https://svn.lrde.epita.fr/svn/oln/trunk Index: ChangeLog from Thierry Geraud &lt;thierry.geraud(a)lrde.epita.fr&gt; Augment tutorial on image types. * milena/doc/tutorial/image_types.txt: Augment. image_types.txt | 129 +++++++++++++++++++++++++++++++++++++++++++++++++++++++- 1 files changed, 128 insertions(+), 1 deletion(-) Index: milena/doc/tutorial/image_types.txt --- milena/doc/tutorial/image_types.txt (revision 1763) +++ milena/doc/tutorial/image_types.txt (working copy) @@ -59,6 +59,14 @@ for all v of ima.values | print v. +For such image types, we have several points (sites) associated with a +single value cell. It is rather different from having couples (p,v); +we actually have one value cell -> a set of points. Such data +structure allows for compressing data because each value cell is +shared by several points. + +*** Example 1 + Consider for instance the following image: +-----+ @@ -67,6 +75,8 @@ | 22 | +-----+ +We have one value per run. + The value type is int_u2 (unsigned integer on 2 bit), that is, the type of the interval [0,3] of N. The image is defined by a run-length encoding with a single value per run. Precisely, this image is: @@ -93,6 +103,115 @@ v v v { ((a,1),2), ((b,3),1), ((c,2),2) } +**** A clue? + +***** part 1 + +When we have value cells, it is usually related to the fact that the +image is not point-wise writable. For instance, with the image type +having one value per run, the operation "ima(p) := v" is not possible. +Otherwise, that would mean that a run is split on such a write access, +which is too slow (thus not allowed). + +Some rare image types can be both point-wise writable and value cell +iterable. For instance: + + image<int_u8> data; + map<int_u8, rgb_3x8> lut; // with i != j => lut[i] != lut[j] + map<rgb_3x8, int_u8> reverse_lut; + +The keys, resp. the values, of the lut are the values, resp. the keys, +of the reverse_lut. With this structure, we can have a rather efficient +point-wise writable image. + +ima(p) <- v { + it <- reverse_lut.find(v) + if (it is not found) + ++n + lut(n) <- v + reverse_lut(v) <- n + data(p) <- n + else + data(p) <- it.key +} + +***** part 2 + +When an image is value-wise writable, we can form: + ima.value(red) = blue; +It means that we can quickly access to the cell (or the cells) +having the value 'red'. + +When an image is value-cell writable, we can have: + for v in ima.values() + ima.cell(v) = v' +In that case, a 'v' is a value cell, not a value. + +***** part 3 + +Is the notion of value-wise readable relevant? + ima.value(red) // read-access +gives: + - red, which is dummy! +or - the corresponding point set, which can be great! + +This is another property... + + +*** Example 2 + +Consider this image type: + +{ data + +-----------+ + | 0 2 1 0 2 | + | 0 0 3 2 1 | + | 2 1 2 2 0 | + +-----------+; + lut + +---+---------------+ + | 0 | (255, 0, 0) | red (r) + | 1 | (127,127,127) | medium grey (m) + | 2 | ( 0, 0,255) | blue (b) + | 3 | (127,127,127) | medium grey (m) again + +---+---------------+ +} + +This image is defined with a look-up table; its external +representation actually is: + + +-----------+ + | r b m r b | + | r r m b m | + | b m b b r | + +-----------+ + +(conversely, the internal representation is the 'data' image.) + +We have I::value = rgb_3x8 +so the image appears to be highly quantized (24 bit). + +Yet the number of possible values is limited (here up to 4, even if +two value cells are identical) and stored by the image in the look-up +table. + +It leads to: + code A) rbmrbrrmbmbmbbr + code B) rmb + code C) rmbm + +ima.destination (browsed by code B) is an instance of +value::subset<rgb_3x8>. + +FIXME: OR (?) + +It leads to: + code A) rbmrbrrmbmbmbbr + code B) impossible because 24bit data + code C) rmbm + +and I.destination gives value::set<rgb_3x8>. + ** associated types @@ -232,7 +351,15 @@ | + -- high -**** +**** low + +I::value defines a set of possible values whose cardinality is less +than or equal to 2^16. + +FIXME: OR (?) + +I.destination (same definition) + *** value |