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   1 <div class="blog">
   2 <h1 id="NAME">
   3 Providing direct memory access to SDL_Surface's pixels
   4 </h1>
   5 <div class="CONTENT">
   6 <p>In an attempt to make pixel access easier on SDL_Surface pixels. I have started work on <a href="http://github.com/kthakore/SDL_perl/commit/7ee1a1a7f162080a6fa5274e95b80961486e21e5">SDLx::Surface</a>. So far I have only start on the 32 bpp surfaces.</p><br />
   7 <p>The general idea is to make Pointer Values (PV) of each pixel in the surface and place them into a 2D matrix. First I make pointer values like this: </p><br />
   8 <pre style='color:#d1d1d1;background:#000000;'>SV <span style='color:#d2cd86; '>*</span> get_pixel32 <span style='color:#d2cd86; '>(</span>SDL_Surface <span style='color:#d2cd86; '>*</span>surface<span style='color:#d2cd86; '>,</span> <span style='color:#e66170; font-weight:bold; '>int</span> x<span style='color:#d2cd86; '>,</span> <span style='color:#e66170; font-weight:bold; '>int</span> y<span style='color:#d2cd86; '>)</span>
   9 <span style='color:#b060b0; '>{</span>
  10  <span style='color:#9999a9; '>//Convert the pixels to 32 bit </span>
  11  Uint32 <span style='color:#d2cd86; '>*</span>pixels <span style='color:#d2cd86; '>=</span> <span style='color:#d2cd86; '>(</span>Uint32 <span style='color:#d2cd86; '>*</span><span style='color:#d2cd86; '>)</span>surface<span style='color:#d2cd86; '>-</span><span style='color:#d2cd86; '>></span>pixels<span style='color:#b060b0; '>;</span>
  12
  13  <span style='color:#9999a9; '>//Get the requested pixel  </span>
  14  Uint32<span style='color:#d2cd86; '>*</span> u_ptr <span style='color:#d2cd86; '>=</span>  pixels <span style='color:#d2cd86; '>+</span> <span style='color:#d2cd86; '>(</span> y <span style='color:#d2cd86; '>*</span> surface<span style='color:#d2cd86; '>-</span><span style='color:#d2cd86; '>></span>w <span style='color:#d2cd86; '>)</span> <span style='color:#d2cd86; '>+</span> x <span style='color:#b060b0; '>;</span>
  15
  16         SV<span style='color:#d2cd86; '>*</span> sv <span style='color:#d2cd86; '>=</span> newSVpv<span style='color:#d2cd86; '>(</span><span style='color:#02d045; '>"</span><span style='color:#00c4c4; '>a</span><span style='color:#02d045; '>"</span><span style='color:#d2cd86; '>,</span><span style='color:#008c00; '>1</span><span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>//Make a temp SV* value on the go</span>
  17         SvCUR_set<span style='color:#d2cd86; '>(</span>sv<span style='color:#d2cd86; '>,</span> <span style='color:#e66170; font-weight:bold; '>sizeof</span><span style='color:#d2cd86; '>(</span>Uint32<span style='color:#d2cd86; '>)</span><span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>//Specify the new CUR length</span>
  18  SvLEN_set<span style='color:#d2cd86; '>(</span>sv<span style='color:#d2cd86; '>,</span> <span style='color:#e66170; font-weight:bold; '>sizeof</span><span style='color:#d2cd86; '>(</span>Uint32<span style='color:#d2cd86; '>)</span><span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>//Specify the LEN length</span>
  19         SvPV_set<span style='color:#d2cd86; '>(</span>sv<span style='color:#d2cd86; '>,</span><span style='color:#d2cd86; '>(</span><span style='color:#e66170; font-weight:bold; '>char</span><span style='color:#d2cd86; '>*</span><span style='color:#d2cd86; '>)</span>u_ptr<span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>// set the actual pixel's pointer as the memory space to use</span>
  20
  21  <span style='color:#e66170; font-weight:bold; '>return</span> sv<span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>//make a modifiable reference using u_ptr's place as the memory :)</span>
  22
  23 <span style='color:#b060b0; '>}</span>
  24 </pre><br />
  25 <p>Next I loop through all the pixels and put them in a 2D array format, shown below: </p><pre style='color:#d1d1d1;background:#000000;'>AV <span style='color:#d2cd86; '>*</span> construct_p_matrix <span style='color:#d2cd86; '>(</span> SDL_Surface <span style='color:#d2cd86; '>*</span>surface <span style='color:#d2cd86; '>)</span>
  26 <span style='color:#b060b0; '>{</span>
  27     AV <span style='color:#d2cd86; '>*</span> matrix <span style='color:#d2cd86; '>=</span> newAV<span style='color:#d2cd86; '>(</span><span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span>
  28      <span style='color:#e66170; font-weight:bold; '>int</span> i<span style='color:#d2cd86; '>,</span> j<span style='color:#b060b0; '>;</span>
  29      <span style='color:#e66170; font-weight:bold; '>for</span><span style='color:#d2cd86; '>(</span>  i <span style='color:#d2cd86; '>=</span><span style='color:#008c00; '>0</span> <span style='color:#b060b0; '>;</span> i <span style='color:#d2cd86; '>&lt;</span> surface<span style='color:#d2cd86; '>-</span><span style='color:#d2cd86; '>></span>w<span style='color:#b060b0; '>;</span> i<span style='color:#d2cd86; '>+</span><span style='color:#d2cd86; '>+</span><span style='color:#d2cd86; '>)</span>
  30       <span style='color:#b060b0; '>{</span>
  31         AV <span style='color:#d2cd86; '>*</span> matrix_row <span style='color:#d2cd86; '>=</span> newAV<span style='color:#d2cd86; '>(</span><span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span>
  32         <span style='color:#e66170; font-weight:bold; '>for</span><span style='color:#d2cd86; '>(</span> j <span style='color:#d2cd86; '>=</span><span style='color:#008c00; '>0</span> <span style='color:#b060b0; '>;</span> j <span style='color:#d2cd86; '>&lt;</span> surface<span style='color:#d2cd86; '>-</span><span style='color:#d2cd86; '>></span>h<span style='color:#b060b0; '>;</span> j<span style='color:#d2cd86; '>+</span><span style='color:#d2cd86; '>+</span><span style='color:#d2cd86; '>)</span>
  33         <span style='color:#b060b0; '>{</span>
  34            av_push<span style='color:#d2cd86; '>(</span>matrix_row<span style='color:#d2cd86; '>,</span> get_pixel32<span style='color:#d2cd86; '>(</span>surface<span style='color:#d2cd86; '>,</span> i<span style='color:#d2cd86; '>,</span>j<span style='color:#d2cd86; '>)</span> <span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span>
  35         <span style='color:#b060b0; '>}</span>
  36         av_push<span style='color:#d2cd86; '>(</span>matrix<span style='color:#d2cd86; '>,</span> newRV_noinc<span style='color:#d2cd86; '>(</span><span style='color:#d2cd86; '>(</span>SV<span style='color:#d2cd86; '>*</span><span style='color:#d2cd86; '>)</span> matrix_row<span style='color:#d2cd86; '>)</span> <span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span>
  37       <span style='color:#b060b0; '>}</span>
  38
  39  <span style='color:#e66170; font-weight:bold; '>return</span> matrix<span style='color:#b060b0; '>;</span>
  40 <span style='color:#b060b0; '>}</span>
  41 </pre><br />
  42 <p>You can see the complete <a href="http://github.com/kthakore/SDL_perl/blob/7ee1a1a7f162080a6fa5274e95b80961486e21e5/src/SDLx/Surface.xs">here</a>.</p><br />
  43 <p>In Perl I can do a get access on this pixel using: </p><br />
  44 <br />
  45 <pre style='color:#d1d1d1;background:#000000;'><span style='color:#e66170; font-weight:bold; '>my</span> $surf32_matrix <span style='color:#d2cd86; '>=</span> <span style='color:#904050; '>SDLx</span><span style='color:#d2cd86; '>::</span><span style='color:#904050; '>Surface</span><span style='color:#d2cd86; '>:</span><span style='color:#d2cd86; '>:</span>pixel_array<span style='color:#d2cd86; '>(</span>$screen_surface<span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span>
  46    <span style='color:#e66170; font-weight:bold; '>print</span> <span style='color:#e66170; font-weight:bold; '>unpack</span> <span style='color:#00c4c4; '>'b*'</span><span style='color:#d2cd86; '>,</span> $surf32_matrix<span style='color:#d2cd86; '>-></span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '># pixel value at x = 0 and y =0</span>
  47 <span style='color:#9999a9; '>#OUTPUT:</span>
  48 <span style='color:#9999a9; '># 11111111000000000000000000000000</span>
  49 </pre><br />
  50 <p>The structure of the PV is using Devel::Peek is : </p><br />
  51 <pre style='color:#d1d1d1;background:#000000;'><span style='color:#e66170; font-weight:bold; '>print</span> Dump $surf32_matrix<span style='color:#d2cd86; '>-></span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#b060b0; '>;</span>
  52 <span style='color:#9999a9; '>#OUTPUT:</span>
  53 <span style='color:#9999a9; '>#SV = PV(0xed0dbc) at 0xeb5344</span>
  54 <span style='color:#9999a9; '>#  REFCNT = 1</span>
  55 <span style='color:#9999a9; '>#  FLAGS = (POK,pPOK)</span>
  56 <span style='color:#9999a9; '>#  PV = 0x9e04ac "\0\0\377\0"</span>
  57 <span style='color:#9999a9; '>#  CUR = 4</span>
  58 <span style='color:#9999a9; '>#  LEN = 4</span>
  59 </pre><br />
  60 <p>The problem is in setting the value of this pointer value. I have tried the following things with no success:</p><br />
  61 <pre style='color:#d1d1d1;background:#000000;'><span style='color:#e66170; font-weight:bold; '>if</span> <span style='color:#d2cd86; '>(</span> <span style='color:#904050; '>SDL</span><span style='color:#d2cd86; '>::</span><span style='color:#904050; '>Video</span><span style='color:#d2cd86; '>:</span><span style='color:#d2cd86; '>:</span>MUSTLOCK<span style='color:#d2cd86; '>(</span>$screen_surface<span style='color:#d2cd86; '>)</span> <span style='color:#d2cd86; '>)</span> <span style='color:#b060b0; '>{</span>
  62     <span style='color:#e66170; font-weight:bold; '>return</span> <span style='color:#e66170; font-weight:bold; '>if</span> <span style='color:#d2cd86; '>(</span> <span style='color:#904050; '>SDL</span><span style='color:#d2cd86; '>::</span><span style='color:#904050; '>Video</span><span style='color:#d2cd86; '>:</span><span style='color:#d2cd86; '>:</span>lock_surface<span style='color:#d2cd86; '>(</span>$screen_surface<span style='color:#d2cd86; '>)</span> <span style='color:#d2cd86; '>&lt;</span> <span style='color:#00a800; '>0</span> <span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>#required for pixel operations</span>
  63 <span style='color:#b060b0; '>}</span>
  64
  65 <span style='color:#9999a9; '>#USING pack</span>
  66
  67 <span style='color:#e66170; font-weight:bold; '>my</span> $green <span style='color:#d2cd86; '>=</span> <span style='color:#e66170; font-weight:bold; '>pack</span> <span style='color:#00c4c4; '>'b*'</span><span style='color:#d2cd86; '>,</span> <span style='color:#00c4c4; '>'11111111000000000000000000000000'</span><span style='color:#b060b0; '>;</span>
  68 <span style='color:#e66170; font-weight:bold; '>substr</span><span style='color:#d2cd86; '>(</span> $surf32_matrix<span style='color:#d2cd86; '>-></span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#d2cd86; '>,</span> <span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>,</span> <span style='color:#00a800; '>8</span> <span style='color:#d2cd86; '>*</span> <span style='color:#00a800; '>4</span><span style='color:#d2cd86; '>,</span> $green<span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>#no change</span>
  69 <span style='color:#9999a9; '>#substr( $surf32_matrix->[0][0], 0, 8 * 4, 0xFF000000); segfault</span>
  70 <span style='color:#e66170; font-weight:bold; '>substr</span><span style='color:#d2cd86; '>(</span> ${$surf32_matrix<span style='color:#d2cd86; '>-></span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span>}<span style='color:#d2cd86; '>,</span> <span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>,</span> <span style='color:#00a800; '>8</span> <span style='color:#d2cd86; '>*</span> <span style='color:#00a800; '>4</span><span style='color:#d2cd86; '>,</span> <span style='color:#00a800; '>0xFF000000</span><span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>#no change</span>
  71 <span style='color:#9999a9; '>#$surf32_matrix->[0][0] = $green; SEGFAULT's cannot write to memory</span>
  72 ${$surf32_matrix<span style='color:#d2cd86; '>-></span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span><span style='color:#d2cd86; '>[</span><span style='color:#00a800; '>0</span><span style='color:#d2cd86; '>]</span>} <span style='color:#d2cd86; '>=</span> $green<span style='color:#b060b0; '>;</span> <span style='color:#9999a9; '>#no change</span>
  73
  74
  75 <span style='color:#904050; '>SDL</span><span style='color:#d2cd86; '>::</span><span style='color:#904050; '>Video</span><span style='color:#d2cd86; '>:</span><span style='color:#d2cd86; '>:</span>unlock_surface<span style='color:#d2cd86; '>(</span>$screen_surface<span style='color:#d2cd86; '>)</span>
  76   <span style='color:#e66170; font-weight:bold; '>if</span> <span style='color:#d2cd86; '>(</span> <span style='color:#904050; '>SDL</span><span style='color:#d2cd86; '>::</span><span style='color:#904050; '>Video</span><span style='color:#d2cd86; '>:</span><span style='color:#d2cd86; '>:</span>MUSTLOCK<span style='color:#d2cd86; '>(</span>$screen_surface<span style='color:#d2cd86; '>)</span> <span style='color:#d2cd86; '>)</span><span style='color:#b060b0; '>;</span>
  77 </pre><br />
  78 <p>You can see an example <a href="http://paste.scsys.co.uk/45111">here</a>.</p><br />
  79 <p>Any help will be greatly appreciated. </p><div class="blogger-post-footer"><img width='1' height='1' src='https://blogger.googleusercontent.com/tracker/3102167581424744259-1929043568015240773?l=yapgh.blogspot.com' alt='' /></div>
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