AdobeRGB wide‑gamut ICC profile using the ColorNeg‑DC mode discrete blue speckles

[Qlong]

问题：
在 Win11 系统下，使用 MakeTiff 输出嵌入 AdobeRGB 广色域 ICC 标签的 16bit 线性 TIFF 底片翻拍文件，通过 Photoshop 调用 ColorPerfect 的 ColorNeg‑DC 模式处理，画面暗部会出现离散蓝点；切换至 SC 模式蓝点即可消失；完全相同的 TIFF 文件，在 Mac 平台使用相同操作流程不会复现故障。

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On Windows 11, when processing 16‑bit linear TIFF scan‑files of negatives exported by MakeTiff with an embedded AdobeRGB wide‑gamut ICC profile using the ColorNeg‑DC mode of ColorPerfect within Photoshop, discrete blue speckles appear in the shadow areas of the image. Switching to SC mode makes these blue speckles disappear. The exact same TIFF file processed with the identical workflow on macOS does not reproduce this issue.

[C.Oldendorf]

The cause of the blue speckles has now been identified, and it is not a ColorPerfect rendering bug.

The important point is that a 16-bit linear TIFF created by MakeTiff must reach ColorPerfect without any color-space conversion being applied to the pixel data first.

In this particular case the TIFF was created by MakeTiff with an embedded Adobe RGB (1998) profile. If Photoshop — or Adobe Camera Raw before Photoshop — converts that image to another RGB working space while opening it, the numerical RGB values of the linear image are changed before ColorPerfect sees them.

For an ordinary positive image that is a normal color-management operation. For this type of linear negative input, however, those values are the data ColorPerfect has to interpret. Once they have been converted, the original input is no longer intact.

That explains the blue artefacts seen in ColorNeg-DC. The fact that they disappear in SC mode does not mean that DC itself is defective; the two modes react differently to input that has already been altered.

There are two separate places in Photoshop that therefore need attention.


1. Photoshop Color Settings

Under:

Edit > Color Settings

the RGB color-management policy should be set to:

Preserve Embedded Profiles

For troubleshooting I also recommend enabling:

Ask When Opening

for profile mismatches.

This makes Photoshop tell you when something is about to happen instead of performing a conversion silently.

The RGB working space itself does not have to match every image you open. An Adobe RGB TIFF can remain Adobe RGB even if another RGB working space is configured, provided Photoshop is set to preserve the embedded profile rather than convert the document to the working space.

See the attached PS-color-settings.png.


2. Adobe Camera Raw

This is the less obvious part.

If Photoshop is configured to open TIFF files through Adobe Camera Raw, the TIFF can be processed by ACR before it ever reaches Photoshop or ColorPerfect.

ACR has its own output/workflow color settings, so it can change the image even when Photoshop's normal Color Settings appear to be correct.

The simplest solution for ColorPerfect input files is therefore to prevent TIFF files from being opened automatically through ACR.

In Camera Raw Preferences, set TIFF handling to:

TIFF: Disable TIFF support

and, if desired, do the same for JPEG.

See the attached ACR-Settings.png.

There is also a simple way of bypassing ACR regardless of that preference:

File > Open As... > select TIFF > Open

This opens the TIFF directly in Photoshop rather than routing it through Camera Raw.


Safe workflow

MakeTiff linear TIFF
→ no RGB conversion in ACR
→ no RGB conversion on opening in Photoshop
→ ColorPerfect

Adobe RGB, sRGB, or another suitable RGB profile is not in itself the deciding issue here.

What matters is that the RGB numbers in the MakeTiff file are not converted before ColorPerfect processes them.

Many thanks to Qlong for testing this so thoroughly. The ACR part in particular is easy to overlook, and I will need to make this point much clearer in future instructions.

PS-color-settings.png (19.98 KiB) Viewed 379 times

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ACR-Settings.png (23.02 KiB) Viewed 379 times

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现在已经可以确定蓝色斑点问题的真正原因了，而且它并不是 ColorPerfect 本身的渲染错误。

关键在于：由 MakeTiff 生成的 16 位线性 TIFF 在进入 ColorPerfect 之前，不能先被 Photoshop 或 Adobe Camera Raw 进行色彩空间转换。

在这个例子中，MakeTiff 生成的 TIFF 文件嵌入了 Adobe RGB (1998) ICC 配置文件。如果 Photoshop，或者在 Photoshop 之前运行的 Adobe Camera Raw，把该图像转换到另一个 RGB 工作空间，那么线性 TIFF 中原始的 RGB 数值就会在 ColorPerfect 读取图像之前被改变。

对于普通的正片图像来说，这属于正常的色彩管理操作；但对于这种供 ColorPerfect 处理的线性负片数据来说，这些 RGB 数值本身就是 ColorPerfect 需要解释的数据。一旦发生颜色空间转换，原始输入数据就已经被改变，之后再由 ColorPerfect 处理也无法恢复到转换前的状态。

这也解释了为什么在 ColorNeg-DC 模式下会出现蓝色斑点。切换到 SC 模式后斑点消失，并不意味着 DC 模式本身存在缺陷，而是两个模式对于已经被修改过的输入数据反应不同。

因此，在 Photoshop 中实际上有两个彼此独立的设置需要检查。


1. Photoshop 的 Color Settings（颜色设置）

进入：

Edit > Color Settings

RGB 的 Color Management Policies 应设置为：

Preserve Embedded Profiles

为了排查问题，我还建议启用配置文件不匹配时的：

Ask When Opening

这样 Photoshop 在准备处理配置文件不匹配时会主动提示，而不是在后台静默进行转换。

需要强调的是，Photoshop 的默认 RGB Working Space 并不需要与每一个打开的图像相同。

例如，一个带有 Adobe RGB 配置文件的 TIFF，即使 Photoshop 当前设置了其他 RGB 工作空间，也完全可以继续保持 Adobe RGB。关键是 Color Management Policy 必须选择 Preserve Embedded Profiles，而不是把文件转换成 Photoshop 的默认工作空间。

参见附件 PS-color-settings.png。


2. Adobe Camera Raw

这一部分更容易被忽略。

如果 Photoshop 被设置为通过 Adobe Camera Raw 打开 TIFF 文件，那么该 TIFF 在真正进入 Photoshop 和 ColorPerfect 之前，就已经先经过了 ACR。

ACR 有自己独立的输出／工作流程色彩空间设置。因此，即使 Photoshop 主程序中的 Color Settings 设置正确，ACR 仍然可能提前改变图像的 RGB 数据。

对于准备交给 ColorPerfect 处理的 TIFF，最简单可靠的方法是不要让 ACR 自动处理 TIFF 文件。

在 Camera Raw Preferences 中，将 TIFF 设置为：

TIFF: Disable TIFF support

如果没有需要，也可以同样关闭 JPEG 的 ACR 自动处理。

参见附件 ACR-Settings.png。

另外还有一种可以明确绕过 ACR 的打开方式：

File > Open As... > 选择 TIFF > Open

这样 TIFF 会直接进入 Photoshop，而不会先经过 Camera Raw。


安全的处理流程

MakeTiff 线性 TIFF
→ ACR 不进行 RGB 转换
→ Photoshop 打开时不进行 RGB 转换
→ ColorPerfect

所以这里真正决定结果的，并不是一定要使用 Adobe RGB、sRGB 或某个特定 RGB 色彩空间。

真正重要的是：MakeTiff 文件中的 RGB 数值在进入 ColorPerfect 之前必须保持不变。

也非常感谢 Qlong 对这个问题进行了如此细致的测试。尤其是 Adobe Camera Raw 这一层确实非常容易被忽略，我以后也需要在相关说明中把这一点写得更加明确。