What format of BMP does the SDL_LoadBMP support? Basically, I’m curious if
I generate a BMP written out in A8R8G8B8 format, can I load that BMP and
have an alpha channel, without needing SDL_Image and loading it as a TGA
instead?
I’ve tried loading it in to memory and loading it from there:
rw = SDLInterop.RWFromMem(arr, arr.Length)
temp = SDLInterop.LoadBMP_RW(rw, 1)
And then I call SDL_DisplayFormatAlpha, but the resulting image does not
have per-pixel transparency. Is what I’m trying to do even possible?
What format of BMP does the SDL_LoadBMP support? Basically, I’m curious if
I generate a BMP written out in A8R8G8B8 format, can I load that BMP and
have an alpha channel, without needing SDL_Image and loading it as a TGA
instead?
…
And then I call SDL_DisplayFormatAlpha, but the resulting image does not
have per-pixel transparency. Is what I’m trying to do even possible?
I'm using PNG files with SDL Image and no problem loading the alpha
channel from them. The greatest point on using PNG files is that they
are MUCH smaller than BMPs, and the image quality is way better than
with JPEG files.
Besides, does BMP files actually (or officially, for that matter)
support alpha channel?
Best regards,
Paulo V W Radtke - Chien Loco Jogos de Garagem?
I’m using PNG files with SDL Image and no problem
loading the alpha
channel from them. The greatest point on using PNG files is that they
are MUCH smaller than BMPs, and the image quality is way
better than
with JPEG files.
I agree that using SDL_Image would be ideal. After all, loading disparate
image formats is what it was designed to do. Unfortunately, I’m using SDL
on my PocketPC and we are having a heckuva time getting SDL_Image to compile
and work, so for now we’re sticking with the base library.
I believe I have found my own answer. It does appear to load the 32bpp BMP
files, but I’m more than a little rusty in RGBA->RGB blits in SDL, so I’m
not 100% certain the alpha is working. (I seem to remember something about
having to load the image, create another surface, blit the loaded image to
the target surface, etc.) I’m working on it. (Anyone have any definitive
alpha examples?)
Besides, does BMP files actually (or officially, for
that matter)
support alpha channel?
Geeze, if you want to get all “official” and everything. No, I do not
believe it’s officially supported, but many image editors will write 32bpp
BMP files since there’s no TECHNICAL reason they can’t have that colour
depth. Photoshop even lets you choose X8R8G8B8 and A8R8G8B8 if you want.
It’s pretty nice.
Geeze, if you want to get all “official” and everything. No, I do not
believe it’s officially supported, but many image editors will write 32bpp
BMP files since there’s no TECHNICAL reason they can’t have that colour
depth. Photoshop even lets you choose X8R8G8B8 and A8R8G8B8 if you want.
It’s pretty nice.
Technically if the formal BMP specification does not specifies such
information, it means that not everybody may support it. PaintShopPro
does not even recognize a 32 bits BMP as a valid file, the same for the
Windows XP integrated Windows Picture and Fax Viewer - and if there’s
somebody who should know BMP files, Microsoft should be the one. So
maybe there’s a point on why SDL may be missing the alpha channel (may
someone involved with this part step up and confirm it?).
But getting back to the point, i.e., helping you solve your problem,
one solution is to separate the alpha channel on Photoshop and save it
as a gray scale bitmap. Then you load your color BMP file, convert it to
a 32 bits surface and fill the alpha values from the second file which
contains the alpha from the complete image.
Another option is to make a TGA loader, which is very simple - check
somebody who should know BMP files, Microsoft should be the one. So
maybe there’s a point on why SDL may be missing the alpha channel (may
someone involved with this part step up and confirm it?).
It does seem that the alpha channel is loaded as I’m definately getting
some per-pixel alpha effect in my experimentations. Of course, I could be
misinterpreting some other effect. I’m still having a great deal of
difficulty remembering all the tricks to getting alpha-blended images to
the display, but I’m getting there.
But getting back to the point, i.e., helping you solve your problem,
one solution is to separate the alpha channel on Photoshop and save it
as a gray scale bitmap. Then you load your color BMP file, convert it to
a 32 bits surface and fill the alpha values from the second file which
contains the alpha from the complete image.
Would you load the image and the alpha, then create another surface at
32bpp, then blit the image on to the surface, and then use… what, some
kind of SetPixel command to push the alpha over? Just not sure of what
the approach would be here.
Actually, thanks very much for that link. That’s probably one of the
clearest explanations of the file format that I’ve seen. And while I can
definately load the image in to memory, I’m having a great deal of
difficulty in creating the SDL_RWOps structure, due to the large number of
pointers it uses. (As I mentioned before I’m writing this code in the
compact framework for PocketPC, which makes constructing things like that
… challenging.)
I’m going to continue to fiddle with the code, but I do appreciate every
insight I can get in to the issue.
Would you load the image and the alpha, then create another surface at
32bpp, then blit the image on to the surface, and then use… what, some
kind of SetPixel command to push the alpha over? Just not sure of what
the approach would be here.
That's it. You can use the SDL page provided set pixel example and the
alpha mask to change the alpha values.
Actually, thanks very much for that link. That’s probably one of the
clearest explanations of the file format that I’ve seen. And while I can
definately load the image in to memory, I’m having a great deal of
difficulty in creating the SDL_RWOps structure, due to the large number of
pointers it uses. (As I mentioned before I’m writing this code in the
compact framework for PocketPC, which makes constructing things like that
… challenging.)
Actually you can get rid wit RWOps if you make your own TGA loader, you
can use the raw RGB data read from the TGA file to create an SDL surface.
Best regards,
Paulo V W Radtke - Chien Loco Jogos de Garagem?
What format of BMP does the SDL_LoadBMP support? Basically, I’m curious
if I generate a BMP written out in A8R8G8B8 format, can I load that BMP
and have an alpha channel, without needing SDL_Image and loading it as a
TGA instead?
I’ve tried loading it in to memory and loading it from there:
rw = SDLInterop.RWFromMem(arr, arr.Length) temp =
SDLInterop.LoadBMP_RW(rw, 1)
And then I call SDL_DisplayFormatAlpha, but the resulting image does not
have per-pixel transparency. Is what I’m trying to do even possible?
Todd Lang
You can use this patch to read ARGB BMP:
GautierOn Mon, 28 Feb 2005 22:19:54 -0500, Todd Lang wrote:
No, I do not