public void prepareForWrite(OutputStream os, int logicalScreenWidth, int logicalScreenHeight) throws Exception {
    	// Header first
    	writeHeader(os, true);
    	this.logicalScreenWidth = logicalScreenWidth;
    	this.logicalScreenHeight = logicalScreenHeight;
    	// We are going to write animated GIF, so enable animated flag
    	animated = true;
    }
This is intended to be called first when writing an animated GIF
frame by frame.

@param os OutputStream for the animated GIF
@param logicalScreenWidth width of the logical screen. If it is less than
       or equal zero, it will be determined from the first frame
@param logicalScreenHeight height of the logical screen. If it is less than
       or equal zero, it will be determined from the first frame
@throws Exception
subs/gif-revert/src/main/java/com/engineer/gif/revert/lib/AnimatedGIFWriter.java