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40 lines
1.5 KiB
Markdown
40 lines
1.5 KiB
Markdown
4 years ago
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# Hacking!
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## Rows
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There are four kinds of `y`s: physical, rational, logical, and virtual. Physical
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and rational `y`s are independent of any particular plane. A physical `y`
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refers to a particular row of the terminal. A rational `y` refers to a particular
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row of the rendering area. They are related by:
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* physical `y` - margin `top` == rational `y`
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* rational `y` + margin `top` == physical `y`
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In the absence of a `top` margin, physical `y` == rational `y`.
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Logical and virtual `y`s are relative to a plane (possibly the standard plane).
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A logical `y` refers to a row of a plane, independent of scrolling. A virtual
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`y` refers to a row-sized chunk of the plane's framebuffer, which might be
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mapped to any row within the plane. They are related by:
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* (logical `y` + plane `logrow`) % plane `leny` == virtual `y`
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* (virtual `y` + plane `leny` - plane `logrow`) % plane `leny` == logical `y`
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All API points expressing a `y`, whether writing it (e.g. `ncplane_cursor_yx()`)
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or reading it (e.g. `ncplane_cursor_move_yx()`), are working with a logical `y`.
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The `y` member of an `ncplane` is also a logical `y`.
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Whenever we initiate a write past the end of the line, and the virtual `y` is
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equal to `ncplane->lenx - 1`, we must scroll. Scrolling:
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* plane `logrow` = (plane `logrow` + 1) % plane `leny`
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As a result, logical `y` is unchanged, but virtual `y` has advanced.
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Virtual `y` is useful for only two things:
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* Determining whether to scroll, and
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* Indexing into the plane's framebuffer
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Thus we usually keep `y` logical.
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