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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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41
java/org/libjpegturbo/turbojpeg/TJTransform.java
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@@ -48,13 +48,13 @@ public class TJTransform extends Rectangle {
public static final int OP_NONE = 0;
/**
* Flip (mirror) image horizontally. This transform is imperfect if there
* are any partial MCU blocks on the right edge.
* are any partial iMCUs on the right edge.
* @see #OPT_PERFECT
*/
public static final int OP_HFLIP = 1;
/**
* Flip (mirror) image vertically. This transform is imperfect if there are
* any partial MCU blocks on the bottom edge.
* any partial iMCUs on the bottom edge.
* @see #OPT_PERFECT
*/
public static final int OP_VFLIP = 2;
@@ -66,26 +66,26 @@ public class TJTransform extends Rectangle {
public static final int OP_TRANSPOSE = 3;
/**
* Transverse transpose image (flip/mirror along upper right to lower left
* axis). This transform is imperfect if there are any partial MCU blocks in
* the image.
* axis). This transform is imperfect if there are any partial iMCUs in the
* image.
* @see #OPT_PERFECT
*/
public static final int OP_TRANSVERSE = 4;
/**
* Rotate image clockwise by 90 degrees. This transform is imperfect if
* there are any partial MCU blocks on the bottom edge.
* there are any partial iMCUs on the bottom edge.
* @see #OPT_PERFECT
*/
public static final int OP_ROT90 = 5;
/**
* Rotate image 180 degrees. This transform is imperfect if there are any
* partial MCU blocks in the image.
* partial iMCUs in the image.
* @see #OPT_PERFECT
*/
public static final int OP_ROT180 = 6;
/**
* Rotate image counter-clockwise by 90 degrees. This transform is imperfect
* if there are any partial MCU blocks on the right edge.
* if there are any partial iMCUs on the right edge.
* @see #OPT_PERFECT
*/
public static final int OP_ROT270 = 7;
@@ -94,20 +94,20 @@ public class TJTransform extends Rectangle {
/**
* This option causes {@link TJTransformer#transform
* TJTransformer.transform()} to throw an exception if the transform is not
* perfect. Lossless transforms operate on MCU blocks, the size of which
* depends on the level of chrominance subsampling used. If the image's
* width or height is not evenly divisible by the MCU block size (see
* {@link TJ#getMCUWidth TJ.getMCUWidth()} and {@link TJ#getMCUHeight
* TJ.getMCUHeight()}), then there will be partial MCU blocks on the right
* and/or bottom edges. It is not possible to move these partial MCU blocks
* to the top or left of the image, so any transform that would require that
* is "imperfect." If this option is not specified, then any partial MCU
* blocks that cannot be transformed will be left in place, which will create
* odd-looking strips on the right or bottom edge of the image.
* perfect. Lossless transforms operate on iMCUs, the size of which depends
* on the level of chrominance subsampling used. If the image's width or
* height is not evenly divisible by the iMCU size (see {@link TJ#getMCUWidth
* TJ.getMCUWidth()} and {@link TJ#getMCUHeight TJ.getMCUHeight()}), then
* there will be partial iMCUs on the right and/or bottom edges. It is not
* possible to move these partial iMCUs to the top or left of the image, so
* any transform that would require that is "imperfect." If this option is
* not specified, then any partial iMCUs that cannot be transformed will be
* left in place, which will create odd-looking strips on the right or bottom
* edge of the image.
*/
public static final int OPT_PERFECT = (1 << 0);
/**
* Discard any partial MCU blocks that cannot be transformed.
* Discard any partial iMCUs that cannot be transformed.
*/
public static final int OPT_TRIM = (1 << 1);
/**
@@ -164,11 +164,10 @@ public class TJTransform extends Rectangle {
* Create a new lossless transform instance with the given parameters.
*
* @param x the left boundary of the cropping region. This must be evenly
* divisible by the MCU block width (see {@link TJ#getMCUWidth
* TJ.getMCUWidth()})
* divisible by the iMCU width (see {@link TJ#getMCUWidth TJ.getMCUWidth()})
*
* @param y the upper boundary of the cropping region. This must be evenly
* divisible by the MCU block height (see {@link TJ#getMCUHeight
* divisible by the iMCU height (see {@link TJ#getMCUHeight
* TJ.getMCUHeight()})
*
* @param w the width of the cropping region. Setting this to 0 is the