Unity-Technologies/ml-agents · error · IndexOutOfRangeException

width value must be in range [0, ]

Error message

width value {w} must be in range [0, {m_TensorShape.Width() - 1}]

What it means

ObservationWriter's indexed setter validates that the width coordinate falls within the observation tensor's width before writing. Writing outside that range would corrupt adjacent data in the flat backing array, so an IndexOutOfRangeException is thrown.

Solutions

1. Clamp loop bounds to the observation spec's width: for (int w = 0; w < spec.Width(); w++)
2. Use ObservationWriter helper APIs (WriteTex2D, Add) that stay within the tensor bounds
3. Ensure ObservationSpec width matches the actual data layout being written
4. Check for off-by-one loops (w <= width instead of w < width)

Example fix

// before
for (int w = 0; w <= tex.width; w++) writer[0, w, 0] = pixel;
// after
for (int w = 0; w < obsSpec.Width(); w++) writer[0, w, 0] = pixel;

// Unity C#: in custom sensor Write
if (w < 0 || w >= obsSpec.Width())
{
    Debug.LogError($"w={w} outside [0,{obsSpec.Width() - 1}]");
    return;
}
writer[h, w, ch] = value;

try
{
    writer[h, w, ch] = value;
}
catch (IndexOutOfRangeException e) when (e.Message.Contains("width value"))
{
    Debug.LogError("Width out of bounds writing observation: " + e.Message);
}

When it happens

Trigger: A custom sensor's Write() implementation calling Write(h, w, ch, value) with w < 0 or w >= observation width; iterating source texture columns wider than the declared observation width; off-by-one loop bounds.

Common situations: Custom visual sensors whose render texture is wider than the observation spec width; writing in column-major order with wrong limits; resizing an observation without updating the writing loops.

Related errors

• channel value must be in range [0, ]
• height value must be in range [0, ]
• Cannot move cell at row=
• col was , but must be between 0 and .
• row was , but must be between 0 and .

AI-assisted analysis of Unity-Technologies/ml-agents@3ecb446f75 (2026-09-02). Data as JSON: /api/errors/f60bd2b2732f892d. Report an issue: GitHub.

Appendix: source

Thrown at com.unity.ml-agents/Runtime/Sensors/ObservationWriter.cs:147

        /// 3D write access at the specified height, width, and channel.
        /// </summary>
        /// <param name="h">Height</param>
        /// <param name="w">Width</param>
        /// <param name="ch">Channels</param>
        public float this[int ch, int h, int w]
        {
            set
            {
                if (m_Data != null)
                {
                    if (h < 0 || h >= m_TensorShape.Height())
                    {
                        throw new IndexOutOfRangeException($"height value {h} must be in range [0, {m_TensorShape.Height() - 1}]");
                    }

                    if (w < 0 || w >= m_TensorShape.Width())
                    {
                        throw new IndexOutOfRangeException($"width value {w} must be in range [0, {m_TensorShape.Width() - 1}]");
                    }

                    if (ch < 0 || ch >= m_TensorShape.Channels())
                    {
                        throw new IndexOutOfRangeException($"channel value {ch} must be in range [0, {m_TensorShape.Channels() - 1}]");
                    }

                    var index = m_TensorShape.Index(m_Batch, ch + m_Offset, h, w);
                    m_Data[index] = value;
                }
                else
                {
                    if (ch + m_Offset < 0 || ch + m_Offset >= m_TensorShape.Channels() ||
                        h < 0 || h >= m_TensorShape.Height() ||
                        w < 0 || w >= m_TensorShape.Width())
                        return;

                    m_Proxy.data.CompleteAllPendingOperations();

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