GLFEEDBACKBUFFER(3G)

💖 GLFEEDBACKBUFFER 💖

okay bestie, gather round, because we're talking about glFeedbackBuffer from the 3G manual — yes the OpenGL one, the iconic one, the one that lets you spy on what the GL would have drawn instead of actually drawing it. it's giving surveillance but make it ✨graphics✨.

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🌸 NAME 🌸

glFeedbackBuffer — controls feedback mode 🎀

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✨ C SPECIFICATION ✨

void glFeedbackBuffer( GLsizei size,
                       GLenum type,
                       GLfloat *buffer )

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🦋 PARAMETERS 🦋

• size — the max number of values that can be written into buffer. like, the vibe capacity. 📏
• type — a symbolic constant that describes what info gets returned for each vertex. the accepted ones are GL_2D, GL_3D, GL_3D_COLOR, GL_3D_COLOR_TEXTURE, and GL_4D_COLOR_TEXTURE. no others, periodt. 👑
• buffer — returns the feedback data. this is where the tea gets poured. 🍵

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💅 DESCRIPTION 💅

So glFeedbackBuffer controls feedback. and feedback, just like selection, is a GL mode. you select it by calling glRenderMode with GL_FEEDBACK. when the GL is in feedback mode, no pixels are produced by rasterization — instead, all the info about the primitives that would have been rasterized gets fed back to the application using the GL. it's the "not me being a renderer, me being a reporter" era. 😭

glFeedbackBuffer has three arguments: buffer is a pointer to an array of floating-point values that feedback info gets placed into. size tells you how big that array is. type is the symbolic constant describing what's fed back for each vertex.

🚨 important, no cap: glFeedbackBuffer must be issued before feedback mode is enabled (that's by calling glRenderMode with argument GL_FEEDBACK). setting GL_FEEDBACK without establishing the feedback buffer, or calling glFeedbackBuffer while the GL is already in feedback mode, is an error. don't do it. 🙈

When glRenderMode is called while in feedback mode, it returns the number of entries placed in the feedback array, and resets the feedback array pointer to the base of the feedback buffer. the returned value never exceeds size. and if the feedback data needed more room than was available in buffer, glRenderMode returns a negative value 💀. to take the GL out of feedback mode, call glRenderMode with a parameter value that isn't GL_FEEDBACK.

While in feedback mode, every primitive, bitmap, or pixel rectangle that would be rasterized generates a block of values that get copied into the feedback array. if doing that would make the number of entries exceed the max, the block gets partially written so as to fill the array (if there's any room left at all), and an overflow flag is set. each block starts with a code indicating the primitive type, then values describing the primitive's vertices and associated data. entries are also written for bitmaps and pixel rectangles.

Feedback happens after polygon culling and glPolygonMode interpretation of polygons has taken place, so polygons that are culled are not returned in the feedback buffer. it can also happen after polygons with more than three edges get broken up into triangles, if the GL implementation renders polygons by performing this decomposition. fyi. 🫶

The glPassThrough command can be used to insert a marker into the feedback buffer. see glPassThrough.

Following is the grammar for the blocks of values written into the feedback buffer. each primitive is indicated with a unique identifying value followed by some number of vertices. polygon entries include an integer value indicating how many vertices follow. a vertex is fed back as some number of floating-point values, as determined by type. colors are fed back as four values in RGBA mode and one value in color index mode.

feedbackList ← feedbackItem feedbackList | feedbackItem

feedbackItem ← point | lineSegment | polygon | bitmap |
pixelRectangle | passThru

point ← GL_POINT_TOKEN vertex

lineSegment ← GL_LINE_TOKEN vertex vertex | GL_LINE_RESET_TOKEN
vertex vertex

polygon ← GL_POLYGON_TOKEN n polySpec

polySpec ← polySpec vertex | vertex vertex vertex

bitmap ← GL_BITMAP_TOKEN vertex

pixelRectangle ← GL_DRAW_PIXEL_TOKEN vertex | GL_COPY_PIXEL_TOKEN
vertex

passThru ← GL_PASS_THROUGH_TOKEN value

vertex ← 2d | 3d | 3dColor | 3dColorTexture | 4dColorTexture

2d ← value value

3d ← value value value

3dColor  ← value value value color

3dColorTexture ← value value value color tex

4dColorTexture ← value value value value color tex

color ← rgba | index

rgba ← value value value value

index ← value

tex ← value value value value

value is a floating-point number, and n is a floating-point integer giving the number of vertices in the polygon. GL_POINT_TOKEN, GL_LINE_TOKEN, GL_LINE_RESET_TOKEN, GL_POLYGON_TOKEN, GL_BITMAP_TOKEN, GL_DRAW_PIXEL_TOKEN, GL_COPY_PIXEL_TOKEN and GL_PASS_THROUGH_TOKEN are symbolic floating-point constants. GL_LINE_RESET_TOKEN is returned whenever the line stipple pattern is reset. the data returned as a vertex depends on the feedback type. 👑

The following gives the correspondence between type and the number of values per vertex. k is 1 in color index mode and 4 in RGBA mode. (iconic variable, we stan 🎀)

• Type: GL_2D — Coordinates: x, y — Color: (none) — Texture: (none) — Total Number of Values: 2
• Type: GL_3D — Coordinates: x, y, z — Color: (none) — Texture: (none) — Total Number of Values: 3
• Type: GL_3D_COLOR — Coordinates: x, y, z — Color: k — Texture: (none) — Total Number of Values: 3+k
• Type: GL_3D_COLOR_TEXTURE — Coordinates: x, y, z — Color: k — Texture: 4 — Total Number of Values: 7+k
• Type: GL_4D_COLOR_TEXTURE — Coordinates: x, y, z, w — Color: k — Texture: 4 — Total Number of Values: 8+k

Feedback vertex coordinates are in window coordinates, except w, which is in clip coordinates. feedback colors are lighted, if lighting is enabled. feedback texture coordinates are generated, if texture coordinate generation is enabled. they are always transformed by the texture matrix. no exceptions bestie. 💜

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🎀 NOTES 🎀

glFeedbackBuffer, when used in a display list, is not compiled into the display list but is executed immediately. it simply refuses to be archived, main character energy fr. 🔥

When the GL_ARB_multitexture extension is supported, glFeedbackBuffer returns only the texture coordinates of texture unit GL_TEXTURE0_ARB. just the one. that's it. 🫶

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💀 ERRORS 💀

• GL_INVALID_ENUM is generated if type is not an accepted value.
• GL_INVALID_VALUE is generated if size is negative.
• GL_INVALID_OPERATION is generated if glFeedbackBuffer is called while the render mode is GL_FEEDBACK, or if glRenderMode is called with argument GL_FEEDBACK before glFeedbackBuffer is called at least once.
• GL_INVALID_OPERATION is generated if glFeedbackBuffer is executed between the execution of glBegin and the corresponding execution of glEnd.

🚨 all four of those? errors. actual errors. not a drill, not a vibe, not a little oopsie. 😭

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⭐ ASSOCIATED GETS ⭐

• glGet with argument GL_RENDER_MODE
• glGet with argument GL_FEEDBACK_BUFFER_POINTER
• glGet with argument GL_FEEDBACK_BUFFER_SIZE
• glGet with argument GL_FEEDBACK_BUFFER_TYPE

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🌈 SEE ALSO 🌈

glBegin(3G), glLineStipple(3G), glPassThrough(3G), glPolygonMode(3G), glRenderMode(3G), glSelectBuffer(3G)

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and that's the whole page bestie, every last flag and token accounted for, nothing skipped, nothing invented, we ate and left zero crumbs 💅✨

this was GLFEEDBACKBUFFER(3G) — the 3G manual, signed off the same way it started, because the glow is eternal. bye babe, go set up that feedback buffer before you enable feedback mode, i'm literally begging you 🎀🦋💖