URL: https://linuxfr.org/users/serge_ss_paille/journaux/pixel-art-et-c-14
Title: Pixel Art et C++14
Authors: serge_sans_paille
Date: 2018年04月03日T11:26:30+02:00
License: CC By-SA
Tags: c++14 et frozen
Score: 22
Petit délire illustrant l'usage de ``constexpr`` et de la biliothèsque [frozen](https://github.com/serge-sans-paille/frozen).
Première étape : faire un dessin en pixel art, par exemple:
```c++
 constexpr unsigned char bytes[] = " ### ### "
 " ##### ########## ##### "
 " ####### ####### "
 " #### #### "
 " ### ### "
 " # # "
 " # # "
 " # ### ### # "
 " # ##### ##### # "
 " # #### # # #### # "
 " # #### # # #### # "
 " # ##### ##### # "
 " # ### ## ### # "
 " # ## # "
 " #### RRR RRR #### "
 " ######RRRRR RRRRR###### "
 " ######RRRRRRRRRRRR###### "
 " ######RRRRRRRRRRRR###### "
 " ######RRRRRRRRRRRR###### "
 " #######RRRRRRRRRR####### "
 " ##### RRRRRRRR #### "
 " ### RRRRRR ## "
 " RRRR ";
```
On remarquera l'énorme code de couleur espace → blanc, ``#`` → noir et ``R`` → rouge.
Ensuite on encode cette colormap dans une petite table de hash (oui dans ce cas un tableau de char suffirait) :
```c++
#include <frozen/map.h>
constexpr frozen::map<char, std::array<char, 3>, 5> Tans{
 {'R', {(char)0xFF, (char)0x00, (char)0x00}},
 {'G', {(char)0x00, (char)0xFF, (char)0x00}},
 {'B', {(char)0x00, (char)0x00, (char)0xFF}},
 {'#', {(char)0x00, (char)0x00, (char)0x00}},
 {' ', {(char)0xFF, (char)0xFF, (char)0xFF}},
};
```
Et comme on est tout foufou, on demande au compilateur C++ de traduire cette image pixel art en image au format PPM, à *compile time*
```c++
constexpr unsigned itoa(unsigned char * start, unsigned i) {
 constexpr unsigned step = sizeof(unsigned) * 3;
 for(unsigned k = 0; k < step; ++k) *start++ = ' '; do { *--start = '0' + i % 10; i /= 10; } while(i); return step; } template <unsigned H, unsigned W> struct ppm {
 unsigned char bytes[9 /* fixed header*/ + sizeof(unsigned) * 3 * 2 /* to hold sizes */ + 3 * H * W];
 constexpr ppm(unsigned char const *data) : bytes{0} {
 unsigned j = 0;
 bytes[j++] = 'P';
 bytes[j++] = '6';
 bytes[j++] = ' ';
 j += itoa(bytes + j, H);
 bytes[j++] = ' ';
 j += itoa(bytes + j, W);
 bytes[j++] = ' ';
 bytes[j++] = '2';
 bytes[j++] = '5';
 bytes[j++] = '5';
 bytes[j++] = '\n';
 for (unsigned i = 0; i < H * W; ++i) { auto const code = Tans.find(data[i])->second;
 bytes[j + 3 * i + 0] = code[0];
 bytes[j + 3 * i + 1] = code[1];
 bytes[j + 3 * i + 2] = code[2];
 }
 }
 void save(char const path[]) const {
 std::ofstream out{path, std::ios::binary};
 out.write((char *)bytes, sizeof bytes);
 }
};
```
On vient donc de créer une image dans un format binaire, en embarquant les sources de cette binaire dans les sources. À quand un encodeur jpeg ``constexpr``?
PS: sources complètes [par ici](https://github.com/serge-sans-paille/frozen/blob/master/examples/pixel_art.cpp)
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