ReadPixels.cs

// OnPreRender/OnPostRender will not call in SRP
void OnPostRender()
{
        //Debug.LogFormat("frameTimer : {0} , frameInterval : {1}", frameTimer, frameInterval);
        if (isREC && (frameTimer += Time.deltaTime) > frameInterval)
        {
            frameTimer -= frameInterval;

            frameBuffer.ReadPixels(camRect, 0, 0);
            Debug.Log("EncodeToJPG Now");
            File.WriteAllBytes(NextImgFilePath(), frameBuffer.EncodeToJPG());
        }
}

string NextImgFilePath()
{
        return String.Format(imgFilePathFormat, framesCount++);
}