bug: colorconvert and the other OCIO operations write 0.5 + 10 * src into channels past the fourth instead of copying them

src/libOpenImageIO/color_ocio.cpp の colorconvert_impl の周辺（issue では3089～3097行目が示されています）と、src/include/OpenImageIO/imagebufalgo.h にある対応する約束を確認してください。異なる宛先への4チャンネルを超えるカラー変換について、4番目より後のチャンネルが変更されないことを確認する回帰テストを追加または調整し、該当するテストを実行してください。残りのチャンネルが変更されずにコピーされ、回帰テストが成功すれば完了です。

Describe the bug

ImageBufAlgo::colorconvert and every other operation built on the same
engine corrupt channels past the fourth. The code that is supposed to copy
them states the intent in its own comment and then does something else.

src/libOpenImageIO/color_ocio.cpp:3089-3097 on main, the last statement of
the per-scanline body in colorconvert_impl:

if (channelsToCopy < roi.chend && (&R != &A)) {
    // If there are "leftover" channels, just copy them
    // unaltered from the source.
    a.rerange(roi.xbegin, roi.xend, j, j + 1, k, k + 1);
    r.rerange(roi.xbegin, roi.xend, j, j + 1, k, k + 1);
    for (; !r.done(); ++r, ++a)
        for (int c = channelsToCopy; c < roi.chend; ++c)
            r[c] = 0.5 + 10 * a[c];
}

channelsToCopy is std::min(4, roi.nchannels()) (:3028), so with the
default ROI this is every channel from index 4 upward. Those channels come out
as 0.5 + 10 * source rather than as the source value.

I expected them to be copied unaltered, which is what the comment says, and
what the public header promises six times — once for each affected operation:

• src/include/OpenImageIO/imagebufalgo.h:2117 — colorconvert
• :2190 — colormatrixtransform
• :2222 — ociolook
• :2271 — ociodisplay
• :2330 — ociofiletransform
• :2366 — ocionamedtransform

each reading "Any additional channels will be simply copied unaltered."

All six funnel into colorconvert_impl through the processor overload of
colorconvert, so all six are affected.

Three conditions have to hold together, which is presumably why this has
lasted:

1. more than four channels in the ROI;
2. the destination is a different ImageBuf from the source — the guard tests
   &R != &A;
3. the processor is not a no-op. An identity conversion returns earlier
   through ImageBufAlgo::copy (:3194-3197), and an in-place no-op returns
   at :3186-3187.

The RGB/RGBA fast path does not hide it: colorconvert_impl_float_rgba is
gated on both buffers having exactly four channels (:3206-3208), so it never
carries an image that would reach this block.

In practice condition 1 means most multi-AOV EXRs, so this affects a common
case rather than an exotic one. The Evidence section below shows it on a
stock OpenEXR test image whose depth, disparity, motion-vector and mask
channels are all corrupted by a single --colorconvert.

This is not a guess about intent. The block entered the file in PR #2987,
"Clarify behavior of color conversion on image with > 4 channels", commit
1b884e123485eef66b27c110f0374bd91bcafda6 (2021-05-31). The line was
0.5 + 10 * a[c] in that diff as merged, and the same pull request added
the "copied unaltered" sentence to imagebufalgo.h. git log -L shows the
line untouched since. It has shipped in every release from v2.3.7.2 onward —
about five years.

There is no test or documentation anywhere in the repository that refers to
this expression: a search for 0.5 + 10 across src/ and testsuite/
returns this one line.

OpenImageIO version and dependencies

OIIO 3.2.2.0 | Windows/x86_64
    Build compiler: MSVS 1951 | C++17/199711
    HW features enabled at build: sse2,sse3,ssse3,sse41,sse42
    No CUDA support (disabled / unavailable at build time)
Dependencies: BZip2 1.0.8, DCMTK NONE, FFmpeg NONE, fmt 12.1.0, Freetype 2.13.3, GIF 5.2.2, Imath 3.2.2, JXL NONE, libdeflate 1.25, Libheif NONE, libjpeg-turbo 3.1.3, LibRaw NONE, libuhdr NONE, OpenColorIO 2.5.1, OpenCV NONE, OpenEXR 3.4.7, OpenJPEG 2.5.4, openjph 0.26.3, PNG 1.6.55, Ptex NONE, Ptex NONE, Robinmap 1.4.1, TBB NONE, TIFF 4.7.1, WebP 1.6.0, ZLIB 1.3.1, zstd 1.5.7

The quoted block is byte-identical in current main, v3.2.2.0 and v3.1.19.0.
Line numbers: main and v3.2.2.0 color_ocio.cpp:3089-3097 with the
assignment at :3096; v3.1.19.0 :2557-2565 with the assignment at :2564.

To Reproduce

Build and run the program at the end of this report. Public
OpenImageIO API only. It fills a six-channel float image whose channels 4 and
5 hold 0.125 and 0.875, converts ACEScg to sRGB - Display into a distinct
destination, and prints the result, with three controls.

It prints both the headers it compiled against and the version the linked
library reports, so the measurement cannot be against a different build by
accident.

compiled against OIIO 3.2.2.0; linked lib reports "3.2.2.0" (30202)
OCIO config: ocio://default
config has 27 color spaces
processor ACEScg -> sRGB - Display: isNoOp=0 (real transform)
processor ACEScg -> ACEScg: isNoOp=1

input fill:
  6 channels, as filled              c0=0.18      c1=0.18      c2=0.18      c3=1         c4=0.125     c5=0.875

Case 1  6ch, DISTINCT dst, real transform  (expect c4/c5 unchanged)
  result                             c0=0.461355  c1=0.461355  c2=0.461355  c3=1         c4=1.75      c5=9.25
  expected (a copy)                  c4=0.125     c5=0.875
  expected if 0.5+10*x               c4=1.75      c5=9.25
  VERDICT: copy=0  0.5+10*x=1  -> CORRUPTION REPRODUCED

Case 2  4ch control, DISTINCT dst, real transform  (no leftover channels)
  result                             c0=0.461355  c1=0.461355  c2=0.461355  c3=1

Case 3  6ch, DISTINCT dst, IDENTITY transform  (early-outs via copy)
  result                             c0=0.18      c1=0.18      c2=0.18      c3=1         c4=0.125     c5=0.875

Case 4  6ch, IN-PLACE dst==src, real transform  (&R==&A guard)
  result                             c0=0.461355  c1=0.461355  c2=0.461355  c3=1         c4=0.125     c5=0.875

0.125 and 0.875 come back as 1.75 and 9.25, which are exactly
0.5 + 10 * 0.125 and 0.5 + 10 * 0.875. Both are exactly representable in
float and half, so the equality is exact rather than approximate; an integer
destination would quantise or clamp instead.

The three controls all behave correctly, which confirms the trigger is the
conjunction above and not something else.

Evidence

This is not only a synthetic case. Here it is on a real multi-AOV frame from
the OpenEXR test images Beachball/singlepart.0001.exr, 911x876, 20
channels, half, with a non-zero data window origin. No C++ at all:

oiiotool singlepart.0001.exr --colorconvert ACEScg "sRGB - Display" \
         --cut 1x1+1100+680 -d float -o conv.exr
oiiotool --dumpdata conv.exr

Every one of the sixteen channels past the fourth comes back as
0.5 + 10 * original, at pixel (1100, 680):

Sixteen of sixteen, and the same at every other pixel I checked. A depth of
9 becomes 90.5, an alpha of 1 becomes 10.5, and the second eye of the stereo
pair carried in left.R/left.G/left.B is scaled along with
everything else, because those channels are past the fourth too.

The same thing on a synthetic file, for a minimal case:

// six.exr :    2 x    1, 6 channel, float openexr
float 0[1][2][6] =
{
  { /* (0, 0): */ { 0.461354554, 0.461354613, 0.461354613, 1.000000000, 1.750000000, 9.250000000 },
    /* (1, 0): */ { 0.461354554, 0.461354613, 0.461354613, 1.000000000, 1.750000000, 9.250000000 } },
};

Channels 4 and 5 were 0.125 and 0.875 before the conversion.

The fix

What the comment already says:

r[c] = a[c];

One caveat on scope, in case it affects how you want to fix it:
channelsToCopy is a count of ROI channels but is compared against the
absolute roi.chend, and the load and store loops at :3050-3051 and
:3087-3088 index from 0 and ignore roi.chbegin. With roi.chbegin > 0 the
boundary therefore moves. Everything above is stated for the default ROI,
which is what the reproduction uses; I have not tried to work out what the
intended behaviour is for a shifted channel range, and have deliberately not
folded it into this report.

Found while writing Rust bindings. The binding currently repairs those
channels from the source after each colour call and has a regression test that
fails without the repair.

I am happy to open a pull request for the one-line change if that is more
useful than an issue, it would need me to sort out a CLA first, so say the
word if you would rather have the patch than the report.

// Reproducer: ImageBufAlgo::colorconvert() corrupts channels past the fourth
// when the destination is a different ImageBuf and the transform is not a no-op.
//
// src/libOpenImageIO/color_ocio.cpp, colorconvert_impl():
//     if (channelsToCopy < roi.chend && (&R != &A)) {
//         // If there are "leftover" channels, just copy them
//         // unaltered from the source.
//         ...
//         r[c] = 0.5 + 10 * a[c];     <-- not a copy
//     }
//
// Public API only. Prints channels 4 and 5 for several cases.

#include <OpenImageIO/color.h>
#include <OpenImageIO/imagebuf.h>
#include <OpenImageIO/imagebufalgo.h>
#include <OpenImageIO/imageio.h>

#include <cstdio>
#include <cstdlib>
#include <string>
#include <vector>

using namespace OIIO;

static const int XRES = 4;
static const int YRES = 2;

// Channel fill values. Channels 4 and 5 are the ones under test.
static const float FILL[6] = { 0.18f, 0.18f, 0.18f, 1.0f, 0.125f, 0.875f };

static ImageBuf
make_buf(int nchannels)
{
    ImageSpec spec(XRES, YRES, nchannels, TypeDesc::FLOAT);
    if (nchannels == 6)
        spec.channelnames = { "R", "G", "B", "A", "Z", "mask" };
    ImageBuf buf(spec);
    if (!ImageBufAlgo::fill(buf, cspan<float>(FILL, FILL + nchannels))) {
        std::printf("FATAL: fill failed: %s\n", buf.geterror().c_str());
        std::exit(1);
    }
    return buf;
}

static void
report(const char* label, const ImageBuf& buf, int nchannels)
{
    std::vector<float> px(nchannels, 0.0f);
    buf.getpixel(0, 0, px.data(), nchannels);
    std::printf("  %-34s", label);
    for (int c = 0; c < nchannels; ++c)
        std::printf(" c%d=%-9.6g", c, px[c]);
    std::printf("\n");
}

int
main()
{
    // Headers compiled against, and the library actually linked
    std::printf("compiled against OIIO %s; linked lib reports \"%s\" (%d)\n",
                OIIO_VERSION_STRING,
                std::string(get_string_attribute("version")).c_str(),
                openimageio_version());

    ColorConfig config;
    if (config.has_error()) {
        std::printf("FATAL: ColorConfig: %s\n", config.geterror().c_str());
        return 1;
    }
    std::printf("OCIO config: %s\n", config.configname().c_str());

    const char* FROM = "ACEScg";
    const char* TO   = "sRGB - Display";

    // Confirm both spaces really exist in whatever config we got.
    std::vector<std::string> names = config.getColorSpaceNames();
    std::printf("config has %d color spaces\n", (int)names.size());
    bool have_from = false, have_to = false;
    for (const std::string& n : names) {
        if (n == FROM)
            have_from = true;
        if (n == TO)
            have_to = true;
    }
    if (!have_from || !have_to) {
        std::printf("FATAL: need both \"%s\" (found=%d) and \"%s\" (found=%d).\n"
                    "Available:\n",
                    FROM, (int)have_from, TO, (int)have_to);
        for (const std::string& n : names)
            std::printf("    %s\n", n.c_str());
        return 1;
    }

    // A real, non-identity transform. Fail loudly if it is a no-op, because a
    // no-op would return early through ImageBufAlgo::copy() and prove nothing.
    ColorProcessorHandle proc = config.createColorProcessor(FROM, TO);
    if (!proc) {
        std::printf("FATAL: createColorProcessor(%s -> %s) failed: %s\n", FROM,
                    TO, config.geterror().c_str());
        return 1;
    }
    if (proc->isNoOp()) {
        std::printf("FATAL: %s -> %s is a no-op; test would be vacuous.\n", FROM,
                    TO);
        return 1;
    }
    std::printf("processor %s -> %s: isNoOp=0 (real transform)\n", FROM, TO);

    // And an identity transform, for the control case.
    ColorProcessorHandle ident = config.createColorProcessor(FROM, FROM);
    if (!ident) {
        std::printf("FATAL: createColorProcessor(%s -> %s) failed: %s\n", FROM,
                    FROM, config.geterror().c_str());
        return 1;
    }
    std::printf("processor %s -> %s: isNoOp=%d\n", FROM, FROM,
                (int)ident->isNoOp());

    std::printf("\ninput fill:\n");
    report("6 channels, as filled", make_buf(6), 6);

    // --- Case 1: the bug. 6 channels, distinct dst, real transform. ---
    std::printf("\nCase 1  6ch, DISTINCT dst, real transform  (expect c4/c5 "
                "unchanged)\n");
    {
        ImageBuf src = make_buf(6);
        ImageBuf dst;
        if (!ImageBufAlgo::colorconvert(dst, src, proc.get(),
                                        /*unpremult=*/false)) {
            std::printf("FATAL: colorconvert failed: %s\n",
                        dst.geterror().c_str());
            return 1;
        }
        report("result", dst, 6);

        std::vector<float> px(6, 0.0f);
        dst.getpixel(0, 0, px.data(), 6);
        std::printf("  %-34s c4=%-9.6g c5=%-9.6g\n", "expected (a copy)",
                    FILL[4], FILL[5]);
        std::printf("  %-34s c4=%-9.6g c5=%-9.6g\n", "expected if 0.5+10*x",
                    0.5f + 10.0f * FILL[4], 0.5f + 10.0f * FILL[5]);
        bool is_copy = px[4] == FILL[4] && px[5] == FILL[5];
        bool is_bug  = px[4] == 0.5f + 10.0f * FILL[4]
                      && px[5] == 0.5f + 10.0f * FILL[5];
        std::printf("  VERDICT: copy=%d  0.5+10*x=%d  -> %s\n", (int)is_copy,
                    (int)is_bug,
                    is_bug ? "CORRUPTION REPRODUCED"
                           : (is_copy ? "correct" : "neither"));
    }

    // --- Case 2: control. 4 channels, distinct dst, real transform. ---
    std::printf("\nCase 2  4ch control, DISTINCT dst, real transform  (no "
                "leftover channels)\n");
    {
        ImageBuf src = make_buf(4);
        ImageBuf dst;
        if (!ImageBufAlgo::colorconvert(dst, src, proc.get(),
                                        /*unpremult=*/false)) {
            std::printf("FATAL: colorconvert failed: %s\n",
                        dst.geterror().c_str());
            return 1;
        }
        report("result", dst, 4);
    }

    // --- Case 3: control. 6 channels, distinct dst, identity transform. ---
    std::printf("\nCase 3  6ch, DISTINCT dst, IDENTITY transform  (early-outs "
                "via copy)\n");
    {
        ImageBuf src = make_buf(6);
        ImageBuf dst;
        if (!ImageBufAlgo::colorconvert(dst, src, ident.get(),
                                        /*unpremult=*/false)) {
            std::printf("FATAL: colorconvert failed: %s\n",
                        dst.geterror().c_str());
            return 1;
        }
        report("result", dst, 6);
    }

    // --- Case 4: control. 6 channels, IN-PLACE, real transform. ---
    std::printf("\nCase 4  6ch, IN-PLACE dst==src, real transform  (&R==&A "
                "guard)\n");
    {
        ImageBuf buf = make_buf(6);
        if (!ImageBufAlgo::colorconvert(buf, buf, proc.get(),
                                        /*unpremult=*/false)) {
            std::printf("FATAL: colorconvert failed: %s\n",
                        buf.geterror().c_str());
            return 1;
        }
        report("result", buf, 6);
    }

    return 0;
}