Idle Time

So you’re writing an Image Unit, and it has a couple of numeric parameters. You expect that Core Image Fun House will show a slider for each of them, and it does—but no matter what you do, the slider’s minimum and maximum are both 0. Furthermore, Core Image Fun House doesn’t show your parameters’ real display names; it simply makes them up from the parameters’ KVC keys.

The problem is that you are specifying those attributes in the wrong place in your Description.plist. And yes, I know you’re specifying them where the project template had them—so was I. The template has it wrong.

One of the filter attributes that Core Image recognizes is CIInputs. The value for this key is an array of dictionaries; each dictionary represents one parameter to the filter. The template has all the parameter attributes in these dictionaries. That makes sense, but it’s not where Core Image looks for them.

In reality, Core Image only looks for three keys in those dictionaries:

• CIAttributeName
• CIAttributeClass
• CIAttributeDefault

Anything else, it simply ignores.

The correct place to put all those other keys (including CIAttributeSliderMin, CIAttributeSliderMax, and CIAttributeDisplayName) is in another dictionary—one for each parameter. These dictionaries go inside the CIFilterAttributes dictionary. In other words, the CIFilterAttributes dictionary should contain:

• CIInputs => Aforementioned array of (now very small) dictionaries
• inputFoo => Dictionary fully describing the inputFoo parameter, including slider attributes and display name
• inputBar => Dictionary fully describing the inputBar parameter, including slider attributes and display name
• inputBaz => Dictionary fully describing the inputBaz parameter, including slider attributes and display name

Finally, an example:

<key>CIFilterAttributes</key>
<dict>
    ⋮
    <key>CIInputs</key>
    <array>
        <dict>
            <key>CIAttributeClass</key>
            <string>CIImage</string>
            <key>CIAttributeName</key>
            <string>inputImage</string>
        </dict>
        <dict>
            <key>CIAttributeClass</key>
            <string>NSNumber</string>
            <key>CIAttributeDefault</key>
            <real>1.0</real>
            <key>CIAttributeName</key>
            <string>inputWhitePoint</string>
        </dict>
        <dict>
            <key>CIAttributeClass</key>
            <string>NSNumber</string>
            <key>CIAttributeDefault</key>
            <real>0.0</real>
            <key>CIAttributeName</key>
            <string>inputBlackPoint</string>
        </dict>
    </array>
    <key>inputWhitePoint</key>
    <dict>
        <key>CIAttributeClass</key>
        <string>NSNumber</string>
        <key>CIAttributeDefault</key>
        <real>1.0</real>
        <key>CIAttributeDisplayName</key>
        <string>White point</string>
        <key>CIAttributeIdentity</key>
        <real>1.0</real>
        <key>CIAttributeMin</key>
        <real>0.0</real>
        <key>CIAttributeMax</key>
        <real>1.0</real>
        <key>CIAttributeName</key>
        <string>inputWhitePoint</string>
        <key>CIAttributeSliderMin</key>
        <real>0.0</real>
        <key>CIAttributeSliderMax</key>
        <real>1.0</real>
        <key>CIAttributeType</key>
        <string>CIAttributeTypeScalar</string>
    </dict>
    <key>inputBlackPoint</key>
    <dict>
        <key>CIAttributeClass</key>
        <string>NSNumber</string>
        <key>CIAttributeDefault</key>
        <real>0.0</real>
        <key>CIAttributeDisplayName</key>
        <string>Black point</string>
        <key>CIAttributeIdentity</key>
        <real>0.0</real>
        <key>CIAttributeMin</key>
        <real>0.0</real>
        <key>CIAttributeMax</key>
        <real>1.0</real>
        <key>CIAttributeName</key>
        <string>inputBlackPoint</string>
        <key>CIAttributeSliderMin</key>
        <real>0.0</real>
        <key>CIAttributeSliderMax</key>
        <real>1.0</real>
        <key>CIAttributeType</key>
        <string>CIAttributeTypeScalar</string>
    </dict>
</dict>

You can see how the descriptions under CIInputs are as terse as possible; everything besides the absolute necessities is specified in the outer dictionaries.