Por que preciso de um tamanho de framebuffer maior que o tamanho do monitor quando a área de destino da transformação é menor

Question

Eu tentei remover 100px restantes & à direita na parte superior de uma tela 1366x768 (como abaixo) usando xrandr transformation. Mas recebi um erro sobre o tamanho insuficiente do framebuffer.

$ xrandr --output LVDS1 --fb 1366x768 --transform 1.171527,0.152543,-117.152659,0.000000,1.171527,0.000000,0.000000,0.000223,1.000000
xrandr: specified screen 1366x768 not large enough for output LVDS1 ( 1602x773+-118+0)

Então eu levantei o tamanho para o mencionado na mensagem 1602x773 ! Isso funciona:

xrandr --output LVDS1 --fb 1602x773 --transform 1.171527,0.152543,-117.152659,0.000000,1.171527,0.000000,0.000000,0.000223,1.000000

Para o que é essa área extra 1366x5+236x768+236x5 e por que é necessário?!

A área de destino da transformação é menor que a origem (tamanho de tela original)! Bem, posso dizer que eles têm a mesma área retangular.

Eu adicionei aqui minha configuração atual, pois ela pode estar relacionada a hardware, driver ou xorg.

### Background Info

Ubuntu 15.10

X.Org X Server 1.17.2
Release Date: 2015-06-16
X Protocol Version 11, Revision 0
Build Operating System: Linux 3.13.0-68-generic x86_64 Ubuntu
Current Operating System: Linux BlueSkies 4.2.0-25-generic #30-Ubuntu SMP Mon Jan 18 12:31:50 UTC 2016 x86_64
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-4.2.0-25-generic root=UUID=9f0530d5-e0eb-43fa-b99b-000fa742cf10 ro persistent quiet splash vt.handoff=7
Build Date: 12 November 2015  05:33:29PM
xorg-server 2:1.17.2-1ubuntu9.1 (For technical support please see http://www.ubuntu.com/support) 
Current version of pixman: 0.32.6
    Before reporting problems, check http://wiki.x.org
    to make sure that you have the latest version.

xrandr program version       1.4.3
Server reports RandR version 1.4

  *-display               
       description: VGA compatible controller
       product: 3rd Gen Core processor Graphics Controller
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 09
       width: 64 bits
       clock: 33MHz
       capabilities: msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0
       resources: irq:27 memory:f6400000-f67fffff memory:e0000000-efffffff ioport:f000(size=64)

### Base info

Screen 0: minimum 8 x 8, current 1366 x 768, maximum 32767 x 32767
LVDS1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
   1366x768      60.07*+  40.02  
   1360x768      59.80    59.96  
   1280x720      60.00  
   1024x768      60.00  
   1024x576      60.00  
   960x540       60.00  
   800x600       60.32    56.25  
   864x486       60.00  
   640x480       59.94  
   720x405       60.00  
   680x384       60.00  
   640x360       60.00  
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
DP3 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
VGA1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

disable_display  N
disable_power_well  1
disable_vtd_wa  N
edp_vswing  0
enable_cmd_parser  1
enable_execlists  0
enable_fbc  -1
enable_hangcheck  Y
enable_ips  1
enable_ppgtt  1
enable_psr  0
enable_rc6  3
fastboot  N
invert_brightness  0
load_detect_test  N
lvds_channel_mode  0
lvds_downclock  0
lvds_use_ssc  -1
mmio_debug  0
modeset  -1
nuclear_pageflip  N
panel_ignore_lid  1
prefault_disable  N
preliminary_hw_support  0
reset  Y
semaphores  -1
use_mmio_flip  0
vbt_sdvo_panel_type  -1
verbose_state_checks  Y