GL_VERTEXID(3G)

💖 GL_VERTEXID 💖

okay bestie so this is a page from the 3G manual section (that's the OpenGL-adjacent girlies, iykyk) and we're talking about one very specific little variable. she's small, she's an integer, she's the main character of your vertex shader. let's get into it ✨

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🌸 NAME 🌸

gl_VertexID — literally just holds the index of the current vertex. that's it. that's her whole job 💅

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🦋 DECLARATION 🦋

in int gl_VertexID ;

(no cap, that stray space before the semicolon is in the original. we respect the original here 🫶)

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⭐ DESCRIPTION ⭐

so gl_VertexID is a vertex language input variable — a shader input, basically — and it holds an integer index for the vertex you're currently shading. think of it as the bouncer handing you a wristband number 🎀

where does that number come from? two ways, bestie:

• implicitly generated by glDrawArrays and other commands that do not reference the content of the GL_ELEMENT_ARRAY_BUFFER — you didn't ask for an index, the system just handed you one 💫
• explicitly generated from the content of the GL_ELEMENT_ARRAY_BUFFER, by commands such as glDrawElements — here the index comes straight out of the buffer 🌈

and here's the lil detail that matters: for the glDrawElements forms that take a basevertex, gl_VertexID will have this value added to the index from the buffer. so it's not just the raw buffer index — basevertex gets folded in. noted. memorised. obsessed 🔥

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✨ VERSION SUPPORT ✨

okay so the original had a whole giant table and we're gonna give you the same facts, just with more glitter and less spreadsheet energy 💀 the columns are OpenGL Shading Language Versions, the row is gl_VertexID:

• 1.10 — - (nope, not yet 🙈)
• 1.20 — - (still nope 😭)
• 1.30 — ✔ (she's here!! 🎉)
• 1.40 — ✔
• 1.50 — ✔
• 3.30 — ✔
• 4.00 — ✔
• 4.10 — ✔
• 4.20 — ✔
• 4.30 — ✔
• 4.40 — ✔
• 4.50 — ✔

so tldr: gl_VertexID said "not me in 1.10 and 1.20" and then showed up for everything from 1.30 onward and never left. consistent queen 👑

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🧚 SEE ALSO 🧚

• gl_InstanceID()

(her bestie. her ride-or-die. go read that one next, fr fr 🫶)

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💜 COPYRIGHT 💜

Copyright © 2011-2014 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.

Copyright © 2011-2014 Khronos Group

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okay that's the whole page bestie, go forth and index your vertices 💅✨

and a lil sign-off: this one came from [FIXME: source], dated 08/09/2021, and lives at GL_VERTEXID(3G) in the 3G manual. mwah 🎀💖