传统的OpenGL纹理格式(texture format)的纹理数据存储和解释是统一的，纹理就是被解释成颜色。而在现代OpenGL应用中，大多数纹理并不是代表颜色，而是代表像shadow maps, normal maps, page tables, occlusion data等等这样的数据。对于后者，我们将纹理数据称作"RGBA"，只是对数据在现有模型上的一种方便的映射，但并不是数据实际上的准确解释。

ARB_shader_ballot

Posted on 2020-06-09 In graphics Views: Disqus:

概述(#183)

ARB_shader_ballot给一组线程(a group of invocations)提供某些形式的线程间(cross-invocation)通信的能力。它要么是通过广播某个线程作用域里的值，要么是通过一个位数组(bitarray)表示一组线程中每个线程作用域里的可能的值。

ARB_texture_view

Posted on 2020-06-09 In graphics Views: Disqus:

概述(#124)

ARB_texture_view的主要目的是通过创建已有纹理对象的视图(view), 来产生新的纹理对象，以达到共享同一纹理存储(data store)的目的。它可以从以下3个方面创建纹理视图：

ARB_point_parameters

Posted on 2020-06-05 In graphics Views: Disqus:

概述(#14)

ARB_point_parameters允许用户设置点的扩展几何特征。这些点的几何特征主要用来渲染粒子或微小光源，通常被称为光点(Light Points).

vim 速查手册

Posted on 2020-02-29 In utilities Views: Disqus:

Visual Block Selection

选择 {}(curly braces) 之间的行(包括 {})
- v%
选择 {}(curly braces) 之间的行(不包括 {})
- vi{

注意：光标必须放在 { 或者 }

Copy & Paste

Copy to clipboard

Normal 模式
- "+y

Paste from clipboard

X11 window system 有 3 个 selections:

PRIMARY 表示当前的可见 selection
SECONDARY
CLIPBOARD 通常所说的剪贴板，用来完成剪贴，复制和粘贴操作

Vim 有两个专门的寄存器分别与 PRIMARY selection 和 CLIPBOARD selection 对应

"* (quotestar) PRIMARY
"+ (quoteplus) CLIPBOARD

如果要在 Vim 内部剪贴，复制/粘贴就使用 quotestar, 如果要将内容剪贴，复制/粘贴到系统剪贴板，就使用 quoteplus

举个例子，在 WSL Ubuntu 里如果想粘贴 Windows 剪贴板里的内容到 vim (最好是 neovim, 因为 vim 可能未使能 clipboard)

1	echo "Hello, world!" \| clip.exe

Normal 模式
- "+p

如果想粘贴 Ubuntu 剪贴板里的内容

1	echo "Hello, world!" \| xclip

Normal 模式
- "*p

Copy from above or below

在 Insert 模式下

<ctrl-y> copies the character from the line above
<ctrl-e> copies the character from the line below (本行就是这样输入的)

Search & Replace

删除多行 C-style 注释

s;/\*\_.\{-}\*/;;
- ; 因为要匹配 /, 为了省去转义 / 的麻烦，将 search 命令的分隔符由 / 改为 ;
- /\* 匹配开始的 /*
- \_. 匹配任意字符，包括 \n, 所以常用在多行匹配
- \{-} 指非贪婪 (non-greedy) 匹配，即匹配最短的字串，默认是贪婪匹配，匹配最长的字串
- \*/ 匹配结尾的 */
- ;; 指使用空替换匹配结果，即删除
- 以下是 vim 中在单行中使用的通配符 (wildcard), 这些通配符一般不能匹配换行 \n
  - . 匹配除了 \n 的任意字符
  - ^ 行首，即锚点 (anchor)
  - $ 行尾，即锚点
  - \s 匹配 space, tab, 但不匹配 \n
- 以上通配符加上 \_ 后可以在多行匹配中使用，即也可以匹配 \n
  - \_. 匹配任意字符，包括 \n
  - \_^ 多行中第一行行首
  - \_$ 多行中最后一行行尾
  - \_s 匹配 space, tab 和 \n

删除空白行

g/^\s*$/d
- g 指所有行搜索，而不是默认的光标所在行
- ^\s*$ 匹配行首到行尾之间任意个空白字符，即空白行
- d 删除命令 (delete)

搜索不匹配某模式的行，即反向搜索

/^$\(^# .*$$\@!.\)*$
- / 查找命令 (search) 的提示符
- ^$\(The_Regex$\@!.\)*$ 反向查找的命令固定模式，本例中 The_Regex 是 ^# .*$ 即以 # 开头的行，所以整个表达式匹配的就是不以 # 开头的所有行
:v/^# .*$/p
- 与上面的反向查找命令功能相同, 但会将匹配结果显示在 Visual 模式下

大小写转换

s/$^# .*$$/\L\1/
- $The_Regex$ 为了向前索引，即后面 \1 所指的部分，本例中 The_Regex 是 ^# .*$, 意思同上
- \L\1 \L 指 Lowercase, 即将 \1 匹配的结果中的所有字母都换成小写
s/$^# .*$$/\U\1/
- 与上面的表达式功能相反，即将 \1 匹配的结果中的所有字母都换成大写 (Uppercase)

expression register

假如你在 markdown 文件里编辑一个 Table, table 的第1列是 find 的输出：一个很长的文件列表，且文件名长短不一(假如最长的文件名是 100 个字符)，你如何在 100 列插入 vertical bar (|)?

N,Ms/.*/\=printf('%-101s|', submatch(0))/
- N,M 指定替换的范围
- \= 表示使用 expression register, 这里相当于把 printf() 函数的输出导入到这里

TOhtml

vim 有一个内置的扩展 TOhtml, 可以将 vim 的 buffer 自动转成 html 文件，这个功能对于 vimdiff 时尤其有用。

:[range]TOhtml range 没有时默认是整个 buffer (vimdiff 时包括左右两个 window 哦)
:wqa 生成的 html 默认保存为 Diff.html
可以将生成的 html 内容直接粘贴到 jsbin.com, 然后就可以分享给他人

Debugging X11 with tcpdump

Posted on 2020-02-24 In graphics Views: Disqus:

X11 is designed as client-server mode. The communication between the X client and server complies with TCP protocol. Recently I have a Windows X server VcXsrv installed on my Windows 10 and I debug an OpenGL demo glxgears on the WSL2 with tcpdump.

Environment

WSL2 is equipped with its own networking interface like a virtual machine.

$ ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 172.28.233.193  netmask 255.255.240.0  broadcast 172.28.239.255
        inet6 fe80::215:5dff:fe27:9562  prefixlen 64  scopeid 0x20<link>
        ether 00:15:5d:27:95:62  txqueuelen 1000  (Ethernet)
        RX packets 221579  bytes 15160108 (15.1 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 306691  bytes 15714800233 (15.7 GB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 392  bytes 43340 (43.3 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 392  bytes 43340 (43.3 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

$ /mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe -C "ipconfig"

Windows IP Configuration


Ethernet adapter vEthernet (WSL):

   Connection-specific DNS Suffix  . : 
   Link-local IPv6 Address . . . . . : fe80::412b:ad3:5548:67ba%38
   IPv4 Address. . . . . . . . . . . : 172.28.224.1
   Subnet Mask . . . . . . . . . . . : 255.255.240.0
   Default Gateway . . . . . . . . . : 

Wireless LAN adapter ±¾µØÁ¬½Ó* 1:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . : 

Wireless LAN adapter ±¾µØÁ¬½Ó* 10:

   Media State . . . . . . . . . . . : Media disconnected
   Connection-specific DNS Suffix  . : 

Wireless LAN adapter WLAN:

   Connection-specific DNS Suffix  . : DHCP HOST
   Link-local IPv6 Address . . . . . : fe80::954b:e66f:4065:20af%20
   IPv4 Address. . . . . . . . . . . : 192.168.2.100
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 192.168.2.1

After starting up the X server VcXsrv, you need to export the environmnet variable DISPLAY on the WSL. In case of the vEthernet configuration changed after rebooting you’d better do it like this:

1	export DISPLAY=$(grep 'nameserver' /etc/resolv.conf \| awk '{print $2}'):0

NOTE: The Firewall between the host and WSL2 must be disabled or your X client can not connect VcXsrv.

Debugging

I trace the demo glxgears using gdb and tcpdump at the same time.

gdb

1	gdb -q -tui glxgears

tcpdump

1	sudo tcpdump -vvX not icmp and not arp and not udp and portrange 37900-37999 -w x110224.pcap

vv: verboser than -v
X: show the packet’s content
not icmp: filter out icmp packets
not arp: filter out arp packets
not udp: filter out udp packets
portrange 37900-37999: listening on the ports from 37900 to 37999
w x110224.pcap: save the packet captures into the file

1	tcpdump -X -r x110224.pcap

reading from file /home/luc/github/x110224.pcap, link-type EN10MB (Ethernet)
09:11:56.659816 IP 172.28.233.193.37950 > 172.28.224.1.x11: Flags [P.], seq 2796598646:2796598658, ack 1053772981, win 259, length 12
	0x0000:  4500 0034 1d84 4000 4006 fb43 ac1c e9c1  E..4..@.@..C....
	0x0010:  ac1c e001 943e 1770 a6b0 b576 3ecf 4cb5  .....>.p...v>.L.
	0x0020:  5018 0103 2223 0000 6200 0300 0400 0000  P..."#..b.......
	0x0030:  4452 4932                                DRI2
09:11:56.660218 IP 172.28.224.1.x11 > 172.28.233.193.37950: Flags [P.], seq 1:33, ack 12, win 8211, length 32
	0x0000:  4500 0048 d9d7 4000 8006 fedb ac1c e001  E..H..@.........
	0x0010:  ac1c e9c1 1770 943e 3ecf 4cb5 a6b0 b582  .....p.>>.L.....
	0x0020:  5018 2013 cf36 0000 0100 0a00 0000 0000  P....6..........
	0x0030:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0040:  0000 0000 0000 0000                      ........
09:11:56.660232 IP 172.28.233.193.37950 > 172.28.224.1.x11: Flags [.], ack 33, win 259, length 0
	0x0000:  4500 0028 1d85 4000 4006 fb4e ac1c e9c1  E..(..@.@..N....
	0x0010:  ac1c e001 943e 1770 a6b0 b582 3ecf 4cd5  .....>.p....>.L.
	0x0020:  5010 0103 2217 0000                      P..."...

what codes sends and receives these packets? The first two twenty-byted segments are IP header (20 bytes without option) and TCP header (20 bytes without option) separately in these packets.

The source code snippet:

Bool
XQueryExtension(
    register Display *dpy,
    _Xconst char *name,
    int *major_opcode,  /* RETURN */
    int *first_event,   /* RETURN */
    int *first_error)	/* RETURN */
{
    xQueryExtensionReply rep;
    register xQueryExtensionReq *req;

    LockDisplay(dpy);
    GetReq(QueryExtension, req);
    req->nbytes = name ? strlen(name) : 0;
    req->length += (req->nbytes+(unsigned)3)>>2;
    _XSend(dpy, name, (long)req->nbytes);
    (void) _XReply (dpy, (xReply *)&rep, 0, xTrue);
    *major_opcode = rep.major_opcode;
    *first_event = rep.first_event;
    *first_error = rep.first_error;
    UnlockDisplay(dpy);
    SyncHandle();
    return (rep.present);
}

The gdb log:

Starting program: /mnt/c/Users/lulu/Documents/github/demos/src/xdemos/glxgears 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, XextAddDisplay (extinfo=0x7ffff7e77380 <_dri2Info_data>, dpy=0x55555555d2a0, 
    ext_name=0x7ffff7e76780 <dri2ExtensionName> "DRI2", hooks=0x7ffff7e767a0 <dri2ExtensionHooks>, nevents=0, data=0x0)
    at extutil.c:103
XInitExtension (dpy=dpy@entry=0x55555555d2a0, name=name@entry=0x7ffff7e76780 <dri2ExtensionName> "DRI2")
    at InitExt.c:44
XQueryExtension (dpy=dpy@entry=0x55555555d2a0, name=name@entry=0x7ffff7e76780 <dri2ExtensionName> "DRI2", 
    major_opcode=major_opcode@entry=0x7fffffffd984, first_event=first_event@entry=0x7fffffffd988, 
    first_error=first_error@entry=0x7fffffffd98c) at QuExt.c:39
$2 = {
  reqType = 0x62, 
  pad = 0x0, 
  length = 0x3, 
  nbytes = 0x4, 
  pad1 = 0x0, 
  pad2 = 0x0
}
$3 = {
  type = 0x1, 
  pad1 = 0x0, 
  sequenceNumber = 0xa, 
  length = 0x0, 
  present = 0x0, 
  major_opcode = 0x0, 
  first_event = 0x0, 
  first_error = 0x0, 
  pad3 = 0x0, 
  pad4 = 0x0, 
  pad5 = 0x0, 
  pad6 = 0x0, 
  pad7 = 0x0
}

$2 is Request packet content to VcXsrv, $3 is Reply packet content from VcXsrv. Even that we can notice the three-way handshake of TCP from the zero-lengthed packet in x110224.pcap.