same 8bit created surface resulting in a different pitch value

Question

Given:

const int w = 10;
const int h = 10;
const int d = 8;
uint8_t* pixels = new uint8_t[w * h]

SDL_Surface* src = SDL_CreateRGBSurfaceFrom(pixels, w, h, d, w, 0, 0, 0, 0);
SDL_Surface* dst = SDL_ConvertSurface(src, src->format, 0);
// src->pitch == 10;
// dst->pitch == 12;
//std:: cout << src->pitch == dst->pitch << std::endl;

My expected result is that the two pitches must match.

In this case I have the "weird" 12 pitch value for the dst image create from the src.

• Is that due to memory alignment optimization?
• Is there any reasonable explanation about it?
• Are those padding bytes used somehow or just wasted?

Answer 1

The following is based on a quick look at the SDL source code for SDL_ConvertSurface:

SDL_ConvertSurface calls SDL_CreateRGBSurface, which determines the used pitch with SDL_CalculatePitch, which aims to pad the surface to 4-bytes for speed reasons. This makes sense as memory is likely to be fetched in minimum 32-bit chunks (at least on x86).

So yes, this is indeed a memory alignment optimization (12 is padded to 4, 10 is not). The extra allocated bytes don't seem to be used, as they are allocated to be part of the surface's pixel array.