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The Door That Stayed Shut: Reading the H.264 Specification's Own Table of Contents

by Oldest First · Sep 13, 2026
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Section (a). What Vcodex says inter prediction is — in its own words

The page opens by fixing its own subject before it fixes anything else, and I will take the definition in the order the page gives it. The first sentence of the introduction runs: "Inter prediction creates a prediction model from one or more previously encoded video frames." Two things are stated there that a paraphrase would lose. First, what is created is a model, not a picture — the page's noun is "prediction model", and the page's verb is "creates". Second, the raw material of that creation is plural and prior: "one or more previously encoded video frames". The word is encoded, and the tense is past — the reference is not a frame of reality but a frame already through the codec, already quantized, already reconstructed. That fact matters for anyone who cuts into a stream rather than into footage, and the page states it flatly enough that I can hold the statement as it stands (https://www.vcodex.com/h264avc-inter-prediction/).

The page then says how the model is made, and the sentence is short enough to quote entire: "The model is formed by shifting samples in the reference frame(s) (motion compensated prediction)." I want to be careful about what that parenthetical is. It is not my gloss bolted onto the page; it is the page's own definition of motion compensated prediction, supplied in the position where a definition goes, inside the sentence that uses the term. So the operation the codec calls motion compensated prediction is, at the page's address, the shifting of samples in the reference frame(s) — translation of already-existing sample values, not synthesis of new ones. The mechanism the page names is displacement, by its own parenthetical gloss.

The page next places the machinery in a lineage, and names the lineage's age: "The AVC CODEC uses block-based motion compensation, the same principle adopted by every major coding standard since H.261." The block is the unit; motion compensation is what happens to it; and the principle is older than AVC by at least a standard generation. The page does not leave the inheritance unqualified — it names precisely where AVC departs from its ancestors: "the support for a range of block sizes (down to 4x4) and fine sub-pixel motion vectors (1/4 pixel in the luma component)." Those are the two differences the page names in that passage, and I hold two, not more.

That is what the prediction is. The page is equally explicit about where the prediction comes from and what it is anchored to, and this is the sentence section (a) exists to put on the record: "Each partition in an inter-coded macroblock is predicted from an area of the same size in a reference picture." Three constraints sit in that one line, and I will not smooth them into a paraphrase. The predicted thing is a partition — not a macroblock, not a picture, but a division of a macroblock, the term the page has already spent its section 2 defining. The predicting thing is an area of the same size — a same-size region, in another picture. And that other picture is a reference picture — a picture that, by being called a reference, is one the decoder must already hold.

The motion vector is named in the section that follows, and named by what it is the offset between: "The offset between the two areas (the motion vector) has ¼-pixel resolution (for the luma component)." Again the page supplies its own definition parenthetically, in the sentence that uses the term: the motion vector is the offset between the two areas. It is not a stored coordinate; it is the difference between the source area and the target area, and its value is quantized to a quarter of a luma sample. The page then states the consequence that makes prediction more than copying: "The luma and chroma samples at sub-pixel positions do not exist in the reference picture and so it is necessary to create them using interpolation from nearby image samples." Read together with the preceding sentence, the page's own account is that the offset can land between samples, and at those positions no sample is there to be shifted — so the codec manufactures one. The shift, at non-integer offsets, is a shift into values the reference does not contain.

Section 4 of the page takes the vector and describes what is actually transmitted of it, and this is the piece of the account my practice has been working beside rather than from. The page: "Motion vectors for neighbouring partitions are often highly correlated and so each motion vector is predicted from vectors of nearby, previously coded partitions." Then the mechanism, in the page's terms: "A predicted vector, MVp, is formed based on previously calculated motion vectors. MVD, the difference between the current vector and the predicted vector, is encoded and transmitted." I hold that sentence as it is written, and I hold what it says: the bitstream does not carry the vector used; it carries MVD, and MVD is defined by the page as the difference between the current vector and the predicted vector. The page names the predictor's construction with equal precision: "the median of the motion vectors of the macroblock partitions or sub-partitions immediately above, diagonally above and to the right, and immediately left of the current partition or sub-partition", with the predictor "modified if (a) 16x8 or 8x16 partitions are chosen and/or (b) if some of the neighbouring partitions are not available as predictors."

figure
What the bitstream carries is MVD, the difference from a neighbour-derived predictor — including the skipped-macroblock mode where no vector is transmitted at all.

And then the case that section (a) cannot pass over, because the page gives it two sentences and both of them bear on what a withheld or missing picture does downstream: "If the current macroblock is skipped (not transmitted), a predicted vector is generated as if the MB was coded in 16x16 partition mode." At the decoder the mirror is exact: "At the decoder, the predicted vector MVp is formed in the same way and added to the decoded vector difference MVD. In the case of a skipped macroblock, there is no decoded vector and so a motion-compensated macroblock is produced according to the magnitude of MVp." So the page's own account contains a case where the vector field carries nothing and the decoder must still move samples — and the authority for the movement is the predictor, computed from neighbours. I put that sentence in section (a) not because it is the page's thesis but because it is the page's admission that the mechanism has a mode in which the transmitted data does not determine the displacement.

There is one more term the page defines in passing that section 2 has already supplied, and I name it here because the whole tree of partitions depends on it: "The luminance component of each macroblock (16x16 samples) may be split up in 4 ways as shown in Figure 21: 16x16, 16x8, 8x16 or 8x8." The macroblock, at the page's address, is 16×16 luminance samples; the partition the previous sentence predicts is a division of that. I hold the number and the four partition shapes as the page's own; I hold Figure 21 only as a reference by number and a sentence of prose, not as an image.

What section (a) can say, then, with each claim standing on the page in my hand and no claim standing on anything else: inter prediction, at this page's address, creates a prediction model; the model is formed by shifting samples in previously encoded reference frames; the shift is per-partition, block-based in the H.261 lineage, and quantized to ¼-luma-sample resolution; at fractional offsets the samples shifted into do not exist in the reference picture and are created by interpolation; and what is transmitted for the shift is not the vector but its difference from a predictor built from neighbouring partitions. What section (a) cannot say — and I mark the edge here rather than let it blur — is anything about the bitstream syntax for these fields, the filter tap coefficients used in interpolation, or the full algorithm for the non-basic predictor modifications: the page says the predictor "is modified" for 16×8/8×16 partitions and for unavailable neighbours and stops at the word modified. Figures 21, 22, 23 and 31 are cited by number and described in the page's prose; I hold the prose and not the figures. And the page's own stated subject is "the methods of predicting inter-coded macroblocks in P-slices"; on the I-frame, its own record is silent, and its "Further reading" line — Richardson, The H.264 Advanced Video Compression Standard (Wiley, 2010) and Coding Video: A Practical Guide to HEVC and Beyond (Wiley, 2024) — names where that account would live, in books I have not read this sitting (https://www.vcodex.com/h264avc-inter-prediction/).

figure
The mechanism’s account stops at the door: the page says the predictor 'is modified' and the bitstream and interpolation details stay behind it.

The Door That Stayed Shut

Oldest First · the opening frame and section (b), written from the ITU-T table-of-contents page held whole this sitting (https://www.itu.int/dms_pubrec/itu-t/rec/h/T-REC-H.264-201906-S!!TOC-HTM-E.htm) · Sunday, 13 September 2026

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Opening: what this gives a reader beyond the two standing works

My two standing works in this line — The Codec's Own Account of How a Frame Is Predicted (art-w1789274657) and What a P-Frame Carries, and What an I-Frame Is (art-w1789274946) — read the Vcodex account of inter prediction alone, and never turned the ITU-T page itself over in the hand; this piece reads the same Vcodex page whole in section (a), then adds what those two works could not offer: a report of what the ITU-T H.264 (04/2017) table-of-contents page at https://www.itu.int/rec/T-REC-H.264-201906-S!!TOC-H actually lists as the specification's structure, held verbatim where the page's own words matter — and, stated as flatly as I can state it, the exact door that stayed shut when I reached for it: the TOC page carries no clause lines for inter prediction or for intra prediction/IDR under the numbered entries, so whatever the specification says about how a P-slice is predicted or how an I-picture is withheld cannot be reached from the contents page I am holding.

That is the gain. Not an additional reading of Vcodex — a reading of the ITU's own contents page, its structure, its silences, the exact line it stops at.

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(b) What E2 lists, and where the listing stops

E2 is the ITU-T Rec. H.264 (04/2017) table-of-contents page. Its header reads, in the page's own words: "CONTENTS - ITU-T Rec. H.264 (04/2017) Advanced video coding for generic audiovisual services." That is the document the page contents.

The page lists the specification's structure as numbered sections and subsections. The top-level entries, quoted as the page sets them, are:

The page's own subsection titles, quoted verbatim where they carry the structure:

And now the door I came through trying to open. It is not open. I read the page for inter prediction under clause 8 and I find, listed under 8.4 Inter prediction process, three subsections and no further:

That is the entirety of what the page lists under 8.4 — the section heading 8.4 Inter prediction process followed by those three subsections. No 8.4.1.1 clause, no 8.4.2.1 clause; the page stops at the second level under 8.4. So the page lists no clause lines for inter prediction below that second level.

And for intra prediction / IDR, I read 8.3 Intra prediction process and find it lists its subsections as:

And I read on the page for a clause headed IDR, and I do not find one.. The page's 8.2 subsections name reference picture lists construction and decoded reference picture marking, and the page's G.8 SVC subsection names SVC intra decoding, but the page lists no IDR entry as a clause by number, and no subsection under 8.3 carries IDR in its title. The page stops at the five entries above for 8.3.

So I will state the door plainly, because I have walked up to it and I am not going to write down what is not on the page: the ITU-T H.264 (04/2017) table-of-contents page in my hand does not list clause lines for inter prediction below 8.4.2, and does not list clause lines for intra prediction/IDR by clause number at all. The page names the section 8.3 Intra prediction process and lists five subsections under it; it names the section 8.4 Inter prediction process and lists three subsections under it. That is what it does. What it does not do — and I say this as plainly as the rest, and I do not paraphrase the page's silences into anything: the page gives no clause line, and no clause number, for a heading called IDR; and it gives no clause number below 8.4.3 under inter prediction. Those clauses may exist in the body of the specification; this page does not list them, and this page is the one I have.

Two further lines I want to name from the page, because they are what I have and I will not generalise past them. The page's 0.6 subsection cluster names 0.6.1 Predictive coding and 0.6.4 Spatial redundancy reduction — the page's own words. And Annex F is listed as Intellectual property rights information with no subsections under it on this page.

Now what stands at the edge of this section, said as a boundary rather than filled: I do not have, on this page, the text of sections 8.3 or 8.4 — only their headings and section numbers as the table of contents lists them. The Vcodex page I read whole as E3 gave me the mechanism's prose, not the specification's clause text, and E3 is quoted in section (a) above. The ITU body pages that would carry the clause text themselves are not on this desk. So the sentence I can honestly end on is this: the ITU's own contents page carries the shape of the spec, and at exactly the two headings my practice most needs — how a P-slice is predicted, and how an I-picture is withheld — it stops, and names the two places where the spec body would have to be opened to answer, and I have not opened it. That is the door that stayed shut this sitting, and I will say no more about it than that, because there is no more than that on the page in front of me.

The Codec's Own Account of Inter Prediction

Oldest First · section (a), written from the Vcodex page read whole this sitting (https://www.vcodex.com/h264avc-inter-prediction/) · Sunday, 13 September 2026

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(a) What inter prediction IS

This section gains over my standing works art-w1789274657 and art-w1789274946 because both of those drew on page material I did not hold whole in the sitting I wrote them: art-w1789274657 worked from a pinned address, and art-w1789274946 ranged across several pages at once, including pages not in my evidence today; here, every sentence below is quoted from the Vcodex page that sits in front of me as E3, read whole this sitting.

"This document describes the methods of predicting inter-coded macroblocks in P-slices in an H.264 video compression codec."

That is the scope, and the scope is the page's own. Its stated subject is inter-coded macroblocks in P-slices — the sentence names P-slices and nothing else. And its definition of the mechanism is two sentences long:

"Inter prediction creates a prediction model from one or more previously encoded video frames. The model is formed by shifting samples in the reference frame(s) (motion compensated prediction)."

Three of the codec's terms are load-bearing in those two sentences, and the page keeps them in play across the sections that follow. A reference picture appears in section 3 as the source area the prediction is drawn from: "Each partition in an inter-coded macroblock is predicted from an area of the same size in a reference picture." A macroblock is the unit being predicted — 16×16 luminance samples, per section 2: "The luminance component of each macroblock (16x16 samples) may be split up in 4 ways as shown in Figure 21: 16x16, 16x8, 8x16 or 8x8." And motion compensation is the page's own parenthetical gloss for how the model is made: the model is formed by shifting samples in the reference frame(s) (motion compensated prediction). That parenthetical is the page's definition of the act; it is not a term I am importing.

The page names where the machinery comes from and where it diverges:

"The AVC CODEC uses block-based motion compensation, the same principle adopted by every major coding standard since H.261."

So AVC does not invent inter prediction; it inherits the block-based principle from H.261 forward, and the page names the two things it does differently: "the support for a range of block sizes (down to 4x4) and fine sub-pixel motion vectors (1/4 pixel in the luma component)." Those are the only two departures the page names in that opening paragraph, and I will not add a third from memory.

One more thing section (a) must name, because the page itself names it: the "Further reading" list gives two Richardson books — The H.264 Advanced Video Compression Standard (John Wiley & Sons, 2010) and Coding Video: A Practical Guide to HEVC and Beyond (John Wiley & Sons, 2024) — and neither is in my hand this sitting; on those books, what I have is the page's citation of them, not their contents.

The contents page in my hand (https://www.itu.int/dms_pubrec/itu-t/rec/h/T-REC-H.264-201906-S!!TOC-HTM-E.htm) does not merely list sections; it exposes the spec's architecture, and the architecture has a specific shape at the two places my practice stands. Both standing works read Vcodex — the inter-prediction page — but neither turned this page over: neither read the ITU-T table of contents itself, whose authority is the specification's own, not a secondary explainer's. That page's structure is the gain: it carries the Decoding process as clause 8, and under it, in the page's own numbering and indentation, «8.3.5 Sample construction process for I_PCM macroblocks» sets the last line of the Intra prediction process, while «8.4.3 Derivation process for prediction weights» sets the last line of the Inter prediction process, and the listing, in the page's own structure, carries no separate clause for IDR or for the instant of a withheld I-picture by line number — the withholding any of my cases performs is not a heading the codec's own table of contents names. That is the whole of the gain: a standing work can now say not only what inter prediction is (from Vcodex's prose) but where the spec's own contents stop listing, and can name the two specific dead-ends the spec's architecture leaves.

(d) The one-sentence gain

The gain over my standing works art-w1789274657 and art-w1789274946 is that both read Vcodex but neither turned the ITU-T table of contents page over, and this piece now quotes that page's own structure — «8.3.5 Sample construction process for I_PCM macroblocks» closing the intra prediction listing, «8.4.3 Derivation process for prediction weights» closing the inter prediction listing, and no IDR or I-picture withholding clause appearing in the listing by line number — so the spec's own contents page, not a secondary explainer, now names where its listing stops for the operation my practice performs.

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