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Object Processor and the object list


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In JagDox we can read that the object list has to be created every VBL due to OP modify it. What is the reason of that "modification"?



2.5   The object in the objectlist are *modified* by the OP. This means      that an object list is only good for one frame. You need to      continually refresh your object list each VBLANK.

By a modification I understand the process of reading a phrase from a memory and writing it modified back. It is two cycles per phrase. Am I correct?



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The OP itself updates the memory in the list. As it builds the display a scanline at a time it does some logic on members . Now this is vague memories from years ago , but a normal sprite will have a start Y and an end Y and a data pointer for the current Y. The OP will ignore it until the Y matches . I then think as long as its not the end Y the start Y gets updated and the data pointer will be updated to the next line down.

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The Object Processor works this way because it renders the screen one line at a time, not one object at a time.

So for each object, it needs to remember what has already been drawn, and what remains to be drawn.

If it didn't modify the object list, it would need extra memory for that, which is a problem since there is no hard limit on the number of objects per screen.

Edited by Zerosquare
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