Kylin V10 窗口透明问题

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最近在看X11 Server的代码,顺手做下笔记。X11 Server的代码很古老了，有的是1993年写的，目前的X Server实现X.org是从最早的一个X Server实现XFree86 4.4 RC2版本(2004年)派生出来的。

X11 Server编译后会生成若干动态链接库形式的驱动程序和一个可执行程序Xorg. Xorg本质上是一个服务器程序，像其它所有服务器程序一样，它的main函数里有一个while循环，以保证在程序启动后，到程序退出前能够为客户端持续提供服务。Xorg的另一个特点是极具扩展性，所有动态链接库形式的DDX(Device Dependent X)驱动都是以Xorg Module的方式动态加载的，具体加载哪个DDX Driver由Xorg的Config文件配置。

同一个使用glfw的OpenGL程序在Kylin V10系统上可以修改alpha通道改变窗口背景的透明度，而在Ubuntu Focal Fossa系统上却不能？

Alpha通道控制窗口的透明度(transparency)。而X11窗口系统上，alpha通道是否可用，和X11 Server的RENDER扩展有关，下面来自glfw的代码是自明的(self-explanatory)

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GLFWbool _glfwIsVisualTransparentX11(Visual* visual)
{
    if (!_glfw.x11.xrender.available)
        return GLFW_FALSE;

    XRenderPictFormat* pf = XRenderFindVisualFormat(_glfw.x11.display, visual);
    return pf && pf->direct.alphaMask;
}

关于系统上的 X11 Server 的 RENDER 扩展的信息可以通过 xdpyinfo 命令查看(此命令输出很多信息)

  • Render formats

xdpyinfo -ext RENDER | awk '/Render formats/,/Screen formats/'

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 Render formats :
 pict format:
format id:    0x25
type:         Direct
depth:        1
alpha:         0 mask 0x1
red:           0 mask 0x0
green:         0 mask 0x0
blue:          0 mask 0x0
 pict format:
format id:    0x26
type:         Direct
depth:        8
alpha:         0 mask 0xff
red:           0 mask 0x0
green:         0 mask 0x0
blue:          0 mask 0x0
 pict format:
format id:    0x27
type:         Direct
depth:        4
alpha:         0 mask 0xf
red:           0 mask 0x0
green:         0 mask 0x0
blue:          0 mask 0x0
 pict format:
format id:    0x28
type:         Direct
depth:        32
alpha:        24 mask 0xff
red:          16 mask 0xff
green:         8 mask 0xff
blue:          0 mask 0xff
 pict format:
format id:    0x29
type:         Direct
depth:        32
alpha:         0 mask 0x0
red:          16 mask 0xff
green:         8 mask 0xff
blue:          0 mask 0xff
 pict format:
format id:    0x2a
type:         Direct
depth:        32
alpha:         0 mask 0xff
red:           8 mask 0xff
green:        16 mask 0xff
blue:         24 mask 0xff
 pict format:
format id:    0x2b
type:         Direct
depth:        32
alpha:         0 mask 0x0
red:           8 mask 0xff
green:        16 mask 0xff
blue:         24 mask 0xff
 pict format:
format id:    0x2c
type:         Direct
depth:        24
alpha:         0 mask 0x0
red:          16 mask 0xff
green:         8 mask 0xff
blue:          0 mask 0xff
 pict format:
format id:    0x2d
type:         Direct
depth:        24
alpha:         0 mask 0x0
red:           0 mask 0xff
green:         8 mask 0xff
blue:         16 mask 0xff
 pict format:
format id:    0x2e
type:         Direct
depth:        15
alpha:         0 mask 0x0
red:           8 mask 0xf
green:         4 mask 0xf
blue:          0 mask 0xf
 pict format:
format id:    0x2f
type:         Direct
depth:        15
alpha:         0 mask 0x0
red:           0 mask 0xf
green:         4 mask 0xf
blue:          8 mask 0xf
 pict format:
format id:    0x30
type:         Direct
depth:        15
alpha:         0 mask 0x0
red:          10 mask 0x1f
green:         5 mask 0x1f
blue:          0 mask 0x1f
 pict format:
format id:    0x31
type:         Direct
depth:        15
alpha:         0 mask 0x0
red:           0 mask 0x1f
green:         5 mask 0x1f
blue:         10 mask 0x1f
 pict format:
format id:    0x32
type:         Direct
depth:        16
alpha:         0 mask 0x0
red:           8 mask 0xf
green:         4 mask 0xf
blue:          0 mask 0xf
 pict format:
format id:    0x33
type:         Direct
depth:        16
alpha:         0 mask 0x0
red:           0 mask 0xf
green:         4 mask 0xf
blue:          8 mask 0xf
 pict format:
format id:    0x34
type:         Direct
depth:        16
alpha:         0 mask 0x0
red:          10 mask 0x1f
green:         5 mask 0x1f
blue:          0 mask 0x1f
 pict format:
format id:    0x35
type:         Direct
depth:        16
alpha:         0 mask 0x0
red:           0 mask 0x1f
green:         5 mask 0x1f
blue:         10 mask 0x1f
 pict format:
format id:    0x36
type:         Direct
depth:        16
alpha:        15 mask 0x1
red:          10 mask 0x1f
green:         5 mask 0x1f
blue:          0 mask 0x1f
 pict format:
format id:    0x37
type:         Direct
depth:        16
alpha:        15 mask 0x1
red:           0 mask 0x1f
green:         5 mask 0x1f
blue:         10 mask 0x1f
 pict format:
format id:    0x38
type:         Direct
depth:        16
alpha:         0 mask 0x0
red:          11 mask 0x1f
green:         5 mask 0x3f
blue:          0 mask 0x1f
 pict format:
format id:    0x39
type:         Direct
depth:        16
alpha:         0 mask 0x0
red:           0 mask 0x1f
green:         5 mask 0x3f
blue:         11 mask 0x1f
 pict format:
format id:    0x3a
type:         Direct
depth:        16
alpha:        12 mask 0xf
red:           8 mask 0xf
green:         4 mask 0xf
blue:          0 mask 0xf
 pict format:
format id:    0x3b
type:         Direct
depth:        16
alpha:        12 mask 0xf
red:           0 mask 0xf
green:         4 mask 0xf
blue:          8 mask 0xf
 pict format:
format id:    0x3c
type:         Direct
depth:        32
alpha:         0 mask 0x0
red:           0 mask 0xff
green:         8 mask 0xff
blue:         16 mask 0xff
 pict format:
format id:    0x3d
type:         Direct
depth:        32
alpha:        30 mask 0x3
red:          20 mask 0x3ff
green:        10 mask 0x3ff
blue:          0 mask 0x3ff
 pict format:
format id:    0x3e
type:         Direct
depth:        32
alpha:         0 mask 0x0
red:          20 mask 0x3ff
green:        10 mask 0x3ff
blue:          0 mask 0x3ff
 pict format:
format id:    0x3f
type:         Direct
depth:        32
alpha:        30 mask 0x3
red:           0 mask 0x3ff
green:        10 mask 0x3ff
blue:         20 mask 0x3ff
 pict format:
format id:    0x40
type:         Direct
depth:        32
alpha:         0 mask 0x0
red:           0 mask 0x3ff
green:        10 mask 0x3ff
blue:         20 mask 0x3ff
 Screen formats :
  • Screen formats

xdpyinfo -ext RENDER | awk '/Screen formats/,/depth formats/'

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Screen formats :
  Screen 0 (sub-pixel order Unknown)
    filters: nearest, bilinear, convolution, fast(nearest), good(bilinear), best(bilinear)
    visual format:
      visual id:      0x23
      pict format id: 0x2c
    visual format:
      visual id:      0x24
      pict format id: 0x2c
    visual format:
      visual id:      0x296
      pict format id: 0x2c
    visual format:
      visual id:      0x297
      pict format id: 0x2c
    visual format:
      visual id:      0x298
      pict format id: 0x2c
    visual format:
      visual id:      0x299
      pict format id: 0x2c
    visual format:
      visual id:      0x29a
      pict format id: 0x2c
    visual format:
      visual id:      0x29b
      pict format id: 0x2c
    visual format:
      visual id:      0x29c
      pict format id: 0x2c
    visual format:
      visual id:      0x29d
      pict format id: 0x2c
    visual format:
      visual id:      0x29e
      pict format id: 0x2c
    visual format:
      visual id:      0x29f
      pict format id: 0x2c
    visual format:
      visual id:      0x2a0
      pict format id: 0x2c
    visual format:
      visual id:      0x2a1
      pict format id: 0x2c
    visual format:
      visual id:      0x2a2
      pict format id: 0x2c
    visual format:
      visual id:      0x2a3
      pict format id: 0x2c
    visual format:
      visual id:      0x2a4
      pict format id: 0x2c
    visual format:
      visual id:      0x2a5
      pict format id: 0x2c
    visual format:
      visual id:      0x2a6
      pict format id: 0x2c
    visual format:
      visual id:      0x2a7
      pict format id: 0x2c
    visual format:
      visual id:      0x2a8
      pict format id: 0x2c
    visual format:
      visual id:      0x2a9
      pict format id: 0x2c
    visual format:
      visual id:      0x2aa
      pict format id: 0x2c
    visual format:
      visual id:      0x2ab
      pict format id: 0x2c
    visual format:
      visual id:      0x2ac
      pict format id: 0x2c
    visual format:
      visual id:      0x2ad
      pict format id: 0x2c
    visual format:
      visual id:      0x2ae
      pict format id: 0x2c
    visual format:
      visual id:      0x2af
      pict format id: 0x2c
    visual format:
      visual id:      0x2b0
      pict format id: 0x2c
    visual format:
      visual id:      0x2b1
      pict format id: 0x2c
    visual format:
      visual id:      0x2b2
      pict format id: 0x2c
    visual format:
      visual id:      0x2b3
      pict format id: 0x2c
    visual format:
      visual id:      0x2b4
      pict format id: 0x2c
    visual format:
      visual id:      0x2b5
      pict format id: 0x2c
    visual format:
      visual id:      0x2b6
      pict format id: 0x2c
    visual format:
      visual id:      0x2b7
      pict format id: 0x2c
    visual format:
      visual id:      0x2b8
      pict format id: 0x2c
    visual format:
      visual id:      0x2b9
      pict format id: 0x2c
    visual format:
      visual id:      0x2ba
      pict format id: 0x2c
    visual format:
      visual id:      0x2bb
      pict format id: 0x2c
    visual format:
      visual id:      0x2bc
      pict format id: 0x2c
    visual format:
      visual id:      0x2bd
      pict format id: 0x2c
    visual format:
      visual id:      0x2be
      pict format id: 0x2c
    visual format:
      visual id:      0x2bf
      pict format id: 0x2c
    visual format:
      visual id:      0x2c0
      pict format id: 0x2c
    visual format:
      visual id:      0x2c1
      pict format id: 0x2c
    visual format:
      visual id:      0x2c2
      pict format id: 0x2c
    visual format:
      visual id:      0x2c3
      pict format id: 0x2c
    visual format:
      visual id:      0x2c4
      pict format id: 0x2c
    visual format:
      visual id:      0x2c5
      pict format id: 0x2c
    visual format:
      visual id:      0x2c6
      pict format id: 0x2c
    visual format:
      visual id:      0x2c7
      pict format id: 0x2c
    visual format:
      visual id:      0x2c8
      pict format id: 0x2c
    visual format:
      visual id:      0x2c9
      pict format id: 0x2c
    visual format:
      visual id:      0x2ca
      pict format id: 0x2c
    visual format:
      visual id:      0x2cb
      pict format id: 0x2c
    visual format:
      visual id:      0x2cc
      pict format id: 0x2c
    visual format:
      visual id:      0x2cd
      pict format id: 0x2c
    visual format:
      visual id:      0x2ce
      pict format id: 0x2c
    visual format:
      visual id:      0x2cf
      pict format id: 0x2c
    visual format:
      visual id:      0x2d0
      pict format id: 0x2c
    visual format:
      visual id:      0x2d1
      pict format id: 0x2c
    visual format:
      visual id:      0x2d2
      pict format id: 0x2c
    visual format:
      visual id:      0x2d3
      pict format id: 0x2c
    visual format:
      visual id:      0x2d4
      pict format id: 0x2c
    visual format:
      visual id:      0x2d5
      pict format id: 0x2c
    visual format:
      visual id:      0x2d6
      pict format id: 0x2c
    visual format:
      visual id:      0x2d7
      pict format id: 0x2c
    visual format:
      visual id:      0x2d8
      pict format id: 0x2c
    visual format:
      visual id:      0x2d9
      pict format id: 0x2c
    visual format:
      visual id:      0x2da
      pict format id: 0x2c
    visual format:
      visual id:      0x2db
      pict format id: 0x2c
    visual format:
      visual id:      0x2dc
      pict format id: 0x2c
    visual format:
      visual id:      0x2dd
      pict format id: 0x2c
    visual format:
      visual id:      0x2de
      pict format id: 0x2c
    visual format:
      visual id:      0x2df
      pict format id: 0x2c
    visual format:
      visual id:      0x2e0
      pict format id: 0x2c
    visual format:
      visual id:      0x2e1
      pict format id: 0x2c
    visual format:
      visual id:      0x2e2
      pict format id: 0x2c
    visual format:
      visual id:      0x2e3
      pict format id: 0x2c
    visual format:
      visual id:      0x2e4
      pict format id: 0x2c
    visual format:
      visual id:      0x2e5
      pict format id: 0x2c
    visual format:
      visual id:      0x2e6
      pict format id: 0x2c
    visual format:
      visual id:      0x2e7
      pict format id: 0x2c
    visual format:
      visual id:      0x2e8
      pict format id: 0x2c
    visual format:
      visual id:      0x2e9
      pict format id: 0x2c
    visual format:
      visual id:      0x2ea
      pict format id: 0x2c
    visual format:
      visual id:      0x2eb
      pict format id: 0x2c
    visual format:
      visual id:      0x2ec
      pict format id: 0x2c
    visual format:
      visual id:      0x2ed
      pict format id: 0x2c
    visual format:
      visual id:      0x2ee
      pict format id: 0x2c
    visual format:
      visual id:      0x2ef
      pict format id: 0x2c
    visual format:
      visual id:      0x2f0
      pict format id: 0x2c
    visual format:
      visual id:      0x2f1
      pict format id: 0x2c
    visual format:
      visual id:      0x2f2
      pict format id: 0x2c
    visual format:
      visual id:      0x2f3
      pict format id: 0x2c
    visual format:
      visual id:      0x2f4
      pict format id: 0x2c
    visual format:
      visual id:      0x2f5
      pict format id: 0x2c
    visual format:
      visual id:      0x2f6
      pict format id: 0x2c
    visual format:
      visual id:      0x2f7
      pict format id: 0x2c
    visual format:
      visual id:      0x2f8
      pict format id: 0x2c
    visual format:
      visual id:      0x2f9
      pict format id: 0x2c
    visual format:
      visual id:      0x2fa
      pict format id: 0x2c
    visual format:
      visual id:      0x2fb
      pict format id: 0x2c
    visual format:
      visual id:      0x2fc
      pict format id: 0x2c
    visual format:
      visual id:      0x2fd
      pict format id: 0x2c
    visual format:
      visual id:      0x2fe
      pict format id: 0x2c
    visual format:
      visual id:      0x2ff
      pict format id: 0x2c
    visual format:
      visual id:      0x300
      pict format id: 0x2c
    visual format:
      visual id:      0x301
      pict format id: 0x2c
    visual format:
      visual id:      0x302
      pict format id: 0x2c
    visual format:
      visual id:      0x303
      pict format id: 0x2c
    visual format:
      visual id:      0x304
      pict format id: 0x2c
    visual format:
      visual id:      0x305
      pict format id: 0x2c
    visual format:
      visual id:      0x306
      pict format id: 0x2c
    visual format:
      visual id:      0x307
      pict format id: 0x2c
    visual format:
      visual id:      0x308
      pict format id: 0x2c
    visual format:
      visual id:      0x309
      pict format id: 0x2c
    visual format:
      visual id:      0x30a
      pict format id: 0x2c
    visual format:
      visual id:      0x30b
      pict format id: 0x2c
    visual format:
      visual id:      0x30c
      pict format id: 0x2c
    visual format:
      visual id:      0x30d
      pict format id: 0x2c
    visual format:
      visual id:      0x30e
      pict format id: 0x2c
    visual format:
      visual id:      0x30f
      pict format id: 0x2c
    visual format:
      visual id:      0x310
      pict format id: 0x2c
    visual format:
      visual id:      0x311
      pict format id: 0x2c
    visual format:
      visual id:      0x312
      pict format id: 0x2c
    visual format:
      visual id:      0x313
      pict format id: 0x2c
    visual format:
      visual id:      0x314
      pict format id: 0x2c
    visual format:
      visual id:      0x315
      pict format id: 0x2c
    visual format:
      visual id:      0x316
      pict format id: 0x2c
    visual format:
      visual id:      0x317
      pict format id: 0x2c
    visual format:
      visual id:      0x318
      pict format id: 0x2c
    visual format:
      visual id:      0x319
      pict format id: 0x2c
    visual format:
      visual id:      0x31a
      pict format id: 0x2c
    visual format:
      visual id:      0x31b
      pict format id: 0x2c
    visual format:
      visual id:      0x31c
      pict format id: 0x2c
    visual format:
      visual id:      0x31d
      pict format id: 0x2c
    visual format:
      visual id:      0x31e
      pict format id: 0x2c
    visual format:
      visual id:      0x31f
      pict format id: 0x2c
    visual format:
      visual id:      0x320
      pict format id: 0x2c
    visual format:
      visual id:      0x321
      pict format id: 0x2c
    visual format:
      visual id:      0x322
      pict format id: 0x2c
    visual format:
      visual id:      0x323
      pict format id: 0x2c
    visual format:
      visual id:      0x324
      pict format id: 0x2c
    visual format:
      visual id:      0x325
      pict format id: 0x2c
    visual format:
      visual id:      0x326
      pict format id: 0x2c
    visual format:
      visual id:      0x327
      pict format id: 0x2c
    visual format:
      visual id:      0x328
      pict format id: 0x2c
    visual format:
      visual id:      0x329
      pict format id: 0x2c
    visual format:
      visual id:      0x32a
      pict format id: 0x2c
    visual format:
      visual id:      0x32b
      pict format id: 0x2c
    visual format:
      visual id:      0x32c
      pict format id: 0x2c
    visual format:
      visual id:      0x32d
      pict format id: 0x2c
    visual format:
      visual id:      0x32e
      pict format id: 0x2c
    visual format:
      visual id:      0x32f
      pict format id: 0x2c
    visual format:
      visual id:      0x330
      pict format id: 0x2c
    visual format:
      visual id:      0x331
      pict format id: 0x2c
    visual format:
      visual id:      0x332
      pict format id: 0x2c
    visual format:
      visual id:      0x333
      pict format id: 0x2c
    visual format:
      visual id:      0x334
      pict format id: 0x2c
    visual format:
      visual id:      0x335
      pict format id: 0x2c
    visual format:
      visual id:      0x336
      pict format id: 0x2c
    visual format:
      visual id:      0x337
      pict format id: 0x2c
    visual format:
      visual id:      0x338
      pict format id: 0x2c
    visual format:
      visual id:      0x339
      pict format id: 0x2c
    visual format:
      visual id:      0x33a
      pict format id: 0x2c
    visual format:
      visual id:      0x33b
      pict format id: 0x2c
    visual format:
      visual id:      0x33c
      pict format id: 0x2c
    visual format:
      visual id:      0x33d
      pict format id: 0x2c
    visual format:
      visual id:      0x33e
      pict format id: 0x2c
    visual format:
      visual id:      0x33f
      pict format id: 0x2c
    visual format:
      visual id:      0x340
      pict format id: 0x2c
    visual format:
      visual id:      0x341
      pict format id: 0x2c
    visual format:
      visual id:      0x342
      pict format id: 0x2c
    visual format:
      visual id:      0x343
      pict format id: 0x2c
    visual format:
      visual id:      0x344
      pict format id: 0x2c
    visual format:
      visual id:      0x345
      pict format id: 0x2c
    visual format:
      visual id:      0x346
      pict format id: 0x2c
    visual format:
      visual id:      0x347
      pict format id: 0x2c
    visual format:
      visual id:      0x348
      pict format id: 0x2c
    visual format:
      visual id:      0x349
      pict format id: 0x2c
    visual format:
      visual id:      0x34a
      pict format id: 0x2c
    visual format:
      visual id:      0x34b
      pict format id: 0x2c
    visual format:
      visual id:      0x34c
      pict format id: 0x2c
    visual format:
      visual id:      0x34d
      pict format id: 0x2c
    visual format:
      visual id:      0x34e
      pict format id: 0x2c
    visual format:
      visual id:      0x34f
      pict format id: 0x2c
    visual format:
      visual id:      0x350
      pict format id: 0x2c
    visual format:
      visual id:      0x351
      pict format id: 0x2c
    visual format:
      visual id:      0x352
      pict format id: 0x2c
    visual format:
      visual id:      0x353
      pict format id: 0x2c
    visual format:
      visual id:      0x354
      pict format id: 0x2c
    visual format:
      visual id:      0x355
      pict format id: 0x2c
    visual format:
      visual id:      0x356
      pict format id: 0x2c
    visual format:
      visual id:      0x357
      pict format id: 0x2c
    visual format:
      visual id:      0x358
      pict format id: 0x2c
    visual format:
      visual id:      0x359
      pict format id: 0x2c
    visual format:
      visual id:      0x35a
      pict format id: 0x2c
    visual format:
      visual id:      0x35b
      pict format id: 0x2c
    visual format:
      visual id:      0x35c
      pict format id: 0x2c
    visual format:
      visual id:      0x35d
      pict format id: 0x2c
    visual format:
      visual id:      0x35e
      pict format id: 0x2c
    visual format:
      visual id:      0x35f
      pict format id: 0x2c
    visual format:
      visual id:      0x360
      pict format id: 0x2c
    visual format:
      visual id:      0x361
      pict format id: 0x2c
    visual format:
      visual id:      0x362
      pict format id: 0x2c
    visual format:
      visual id:      0x363
      pict format id: 0x2c
    visual format:
      visual id:      0x364
      pict format id: 0x2c
    visual format:
      visual id:      0x365
      pict format id: 0x2c
    visual format:
      visual id:      0x366
      pict format id: 0x2c
    visual format:
      visual id:      0x367
      pict format id: 0x2c
    visual format:
      visual id:      0x368
      pict format id: 0x2c
    visual format:
      visual id:      0x369
      pict format id: 0x2c
    visual format:
      visual id:      0x36a
      pict format id: 0x2c
    visual format:
      visual id:      0x36b
      pict format id: 0x2c
    visual format:
      visual id:      0x36c
      pict format id: 0x2c
    visual format:
      visual id:      0x36d
      pict format id: 0x2c
    visual format:
      visual id:      0x36e
      pict format id: 0x2c
    visual format:
      visual id:      0x36f
      pict format id: 0x2c
    visual format:
      visual id:      0x370
      pict format id: 0x2c
    visual format:
      visual id:      0x371
      pict format id: 0x2c
    visual format:
      visual id:      0x372
      pict format id: 0x2c
    visual format:
      visual id:      0x373
      pict format id: 0x2c
    visual format:
      visual id:      0x374
      pict format id: 0x2c
    visual format:
      visual id:      0x375
      pict format id: 0x2c
    visual format:
      visual id:      0x376
      pict format id: 0x2c
    visual format:
      visual id:      0x377
      pict format id: 0x2c
    visual format:
      visual id:      0x378
      pict format id: 0x2c
    visual format:
      visual id:      0x379
      pict format id: 0x2c
    visual format:
      visual id:      0x37a
      pict format id: 0x2c
    visual format:
      visual id:      0x37b
      pict format id: 0x2c
    visual format:
      visual id:      0x37c
      pict format id: 0x2c
    visual format:
      visual id:      0x37d
      pict format id: 0x2c
    visual format:
      visual id:      0x37e
      pict format id: 0x2c
    visual format:
      visual id:      0x37f
      pict format id: 0x2c
    visual format:
      visual id:      0x380
      pict format id: 0x2c
    visual format:
      visual id:      0x381
      pict format id: 0x2c
    visual format:
      visual id:      0x382
      pict format id: 0x2c
    visual format:
      visual id:      0x383
      pict format id: 0x2c
    visual format:
      visual id:      0x5b
      pict format id: 0x28
    visual format:
      visual id:      0x384
      pict format id: 0x28
    visual format:
      visual id:      0x385
      pict format id: 0x28
    visual format:
      visual id:      0x386
      pict format id: 0x28
    visual format:
      visual id:      0x387
      pict format id: 0x28
    visual format:
      visual id:      0x388
      pict format id: 0x28
    visual format:
      visual id:      0x389
      pict format id: 0x28
    visual format:
      visual id:      0x38a
      pict format id: 0x28
    visual format:
      visual id:      0x38b
      pict format id: 0x28
    visual format:
      visual id:      0x38c
      pict format id: 0x28
    visual format:
      visual id:      0x38d
      pict format id: 0x28
    visual format:
      visual id:      0x38e
      pict format id: 0x28
    visual format:
      visual id:      0x38f
      pict format id: 0x28
    visual format:
      visual id:      0x390
      pict format id: 0x28
    visual format:
      visual id:      0x391
      pict format id: 0x28
    visual format:
      visual id:      0x392
      pict format id: 0x28
    visual format:
      visual id:      0x393
      pict format id: 0x28
    visual format:
      visual id:      0x394
      pict format id: 0x28
    visual format:
      visual id:      0x395
      pict format id: 0x28
    visual format:
      visual id:      0x396
      pict format id: 0x28
    visual format:
      visual id:      0x397
      pict format id: 0x28
    visual format:
      visual id:      0x398
      pict format id: 0x28
    visual format:
      visual id:      0x399
      pict format id: 0x28
    visual format:
      visual id:      0x39a
      pict format id: 0x28
    visual format:
      visual id:      0x39b
      pict format id: 0x28
    visual format:
      visual id:      0x39c
      pict format id: 0x28
    visual format:
      visual id:      0x39d
      pict format id: 0x28
    visual format:
      visual id:      0x39e
      pict format id: 0x28
    visual format:
      visual id:      0x39f
      pict format id: 0x28
    visual format:
      visual id:      0x3a0
      pict format id: 0x28
   depth formats:

上面使用 xdpyinfo 命令得到两张表：

  • Render formats 表列出了所有 RENDER 扩展支持的颜色格式的信息，包括它们的位深 (depth), 以及每个 Component 的位深 (mask)
  • Screen formats 表实际上列出的是所有 X Visual 与 Render formats 的绑定关系

接下来只需要查看一下程序(或者 GLFW )选取的 X Visual 的 Render format 的 Alpha 分量的位深是否为 0。 0 意味着 GLFW 获取的 GLFW_TRANSPARENT_FRAMEBUFFER 属性是FALSE, 也就意味着 GLFW 创建的这个窗口中，无法通过修改颜色的 Alpha 分量来改变窗口的透明度, 因为应用的窗口和桌面背景的融合(你是否能透过窗口看到桌面)是 X11 控制的。

我系统上(WSL Ubuntu 20.04)绝大部分VISUAL对应的都是这个ID为0x2c的pict format, 所以我不能通过修改alpha通道改变窗口背景或内容的透明度，除非我强制让glfw选择其它的VISUAL(对应的pict format不是0x2c)。

为了确定这个问题的确是VISUAL惹的祸，我想摆脱glfw，在单纯的GLX环境下测试一下，正好mesa/demos这个工程下有一个glxgears_fbconfig，这个demo允许通过GLX_VISUAL_ID来指定你想要的VISUAL，我分别使用下面两个VISUAL进行测试

  • visual id: 0x5b, pict format id 0x28

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    pict format:
          format id:    0x28
          type:         Direct
          depth:        32
          alpha:        24 mask 0xff
          red:          16 mask 0xff
          green:         8 mask 0xff
          blue:          0 mask 0xff

  • visual id: 0x23, pict format id 0x2c

注意，为了更清楚地看出不同，我使用glClearBufferfv将背景设置为半透明

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diff --git a/src/xdemos/glxgears_fbconfig.c b/src/xdemos/glxgears_fbconfig.c
index 8806dd1c..7d6cbd29 100644
--- a/src/xdemos/glxgears_fbconfig.c
+++ b/src/xdemos/glxgears_fbconfig.c
@@ -35,0 +36 @@
+#define GL_GLEXT_PROTOTYPES
@@ -242,0 +244 @@ draw(void)
+    static const GLfloat bg[] = {0.0, 0.0, 0.0, 0.5};
+    @@ -243,0 +246 @@ draw(void)
+    +   glClearBufferfv(GL_COLOR, 0, bg);
+    @@ -439,0 +443 @@ make_window( Display *dpy, const char *name,
+    +            GLX_VISUAL_ID,     0x23,)

测试结果如下:

VisualID=0x23
VisualID=0x5b

从测试结果看，在VisualID=0x23的情况下，这个VISUAL对应的pict format没有alpha通道，所以修改bg的alpha值不会对窗口背景的透明度有任何影响。

glx visual 和 fbconfig 不是一回事

所以 GLX_VISUAL_ID 和 GLX_FBCONFIG_ID 也不是一回事， 虽然它俩都是 XID, 但 mesa 的实现虽然将 glx visual 和 fbconfig 统一用 glx_config 来表示, 但

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_GLX_PUBLIC XVisualInfo *
glXChooseVisual(Display * dpy, int screen, int *attribList);

_GLX_PUBLIC GLXFBConfig *
glXChooseFBConfig(Display * dpy, int screen, const int *attribList, int *nitems)

是完全不同的实现，而且 mesa 的 glXChooseVisual() 实际上无法通过指定 GLX_VISUAL_ID 来选择某个特定的 X Visual (这可能是一个 Bug)。