并未暗示这应该是不可能的.但我确定我错过了一些东西.

我花了好几个小时浏览xcb / glx.h,它组织得很好here.在我看来,它就像一个完整的API.但我无法让它发挥作用.

问题

> XCB / GLX API无用吗? xcb / glx.h的目的是什么?
>如何使用API​​?

(注意:这是了解XCB如何工作的持续努力的一部分.)

相关SO thread.

次要的,啰嗦的东西

如果有人愿意采取行动,这里是来自XCB邮件列表中原始帖子的来源,删除了一点并放入一个文件中.

您会注意到xcb_glx_make_context_current返回错误169(不知道这意味着什么),但前提是xcb_glx_create_window为其最后两个参数取0和NULL.这些参数涉及一系列属性,它们似乎是由函数xcb_glx_create_window_attribs返回的,但我无法弄清楚如何使用它…

int main()之前的长辅助函数仅表示两个返回两个整数xcb_glx_fbconfig_t fbconfig xcb_visualid_t glx_visual,对应于第一个“匹配”帧缓冲配置.在我的平台上,这些是0xa7和0x24.它们正是Xlib / GLX例程(实际工作)返回的原因,所以我知道我选择的帧缓冲配置很好.

所以,问题似乎发生在xcb_glx_create_window和xcb_glx_make_context_current之间…

// gcc main2.c -o main -lxcb -lxcb-glx -lGL && ./main2
// TODO free replies

#include <stdio.h>
#include <stdlib.h>
#include <xcb/glx.h>
#include <GL/gl.h>

#define W 1024
#define H 1024

// parameter types returned by xcb_glx_get_fb_configs
#define GLX_DRAWABLE_TYPE               0x8010
#define GLX_RENDER_TYPE                 0x8011
#define GLX_DOUBLEBUFFER                5
#define GLX_RED_SIZE                    8
#define GLX_GREEN_SIZE                  9
#define GLX_BLUE_SIZE                   10
#define GLX_RGBA_BIT                    0x00000001
#define GLX_RGBA_TYPE                   0x8014
#define GLX_STENCIL_SIZE                13
#define GLX_DEPTH_SIZE                  12
#define GLX_BUFFER_SIZE                 2
#define GLX_ALPHA_SIZE                  11
#define GLX_X_RENDERABLE                0x8012

#define GLX_FBCONFIG_ID                 0x8013
#define GLX_VISUAL_ID                   0x800b

#define GLX_WINDOW_BIT                  0x00000001
#define GLX_PIXMAP_BIT                  0x00000002
#define GLX_PBUFFER_BIT                 0x00000004


// ------------------------------------------------------------------------------------------------
// fbconfig and visual?
uint32_t glx_attrs[] = {
  GLX_DOUBLEBUFFER, 1,
  GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT|GLX_PIXMAP_BIT|GLX_PBUFFER_BIT,
  GLX_X_RENDERABLE, 1,  
  GLX_RED_SIZE, 8,
  GLX_GREEN_SIZE, 8,
  GLX_BLUE_SIZE, 8,
  GLX_ALPHA_SIZE, 8,
  GLX_STENCIL_SIZE, 8,
  GLX_DEPTH_SIZE, 24,
  GLX_BUFFER_SIZE, 32,
  GLX_RENDER_TYPE, GLX_RGBA_BIT,
};

// ------------------------------------------------------------------------------------------------
// This function searches for an @param prop_name in the @param property list of properties of size @param prop. Prop is property count and not buffer size.
uint32_t glx_get_property(const uint32_t* property, const uint props, uint32_t prop_name){
  uint i=0;
  while(i < props*2){
      if(property[i] == prop_name)
        return property[i+1];
      else i += 2;
  }
  return -1;
}

// This function chooses and returns specific fbconfig id depending on attributes specified in 
// @param attrib list. @param attribsz is the number of properties(not list size)
int32_t glx_choose_fbconfig(xcb_connection_t* connection, uint32_t screen_num, uint32_t* attrib, uint32_t attribsz){
  xcb_generic_error_t* xerror;

  xcb_glx_get_fb_configs_reply_t* fbconfigs = xcb_glx_get_fb_configs_reply(connection, xcb_glx_get_fb_configs(connection, screen_num), NULL);
  uint32_t* prop = xcb_glx_get_fb_configs_property_list(fbconfigs);

  uint32_t* fbconfig_line   = prop;
  uint32_t  fbconfig_linesz = fbconfigs->num_properties * 2;

  for(uint i=0 ; i<fbconfigs->num_FB_configs; i++){  // for each fbconfig line
      uint good_fbconfig = 1;

    for(uint j=0 ; j<attribsz*2; j += 2){  // for each attrib
    // if property found != property given
        if(glx_get_property(fbconfig_line, fbconfigs->num_properties, attrib[j]) != attrib[j+1]) {
            good_fbconfig = 0; // invalidate this fbconfig entry, sine one of the attribs doesn't match
            break;
        }
    }

    // if all attribs matched, return with fid
    if(good_fbconfig){
        uint32_t fbconfig_id = glx_get_property(fbconfig_line, fbconfigs->num_properties , GLX_FBCONFIG_ID);
        free(fbconfigs);
        return fbconfig_id;
    }

    fbconfig_line += fbconfig_linesz; // next fbconfig line;
  }
  return -1;
}

// This function returns @param attrib value from a line containing GLX_FBCONFIG_ID of @param fid
// It kind of queries particular fbconfig line for a specific property.
uint32_t glx_get_attrib_from_fbconfig(xcb_connection_t* connection, uint32_t screen_num, uint32_t fid, uint32_t attrib){
  xcb_glx_get_fb_configs_reply_t* fbconfigs = xcb_glx_get_fb_configs_reply(connection, xcb_glx_get_fb_configs(connection, screen_num), NULL);
  uint32_t* prop   = xcb_glx_get_fb_configs_property_list(fbconfigs);

  uint i = 0;
  uint fid_found = 0;
  while(i < fbconfigs->length){
    if(prop[i] == GLX_FBCONFIG_ID) {
      if(prop[i+1] == fid){
        fid_found = 1;
        i -= i%(fbconfigs->num_properties * 2); // going to start of the fbconfig  line
        uint32_t attrib_value = glx_get_property(&prop[i], fbconfigs->num_properties, attrib);
        free(fbconfigs);
        return attrib_value;
      }
    }
    i+=2;
  }
  if(fid_found) printf("glx_get_attrib_from_fbconfig: no attrib %u was found in a fbconfig with GLX_FBCONFIG_ID %u\n", attrib, fid);
  else printf("glx_get_attrib_from_fbconfig: GLX_FBCONFIG_ID %u was not found!\n", fid);
  return -1;
}

// ------------------------------------------------------------------------------------------------
int main(){
  xcb_generic_error_t* xerror; // To hold errors!
  int screen_number;

  xcb_connection_t* connection = xcb_connect(NULL, &screen_number);
  xcb_screen_t* screen = xcb_setup_roots_iterator(xcb_get_setup(connection)).data;   // getting the default screen
  printf("screen %d  root %d\n", screen_number, screen->root);

  xcb_colormap_t    colormap    = xcb_generate_id(connection);  // generating XID's for our objects!
  xcb_window_t      window      = xcb_generate_id(connection);
  xcb_glx_context_t glx_context = xcb_generate_id(connection);
  xcb_glx_window_t  glx_window  = xcb_generate_id(connection);

  // ----------------------------------------------------------------------------------------------
  xcb_glx_query_version_reply_t* glx_version = xcb_glx_query_version_reply(connection, xcb_glx_query_version(connection, 0, 0), NULL);
  printf("glx %d.%d  response_type %x  pad0 %x  sequence %x  length %d\n",
    glx_version->major_version, glx_version->minor_version, glx_version->response_type,
    glx_version->pad0, glx_version->sequence, glx_version->length);

  // ----------------------------------------------------------------------------------------------
  xcb_glx_fbconfig_t fbconfig   = glx_choose_fbconfig(connection, screen_number, glx_attrs, sizeof(glx_attrs)/2/sizeof(uint32_t));
  xcb_visualid_t glx_visual     = glx_get_attrib_from_fbconfig(connection, screen_number, fbconfig, GLX_VISUAL_ID);
  printf("fbconfig %x  glx_visual %x\n", fbconfig, glx_visual);

  // ----------------------------------------------------------------------------------------------
  xcb_glx_create_new_context(connection, glx_context, fbconfig, screen_number, GLX_RGBA_TYPE, 0, 1);  // New-style context?
  // xcb_glx_create_context(connection, glx_context, glx_visual, 0, 0, 1);  // Alt method! Old-style context?

  if(!(xcb_glx_is_direct_reply(connection, xcb_glx_is_direct(connection, glx_context), NULL)->is_direct))
    puts("glx context is not direct!");

  // ----------------------------------------------------------------------------------------------
  xcb_create_colormap(connection , XCB_COLORMAP_ALLOC_NONE, colormap, screen->root, glx_visual);  // creating colormap

  // creating a window, using our new colormap
  uint32_t window_mask = XCB_CW_BACK_PIXEL|XCB_CW_EVENT_MASK|XCB_CW_COLORMAP;
  uint32_t window_attrs[] = {0x444444, XCB_EVENT_MASK_EXPOSURE|XCB_EVENT_MASK_KEY_PRESS, colormap};

  xcb_create_window(connection, screen->root_depth, window, screen->root, 0,0, W,H, 0, XCB_WINDOW_CLASS_INPUT_OUTPUT, glx_visual, window_mask, window_attrs);
  xcb_map_window(connection, window);

  xcb_glx_create_window(connection, screen_number, fbconfig, window, glx_window, 0, NULL);
  xcb_flush(connection);

  // ----------------------------------------------------------------------------------------------
  xcb_glx_make_context_current_reply_t* reply_ctx = xcb_glx_make_context_current_reply(connection, xcb_glx_make_context_current(connection, 0, glx_window, glx_window, glx_context), NULL);
  if(!reply_ctx)  puts("ERROR  xcb_glx_make_context_current returned NULL!");
  xcb_glx_context_tag_t glx_context_tag = reply_ctx->context_tag;

  // alternative ?
  // xcb_glx_make_current_reply_t* reply_mc = xcb_glx_make_current_reply(connection, xcb_glx_make_current(connection, glx_window, glx_context, 0), NULL);
  // xcb_glx_context_tag_t glx_context_tag = reply_mc->context_tag;

  // ----------------------------------------------------------------------------------------------
  xcb_glx_get_error_reply(connection, xcb_glx_get_error(connection, glx_context_tag), &xerror);
  if(xerror)  printf("\nERROR  xcb_glx_get_error %d\n", xerror->error_code);

  // ----------------------------------------------------------------------------------------------
  xcb_generic_event_t* event;
  uint running = 1;
  while(running){
    event = xcb_poll_for_event(connection);
    if(event){
      switch (event->response_type) {
        case XCB_EXPOSE:
          glClearColor(0, .5, 1, 1);  // Blue
          glFlush();
          xcb_glx_swap_buffers(connection, glx_context_tag, glx_window);
          puts("Expose!");
          break;
        case XCB_KEY_PRESS: // exit on key press
          running = 0;
          break;
      }
    }
    free(event);
  }

  xcb_disconnect(connection);
}

解决方法:

这里需要了解的重要事项是XCB函数直接映射到X11协议请求.这意味着xcb_glx_ *函数直接映射到X11 GLX协议请求.请参阅https://www.khronos.org/registry/OpenGL/specs/gl/glx1.4.pdf中的“第4章”.它列出了所有可用的GLX请求.例如,glAreTexturesResident从xcb / glx.h(https://xcb.freedesktop.org/manual/glx_8h_source.html)映射到xcb_glx_are_textures_resident_ * API.在Khronos规范中,您可以阅读请求做什么.

What’s the purpose of xcb/glx.h?

XCB-GLX只与X服务器通信,它不执行任何硬件初始化或触摸OpenGL客户端状态.因此,XCB-GLX不能用作GLX API的替代品. [1]

硬件初始化和其他GL内容由openGL lib完成.这就是规范的“另一半”实施的地方.在Linux上,libsa由mesa(https://cgit.freedesktop.org/mesa/mesa/tree/src/glx)提供.你可以看到glx目录中的文件包含Xlib.h,所以我想这就是Xlib依赖的来源.这解释了“GLX API与Xlib紧密结合.因此,X Windows上的OpenGL应用程序必须使用Xlib,因此不能仅使用XCB.” [1].

Is the XCB/GLX API useless?

虽然XCB-GLX API对最终用户XCB应用程序开发人员没什么价值,但它可以用于开发基于XCB的新OpenGL和GLX实现. XCB可以提高OpenGL库的速度和质量. [1]

因此,为了获得纯XCB GLX,有人需要在openGL lib中重新实现GLX:

>公开基于XCB的API以初始化硬件等.
>应该使用xcb_glx API与X服务器进行通信,而不是Xlib.

来自[1]的文档说“GLX系统有两个角色,它与X服务器通信并初始化客户端和硬件状态.”

xcb-glx负责沟通.另一个角色(基于XCB的OpenGL和GLX实现)目前尚未实现,也不太可能实现.

“GLX API是根据Xlib指定的,glX函数使用Xlib Displays,Windows,Visuals等.GLX实现也是使用Xlib构建的.” (参见libGL.so的导出符号).为了完成第二个角色,我们需要一个使用XCB连接,窗口,视觉效果的相同API.

[1] https://xcb.freedesktop.org/opengl/

免责声明:这是我对收集的理解.

标签:linux,x11,framebuffer,glx,xcb
来源: https://codeday.me/bug/20190627/1308961.html