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Home / 🗃️ unorganized / general notes / How to Letterbox in UNIX-like Systems

How to Letterbox in UNIX-like Systems

Why would I want this

  • Screen resolution is non-standard, and I want to cut off some areas to make it look like typical screens

How it is done in Windows

On Windows this is done by the OS using the drivers or by the Intel Graphics Control Panel (in Intel GPUs)

References

How I did it

X Windowing System

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Notes:

  • Panning will crop render area, but it won't crop where the mouse can move.
  • Framebuffer size sets the
    • Without framebuffer size set to 1280x720, mouse will go down with transform

  • Transform translate values will affect how much the mouse can move in the specified direction

  • Functions

    • Mode
      • (Sets Data Resolution)
    • Transform (Resolution, Scale)
    • Framebuffer Size
      • (Sets Canvas Resolution) (i.e. Canvas)
    • Pan Resolution
      • (Sets the Canvas Pan) (Default: Whole Canvas)
    • Scale
      • (Sets the DPI Scale for Data)
      • It is only a shorthand for "transform"

  • If only transform is used (with scale), and if the render goes out of screen, moving the mouse out of screen resets scale to 1x1 and stretches.

    • And of course, without framebuffer, the mouse can go down
  • If transform and framebuffer is used, and if the render goes out of screen, moving the mouse out of screet resets scale to 1x1 and stretches

NOTE:

  • The monitor is set in 1280x720 mode.
  • But it stretches out to the same resolution that 1280x1024 mode takes.

In XRandR (X Rotate and Resize):

  • The following is the Transform Matrix
    • \(\begin{bmatrix} a & b & c \\ d & e & f \\ g & i & h \end{bmatrix}\)
    • The values in Transform Matrix are as follows:
      • a: Scale X * cos Angle
      • b: Scale X * sin Angle
      • c: Transform X
      • d: Scale Y * cos Angle
      • e: Scale Y * sin Angle
      • f: Transform Y
      • g: 0 (for some skewing?)
      • h: 0 (for some skewing?)
      • i: 1 (for some skewing?)
    • Basically, it can shift the perspectives, but not like transform it into a circle.
      • That is, it only works via keystone transformations (and that includes rotations and perspectives).
      • It exists for cases where the projector projects from an angle. (Remember TeachNext)
    • See more: https://unix.stackexchange.com/questions/774967/can-someone-please-explain-how-xrandr-scale-works

The Theory: Projective Mapping
Tools: LPMT (Little Projection Mapping Tool), XRandR Keystone Helper, Desmos (mentioned below in this page)

Modelines

  • Modes:
  • Modelines: Information about available modes

Modeline Syntax:

Modeline <"Name"> <Pixel Clock Frequency> <Horizontal Pixels> <Horizontal Sync Start> <Horizontal Sync End> <Total Horizontal Pixel Clocks> <Vertical Pixels> <Vertical Sync Start> <Vertical Sync End> <Total Vertical Pixel Clocks> [options]

That is, it contains:

  • Mode Name
  • Dot Clock (Pixel Clock)
  • Horizontal Pixels
  • Scan Sync Paramters (Handles Blanking Periods, etc.)
    • Horizontal Sync Start
    • Horizontal Sync End
    • Horizontal Total
  • Vertical Pixels (Handles Blanking Periods, etc.)
  • Scan Sync Parameters
    • Vertical Sync Start
    • Vertical Sync End
    • Vertical Total
  • Options (-HSync, +VSync, etc.)

You get modelines from EDID, but if they don't exist (like with TVs), we have to manually calculate them.

Timing Standards

  • GTF (Generalized Timing Formula)
  • DMT (Discrete Monitor Timings)
  • VESA CVT (Coordinated Video Timing)
  • VESA CVT-RB (Coordinated Video Timing - Reduced Blanking)
  • Manual Modelines

Programs

Modeline Calculators

  • cvt: Calculates VESA CVT (Coordinated Video Timing) modelines.
    • Multiple of 60Hz refresh rate required for reduced blanking (CVT-Reduce Blank / CVT-RB)
    • Warning: Refresh Rate 144.00 is not CVT standard (50, 60, 75 or 85Hz).
  • gtf: Calculates the GTF (Generalized Timing Formula) modelines.

Transform Calculators

  • https://www.desmos.com/calculator/d2ioksn1ms: This allows trying out values
    • Turns out all it does is rotate, resize and reflect
  • xrandr-keystone-helper: This allows moving keystones
    • It is different from the values provided in Desmos
      • Turns out Desmos and XRandr keep different outputs.
      • One of them shows the pan against the framebuffer, and the other shows the framebuffer against the pan. I don't know which shows which.
  • LPMT (Little Projection Mapping Tool)
Left-click: follow link, Right-click: select node, Scroll: zoom
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