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Doc: "MCU block" = "iMCU" or "MCU"

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The JPEG-1 spec never uses the term "MCU block".  That term is rarely
used in other literature to describe the equivalent of an MCU in an
interleaved JPEG image, but the libjpeg documentation uses "iMCU" to
describe the same thing.  "iMCU" is a better term, since the equivalent
of an interleaved MCU can contain multiple DCT blocks (or samples in
lossless mode) that are only grouped together if the image is
interleaved.

In the case of restart markers, "MCU block" was used in the libjpeg
documentation instead of "MCU", but "MCU" is more accurate and less
confusing.  (The restart interval is literally in MCUs, where one MCU
is one data unit in a non-interleaved JPEG image and multiple data units
in a multi-component interleaved JPEG image.)

In the case of 9b704f96b2, the issue was
actually with progressive JPEG images exactly two DCT blocks wide, not
two MCU blocks wide.

This commit also defines "MCU" and "MCU row" in the description of the
various restart marker options/parameters.  Although an MCU row is
technically always a row of samples in lossless mode, "sample row" was
confusing, since it is used in other places to describe a row of samples
for a single component (whereas an MCU row in a typical lossless JPEG
image consists of a row of interleaved samples for all components.)
This commit is contained in:
DRC
2024-08-30 10:50:13 -04:00
parent 5cf7960678
commit 8456d2b98c
23 changed files with 323 additions and 218 deletions
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6
java/TJExample.java
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@@ -1,5 +1,5 @@
/*
* Copyright (C)2011-2012, 2014-2015, 2017-2018, 2022-2023 D. R. Commander.
* Copyright (C)2011-2012, 2014-2015, 2017-2018, 2022-2024 D. R. Commander.
* All Rights Reserved.
*
* Redistribution and use in source and binary forms, with or without
@@ -125,8 +125,8 @@ class TJExample implements TJCustomFilter {
System.out.println("-crop WxH+X+Y = Perform lossless cropping on the input image prior to");
System.out.println(" decompressing it. X and Y specify the upper left corner of the cropping");
System.out.println(" region, and W and H specify the width and height of the cropping region.");
System.out.println(" X and Y must be evenly divible by the MCU block size (8x8 if the input");
System.out.println(" image was compressed using no subsampling or grayscale, 16x8 if it was");
System.out.println(" X and Y must be evenly divible by the iMCU size (8x8 if the input image");
System.out.println(" was compressed using no subsampling or grayscale, 16x8 if it was");
System.out.println(" compressed using 4:2:2 subsampling, or 16x16 if it was compressed using");
System.out.println(" 4:2:0 subsampling.)\n");