RLE is an old technique being used for decades, as is known as
Run-Length-Encoding, which means that for repeating sequence of bytes,
we keep an indicator for the length of the sequence and only one sample
of it, to save storage space.
GIMP can generate lossless-compressed TGA images, with RLE compression
being used. It means that for a compressed image, the data is no longer
arranged in sequence of pixels, but a sequence of pixel packets.
There are two possible pixel packets:
- RLE packets, which are encoded with one byte for indicating the
run-length and another one pixel (3 bytes for TrueColor pixel), so
essentially in runtime, the TGA decoder will use the length to plot
the same pixel in multiple pixels of the output pixel bitmap.
- Raw packets, which are encoded with one byte as indicator for the
length of the whole pixel sequence and N-length pixel sequence
afterwards.
This is not used for any sort of compression by the TGA format, but
still needed to be supported for full compatibility with TGA images
that uses the RLE compression.
GIMP allows a user to export a TGA image with either of these possible
orientations, so we can easily support it by looking at the X origin and
Y origin values to determine where to put the pixels in the bitmap.
