CODEWITHSUNDEEP
c++sdl
user451498
user451498

Reputation:

Is it safe to access SDL_Surface's pitch data member if the surface is locked?

I have the following code in my font rendering test:

SDL_Surface* image = SDL_CreateRGBSurface(SDL_SWSURFACE,
                                          face->glyph->bitmap.width,
                                          face->glyph->bitmap.rows,
                                          8,
                                          0, 0, 0,
                                          0);
if(!image)
{
  throw std::runtime_error("Failed to generate 8 bit image");
}

//There's no better way to do this, right? Being it just sets up a basic grayscale palette
SDL_Color colors[256];
for(int i=0; i<256; ++i)
{
  colors[i].r = i;
  colors[i].g = i;
  colors[i].b = i;
}
SDL_SetColors(image, colors, 0, 256);

SDL_LockSurface(image);

uint8_t* pixels = static_cast<uint8_t*>(image->pixels);
for(int i = 0; i<face->glyph->bitmap.rows; ++i)
{
  for(int j = 0; j<face->glyph->bitmap.width; ++j)
  {
    pixels[i * face->glyph->bitmap.width + j] = face->glyph->bitmap.buffer[
                                           i * face->glyph->bitmap.width + j];
  }
}
###############################################################################
#                           Here's the spotlight                              #
#                                    |                                        #
#                   _________________/                                        #
#                  /                                                          #
###############################################################################
image->pitch = image->w;
SDL_UnlockSurface(image);

Notice I change the pitch to the width, effectively making the byte-per-pixel equal to one. This is exactly what I need, in fact, changing the pitch like I did is the deciding factor in whether my letters come out crooked or not.

My question is: Is this defined, safe, and good practice? Is there a function to help me out that I don't know about? Is there a safer alternative?

Upvotes: 0

Views: 397

Answers (1)

goldibehr
goldibehr

Reputation: 71

You are wrong to write image->pitch. It might work in some situations, but cause corrupted display or segfault in other situations. It all depends where that chunk of memory came from.

I think your calculation of the index into pixels[] is wrong, it should be:

pixels[i * image->pitch + j] = ...

Upvotes: 2

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