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

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.)

java/org/libjpegturbo/turbojpeg/TJ.java

@@ -120,14 +120,24 @@ public final class TJ {
   public static final int SAMP_UNKNOWN = -1;
 
   /**
-   * Returns the MCU block width for the given level of chrominance
-   * subsampling.
+   * Returns the iMCU width for the given level of chrominance subsampling.
+   *
+   * <p>In a typical lossy JPEG image, 8x8 blocks of DCT coefficients for each
+   * component are interleaved in a single scan.  If the image uses chrominance
+   * subsampling, then multiple luminance blocks are stored together, followed
+   * by a single block for each chrominance component.  The combination of the
+   * full-resolution luminance block(s) and the (possibly subsampled)
+   * chrominance blocks corresponding to the same pixels is called a "Minimum
+   * Coded Unit" (MCU.)  In a non-interleaved lossy JPEG image, each component
+   * is stored in a separate scan, and an MCU is a single DCT block, so we use
+   * the term "iMCU" (interleaved MCU) to refer to the equivalent of an MCU in
+   * an interleaved JPEG image.  For the common case of interleaved JPEG
+   * images, an iMCU is the same as an MCU.
    *
    * @param subsamp the level of chrominance subsampling (one of
    * {@link #SAMP_444 SAMP_*})
    *
-   * @return the MCU block width for the given level of chrominance
-   * subsampling.
+   * @return the iMCU width for the given level of chrominance subsampling.
    */
   public static int getMCUWidth(int subsamp) {
     checkSubsampling(subsamp);
@@ -140,14 +150,24 @@ public final class TJ {
 
 
   /**
-   * Returns the MCU block height for the given level of chrominance
-   * subsampling.
+   * Returns the iMCU height for the given level of chrominance subsampling.
+   *
+   * <p>In a typical lossy JPEG image, 8x8 blocks of DCT coefficients for each
+   * component are interleaved in a single scan.  If the image uses chrominance
+   * subsampling, then multiple luminance blocks are stored together, followed
+   * by a single block for each chrominance component.  The combination of the
+   * full-resolution luminance block(s) and the (possibly subsampled)
+   * chrominance blocks corresponding to the same pixels is called a "Minimum
+   * Coded Unit" (MCU.)  In a non-interleaved lossy JPEG image, each component
+   * is stored in a separate scan, and an MCU is a single DCT block, so we use
+   * the term "iMCU" (interleaved MCU) to refer to the equivalent of an MCU in
+   * an interleaved JPEG image.  For the common case of interleaved JPEG
+   * images, an iMCU is the same as an MCU.
    *
    * @param subsamp the level of chrominance subsampling (one of
    * {@link #SAMP_444 SAMP_*})
    *
-   * @return the MCU block height for the given level of chrominance
-   * subsampling.
+   * @return the iMCU height for the given level of chrominance subsampling.
    */
   public static int getMCUHeight(int subsamp) {
     checkSubsampling(subsamp);
@@ -608,8 +628,8 @@ public final class TJ {
    * and refined with subsequent higher-quality scans containing
    * higher-frequency DCT coefficients.  When using Huffman entropy coding, the
    * progressive JPEG format also provides an "end-of-bands (EOB) run" feature
-   * that allows large groups of zeroes, potentially spanning multiple MCU
-   * blocks, to be represented using only a few bytes.
+   * that allows large groups of zeroes, potentially spanning multiple MCUs, to
+   * be represented using only a few bytes.
    *
    * <p><b>Value</b>
    * <ul>
@@ -765,7 +785,7 @@ public final class TJ {
    */
   public static final int PARAM_LOSSLESSPT = 17;
   /**
-   * JPEG restart marker interval in MCU blocks [lossy compression only]
+   * JPEG restart marker interval in MCUs [lossy compression only]
    *
    * <p>The nature of entropy coding is such that a corrupt JPEG image cannot
    * be decompressed beyond the point of corruption unless it contains restart
@@ -775,9 +795,18 @@ public final class TJ {
    * tolerance of the JPEG image, but adding too many restart markers can
    * adversely affect the compression ratio and performance.
    *
+   * <p>In typical JPEG images, an MCU (Minimum Coded Unit) is the minimum set
+   * of interleaved "data units" (8x8 DCT blocks if the image is lossy or
+   * samples if the image is lossless) necessary to represent at least one data
+   * unit per component.  (For example, an MCU in an interleaved lossy JPEG
+   * image that uses 4:2:2 subsampling consists of two luminance blocks
+   * followed by one block for each chrominance component.)  In
+   * single-component or non-interleaved JPEG images, an MCU is the same as a
+   * data unit.
+   *
    * <p><b>Value</b>
    * <ul>
-   * <li> the number of MCU blocks between each restart marker <i>[default:
+   * <li> the number of MCUs between each restart marker <i>[default:
    * <code>0</code> (no restart markers)]</i>
    * </ul>
    *
@@ -786,15 +815,16 @@ public final class TJ {
    */
   public static final int PARAM_RESTARTBLOCKS = 18;
   /**
-   * JPEG restart marker interval in MCU rows (lossy) or sample rows (lossless)
-   * [compression only]
+   * JPEG restart marker interval in MCU rows [compression only]
    *
-   * <p>See {@link #PARAM_RESTARTBLOCKS} for a description of restart markers.
+   * <p>See {@link #PARAM_RESTARTBLOCKS} for a description of restart markers
+   * and MCUs.  An MCU row is a row of MCUs spanning the entire width of the
+   * image.
    *
    * <p><b>Value</b>
    * <ul>
-   * <li> the number of MCU rows or sample rows between each restart marker
-   * <i>[default: <code>0</code> (no restart markers)]</i>
+   * <li> the number of MCU rows between each restart marker <i>[default:
+   * <code>0</code> (no restart markers)]</i>
    * </ul>
    *
    * <p>Setting this parameter to a non-zero value sets