Nuke works in scanlines like this, and can process a whole tree of operations only loading the input lines necessary for the current output row.  The SDK docs explain the architecture somewhat: <a href="https:&#x2F;&#x2F;www.thefoundry.co.uk&#x2F;products&#x2F;nuke&#x2F;developers&#x2F;90&#x2F;ndkdevguide&#x2F;2d&#x2F;architecture.html#fundamental-image-processing-unit-the-row" rel="nofollow">https:&#x2F;&#x2F;www.thefoundry.co.uk&#x2F;products&#x2F;nuke&#x2F;developers&#x2F;90&#x2F;ndk...</a>