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[PR #4877] Optimize rendering runs of spaces when there is no visual change #26008

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opened 2026-01-31 09:13:17 +00:00 by claunia · 0 comments
claunia commented 2026-01-31 09:13:17 +00:00
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Original Pull Request: https://github.com/microsoft/terminal/pull/4877

State: closed
Merged: Yes

cmatrix is somewhat of a pathological case for our infrastructure: it
prints out a bunch of green and white characters and then updates them a
million times a second.

It also maintains a column of space between every green character. When
it prints this column, it prints it in "default" or "white". This ends
up making runs of text that look like this:

(def: G=green B=bright white W=white *=matrix char =space)

G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W

As characters trickle in:

G*W G*W G*W G*W G*W G*W G*W B*W
G*W G*W G*W G*W G*W G*W G*W G W
G*W G*W G*W B*W G*W G*W G*W G W
G*W B*W G*W G W G*W G*W G*W G*W
G*W G W G*W G W G*W B*W G*W G*W
B*W G W G*W G W G*W G W B*W G*W
G W G W G*W G W G*W G W G W B*W
G W G W B*W G W G*W G W G W G W

Every one of those color transitions causes us to break up the run of
text and start rendering it again. This impacts GDI, Direct2D and
ConPTY. In the example above, there are 120 runs.

The problem is, printing a space doesn't use the foreground color!

This commit introduces an optimization. When we're about to break a text
cluster because its attributes changed, we make sure that it's not just
filled with spaces and doesn't differ in any visually-meaningful way
(like underline or strikethrough, considering global invert state).

This lets us optimize both the rendering and the PTY output to look
like this:

G*   *   *   *   *   *   *  B*G
G*   *   *   *   *   *   *
G*   *   *  B*G  *   *   *
G*  B*G  *       *   *   *   *
G*       *       *  B*G  *   *
B*G      *       *      B*G  *
G        *       *          B*G
G       B*G      *

Text will be printed at best line-by-line and at worst only when the
visible properties of the screen actually change. In the example
above, there are only 21 runs.

This speeds up cmatrix remarkably.

PR Checklist

**Original Pull Request:** https://github.com/microsoft/terminal/pull/4877 **State:** closed **Merged:** Yes --- cmatrix is somewhat of a pathological case for our infrastructure: it prints out a bunch of green and white characters and then updates them a million times a second. It also maintains a column of space between every green character. When it prints this column, it prints it in "default" or "white". This ends up making runs of text that look like this: `(def: G=green B=bright white W=white *=matrix char =space)` ``` G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W G W ``` As characters trickle in: ``` G*W G*W G*W G*W G*W G*W G*W B*W G*W G*W G*W G*W G*W G*W G*W G W G*W G*W G*W B*W G*W G*W G*W G W G*W B*W G*W G W G*W G*W G*W G*W G*W G W G*W G W G*W B*W G*W G*W B*W G W G*W G W G*W G W B*W G*W G W G W G*W G W G*W G W G W B*W G W G W B*W G W G*W G W G W G W ``` Every one of those color transitions causes us to break up the run of text and start rendering it again. This impacts GDI, Direct2D *and* ConPTY. In the example above, there are 120 runs. The problem is, printing a space doesn't **use** the foreground color! This commit introduces an optimization. When we're about to break a text cluster because its attributes changed, we make sure that it's not just filled with spaces and doesn't differ in any visually-meaningful way (like underline or strikethrough, considering global invert state). This lets us optimize both the rendering _and_ the PTY output to look like this: ``` G* * * * * * * B*G G* * * * * * * G* * * B*G * * * G* B*G * * * * * G* * * B*G * * B*G * * B*G * G * * B*G G B*G * ``` Text will be printed at best line-by-line and at worst only when the visible properties of the screen actually change. In the example above, there are only 21 runs. This speeds up cmatrix remarkably. ## PR Checklist * [x] Related to #1064 * [x] CLA signed * [ ] Tests added/passed (I haven't run these yet.) * [ ] Requires documentation to be updated * [x] Core contributee
claunia added the pull-request label 2026-01-31 09:13:17 +00:00
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Reference: starred/terminal#26008
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