Efficiency of querying an SDL_Surface for pixel colors
I'm writing a game that has a simple 2D map and needs to determine which "region" on the map the user clicked. Each region has a unique RGB color associated with it. So, given the (x,y) coordinates of a click, I do this:
Uint32 *pixels = (Uint32 *) image_surface_->pixels;
Uint32 pixel = pixels[(y * image_surface_->w) + x];
Uint8 r, g, b;
SDL_GetRGB(pixel, image_surface_->format, &r, &g, &b);
SDL_Color rgb = r, g, b, 255;
where image_surface_ is the SDL_Surface which contains the map that was loaded into the game's SDL_Window.
After the bit of code above, I run through the list of regions and check which of them has a matching color value.
My question is, what is the efficiency of querying an SDL_Surface's pixels? Is it just O(1) as appears to be the case because all you're doing is indexing a pointer? Or is there more going on?
(For the actual game I'm writing, poor efficiency doesn't matter much, but I plan on developing a mini-engine for the writing of 2D (and potentially, 3D games) that use a map, which could theoretically be very large with many regions)
c++ performance sdl sdl-2
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
|
show 5 more comments
I'm writing a game that has a simple 2D map and needs to determine which "region" on the map the user clicked. Each region has a unique RGB color associated with it. So, given the (x,y) coordinates of a click, I do this:
Uint32 *pixels = (Uint32 *) image_surface_->pixels;
Uint32 pixel = pixels[(y * image_surface_->w) + x];
Uint8 r, g, b;
SDL_GetRGB(pixel, image_surface_->format, &r, &g, &b);
SDL_Color rgb = r, g, b, 255;
where image_surface_ is the SDL_Surface which contains the map that was loaded into the game's SDL_Window.
After the bit of code above, I run through the list of regions and check which of them has a matching color value.
My question is, what is the efficiency of querying an SDL_Surface's pixels? Is it just O(1) as appears to be the case because all you're doing is indexing a pointer? Or is there more going on?
(For the actual game I'm writing, poor efficiency doesn't matter much, but I plan on developing a mini-engine for the writing of 2D (and potentially, 3D games) that use a map, which could theoretically be very large with many regions)
c++ performance sdl sdl-2
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
2
It is indeed O(1) but I wouldn't be using functions lie SDL_GetRGB if you can avoid it. Per pixel inner loop stuff is best done without calling a function or needing an if statement (ie branching). You can make the "pixel" pointer uint8_t* so that you can address the individual rgb components like pixel[RED_OFFSET], pixel[BLUE_OFFSET] for example.
– fret
Nov 13 '18 at 3:35
@fret What is the reason I should avoid calling a function?
– ubadub
Nov 13 '18 at 4:58
1
@ubadub function call is not free, and said function have a branch inside it. If target CPU don't have branch prediction - that will not be great to call it too many times in a loop (e.g. iterating through entire image). Not terrible, but you can do better without extra function call or braches.
– keltar
Nov 13 '18 at 8:59
1
@keltar never mind, I got it. Leaving it here for anyone else who needs it:(pixel & image_surface_->format->Rmask) >> image_surface_->format->Rshift;
– ubadub
Nov 14 '18 at 16:43
1
Also, this code is wrong. You should always usesurface->pitchinstead of width for calculating which row you are on. Even then this of course will be broken if you ever need to use it for a non 32 bit surface. Another reason to just do it the right way so you aren't needlessly shackled to specific formats.
– Brad Allred
Nov 15 '18 at 23:23
|
show 5 more comments
I'm writing a game that has a simple 2D map and needs to determine which "region" on the map the user clicked. Each region has a unique RGB color associated with it. So, given the (x,y) coordinates of a click, I do this:
Uint32 *pixels = (Uint32 *) image_surface_->pixels;
Uint32 pixel = pixels[(y * image_surface_->w) + x];
Uint8 r, g, b;
SDL_GetRGB(pixel, image_surface_->format, &r, &g, &b);
SDL_Color rgb = r, g, b, 255;
where image_surface_ is the SDL_Surface which contains the map that was loaded into the game's SDL_Window.
After the bit of code above, I run through the list of regions and check which of them has a matching color value.
My question is, what is the efficiency of querying an SDL_Surface's pixels? Is it just O(1) as appears to be the case because all you're doing is indexing a pointer? Or is there more going on?
(For the actual game I'm writing, poor efficiency doesn't matter much, but I plan on developing a mini-engine for the writing of 2D (and potentially, 3D games) that use a map, which could theoretically be very large with many regions)
c++ performance sdl sdl-2
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
I'm writing a game that has a simple 2D map and needs to determine which "region" on the map the user clicked. Each region has a unique RGB color associated with it. So, given the (x,y) coordinates of a click, I do this:
Uint32 *pixels = (Uint32 *) image_surface_->pixels;
Uint32 pixel = pixels[(y * image_surface_->w) + x];
Uint8 r, g, b;
SDL_GetRGB(pixel, image_surface_->format, &r, &g, &b);
SDL_Color rgb = r, g, b, 255;
where image_surface_ is the SDL_Surface which contains the map that was loaded into the game's SDL_Window.
After the bit of code above, I run through the list of regions and check which of them has a matching color value.
My question is, what is the efficiency of querying an SDL_Surface's pixels? Is it just O(1) as appears to be the case because all you're doing is indexing a pointer? Or is there more going on?
(For the actual game I'm writing, poor efficiency doesn't matter much, but I plan on developing a mini-engine for the writing of 2D (and potentially, 3D games) that use a map, which could theoretically be very large with many regions)
c++ performance sdl sdl-2
c++ performance sdl sdl-2
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
asked Nov 13 '18 at 3:25
ubadububadub
2,046821
2,046821
2
It is indeed O(1) but I wouldn't be using functions lie SDL_GetRGB if you can avoid it. Per pixel inner loop stuff is best done without calling a function or needing an if statement (ie branching). You can make the "pixel" pointer uint8_t* so that you can address the individual rgb components like pixel[RED_OFFSET], pixel[BLUE_OFFSET] for example.
– fret
Nov 13 '18 at 3:35
@fret What is the reason I should avoid calling a function?
– ubadub
Nov 13 '18 at 4:58
1
@ubadub function call is not free, and said function have a branch inside it. If target CPU don't have branch prediction - that will not be great to call it too many times in a loop (e.g. iterating through entire image). Not terrible, but you can do better without extra function call or braches.
– keltar
Nov 13 '18 at 8:59
1
@keltar never mind, I got it. Leaving it here for anyone else who needs it:(pixel & image_surface_->format->Rmask) >> image_surface_->format->Rshift;
– ubadub
Nov 14 '18 at 16:43
1
Also, this code is wrong. You should always usesurface->pitchinstead of width for calculating which row you are on. Even then this of course will be broken if you ever need to use it for a non 32 bit surface. Another reason to just do it the right way so you aren't needlessly shackled to specific formats.
– Brad Allred
Nov 15 '18 at 23:23
|
show 5 more comments
2
It is indeed O(1) but I wouldn't be using functions lie SDL_GetRGB if you can avoid it. Per pixel inner loop stuff is best done without calling a function or needing an if statement (ie branching). You can make the "pixel" pointer uint8_t* so that you can address the individual rgb components like pixel[RED_OFFSET], pixel[BLUE_OFFSET] for example.
– fret
Nov 13 '18 at 3:35
@fret What is the reason I should avoid calling a function?
– ubadub
Nov 13 '18 at 4:58
1
@ubadub function call is not free, and said function have a branch inside it. If target CPU don't have branch prediction - that will not be great to call it too many times in a loop (e.g. iterating through entire image). Not terrible, but you can do better without extra function call or braches.
– keltar
Nov 13 '18 at 8:59
1
@keltar never mind, I got it. Leaving it here for anyone else who needs it:(pixel & image_surface_->format->Rmask) >> image_surface_->format->Rshift;
– ubadub
Nov 14 '18 at 16:43
1
Also, this code is wrong. You should always usesurface->pitchinstead of width for calculating which row you are on. Even then this of course will be broken if you ever need to use it for a non 32 bit surface. Another reason to just do it the right way so you aren't needlessly shackled to specific formats.
– Brad Allred
Nov 15 '18 at 23:23
2
2
It is indeed O(1) but I wouldn't be using functions lie SDL_GetRGB if you can avoid it. Per pixel inner loop stuff is best done without calling a function or needing an if statement (ie branching). You can make the "pixel" pointer uint8_t* so that you can address the individual rgb components like pixel[RED_OFFSET], pixel[BLUE_OFFSET] for example.
– fret
Nov 13 '18 at 3:35
It is indeed O(1) but I wouldn't be using functions lie SDL_GetRGB if you can avoid it. Per pixel inner loop stuff is best done without calling a function or needing an if statement (ie branching). You can make the "pixel" pointer uint8_t* so that you can address the individual rgb components like pixel[RED_OFFSET], pixel[BLUE_OFFSET] for example.
– fret
Nov 13 '18 at 3:35
@fret What is the reason I should avoid calling a function?
– ubadub
Nov 13 '18 at 4:58
@fret What is the reason I should avoid calling a function?
– ubadub
Nov 13 '18 at 4:58
1
1
@ubadub function call is not free, and said function have a branch inside it. If target CPU don't have branch prediction - that will not be great to call it too many times in a loop (e.g. iterating through entire image). Not terrible, but you can do better without extra function call or braches.
– keltar
Nov 13 '18 at 8:59
@ubadub function call is not free, and said function have a branch inside it. If target CPU don't have branch prediction - that will not be great to call it too many times in a loop (e.g. iterating through entire image). Not terrible, but you can do better without extra function call or braches.
– keltar
Nov 13 '18 at 8:59
1
1
@keltar never mind, I got it. Leaving it here for anyone else who needs it:
(pixel & image_surface_->format->Rmask) >> image_surface_->format->Rshift;– ubadub
Nov 14 '18 at 16:43
@keltar never mind, I got it. Leaving it here for anyone else who needs it:
(pixel & image_surface_->format->Rmask) >> image_surface_->format->Rshift;– ubadub
Nov 14 '18 at 16:43
1
1
Also, this code is wrong. You should always use
surface->pitch instead of width for calculating which row you are on. Even then this of course will be broken if you ever need to use it for a non 32 bit surface. Another reason to just do it the right way so you aren't needlessly shackled to specific formats.– Brad Allred
Nov 15 '18 at 23:23
Also, this code is wrong. You should always use
surface->pitch instead of width for calculating which row you are on. Even then this of course will be broken if you ever need to use it for a non 32 bit surface. Another reason to just do it the right way so you aren't needlessly shackled to specific formats.– Brad Allred
Nov 15 '18 at 23:23
|
show 5 more comments
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2
It is indeed O(1) but I wouldn't be using functions lie SDL_GetRGB if you can avoid it. Per pixel inner loop stuff is best done without calling a function or needing an if statement (ie branching). You can make the "pixel" pointer uint8_t* so that you can address the individual rgb components like pixel[RED_OFFSET], pixel[BLUE_OFFSET] for example.
– fret
Nov 13 '18 at 3:35
@fret What is the reason I should avoid calling a function?
– ubadub
Nov 13 '18 at 4:58
1
@ubadub function call is not free, and said function have a branch inside it. If target CPU don't have branch prediction - that will not be great to call it too many times in a loop (e.g. iterating through entire image). Not terrible, but you can do better without extra function call or braches.
– keltar
Nov 13 '18 at 8:59
1
@keltar never mind, I got it. Leaving it here for anyone else who needs it:
(pixel & image_surface_->format->Rmask) >> image_surface_->format->Rshift;– ubadub
Nov 14 '18 at 16:43
1
Also, this code is wrong. You should always use
surface->pitchinstead of width for calculating which row you are on. Even then this of course will be broken if you ever need to use it for a non 32 bit surface. Another reason to just do it the right way so you aren't needlessly shackled to specific formats.– Brad Allred
Nov 15 '18 at 23:23