I have read Wiki http://en.wikipedia.org/wiki/Truevision_TGA and I know that the 5-4 bits of the Eighteenth byte of tga file represent the direction of tga file.
So, there is 4 permutations(00,01,10,11) to show us the direction, which direction does each permutation stand for?
From the PDF Truevision TGAª — FILE FORMAT SPECIFICATION — Version 2.0 linked on the Wikipedia page.
Bits 5 & 4: These bits are used to indicate the order in which pixel data is transferred from the file to the screen. Bit 4 is for left-to-right ordering and bit 5 is for top-to-bottom ordering as shown below.
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