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OpenGL Masking question (from OpenGL newbie)

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What''s the simplest/most efficient way to set OpenGL to read black pixels (RGB 0,0,0) as transparent? I was playing around with glBlendFunc() but couldn''t get anything to work. I''m using 24-bit bitmaps in my images. Thanks for any help/suggestions

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You could use additive blending (be warned, you won''t get exactly what you''re expecting though). My advice is to simply use an alpha channel with alpha testing though. Not only is it faster, it could get you the exact kind of transparency you''re looking for.

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thanks for the advice. however, does this mean I can''t use bitmaps anymore? I was under the impression that bitmaps didn''t support alpha channels. If that is the case, I had better get writing my targa file loader

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well, since you''ve been so helpful so far
how do i go about using alpha channels to do transparency?

thanks again for your time

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If you wanted to make all pixels with an alpha value of 0 transparent, you'd do the following:
  glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0);
The second line causes only pixels with an alpha value greater than zero to be drawn. Look at the documentation for glAlphaFunc for the other options available to you.

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Also, when you create your texture image, it must be 32 bit. Make sure it has an alpah channel where all the pixels you want to be transparent are black, and all those that you want to see are white

'I sure could use a vacation from this bullshit three ring circus sideshow of freaks...' - TOOL
[TheBlackJester]


Edited by - TheBlackJester on February 16, 2002 1:15:35 PM

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well it was. it loaded once, but it was upside down, so i quit and flipped the image in Gimp. now when i try to load it, it won''t. any ideas?

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I'm using nehe's tga loading code:

  
typedef struct
{
GLubyte *ImageData; // image data (up to 32 bits)

GLuint BitDepth; // color depth

GLuint Width; // image width

GLuint Height; // image height

GLuint ImgID; // image ID number if used in an array

}TGAFILE;

TGAFILE *PlayerShip; // this is what i'm sending the function


// and this is how i'm using it

PlayerShip = (TGAFILE *)malloc(sizeof(TGAFILE));
if(diLoadTGA("Player.tga",PlayerShip) == false)
MessageBox(NULL,"Could not find player image file","Startup Error",MB_ICONSTOP|MB_OK);

// this is the function

bool diLoadTGA(char *FileName,TGAFILE *tgaFile)
{
GLubyte TGAheader[12]={0,0,2,0,0,0,0,0,0,0,0,0}; // uncompressed TGA header

GLubyte TGAcompare[12]; // used in comparison with TGA header

GLubyte header[6]; // the first 6 useable bytes from the header

GLuint bytesPerPixel; // bytes per pixel in the TGA file

GLuint imageSize; // used to store size when allocating memory

GLuint temp; // temporary variable used in swap

GLuint type=GL_RGBA; // default GL mode is 32 bpp


FILE *fp = fopen(FileName,"rb"); // try to open the file


if(fp == NULL || // see if file exists

fread(TGAcompare,1,sizeof(TGAcompare),fp)!=sizeof(TGAcompare) || // see if we can read the 12 bytes

memcmp(TGAheader,TGAcompare,sizeof(TGAheader))!=0 || // is the header matching to what we want?

fread(header,1,sizeof(header),fp)!=sizeof(header)) // read the header

{
if(fp == NULL) // see if file exists

return false; // quit

else
{
fclose(fp); // something failed, so close the file

return false; // quit

}
}

tgaFile->Width = header[1] * 256 + header[0]; // calculate TGA width (highbyte*256+lowbyte)

tgaFile->Height = header[3] * 256 + header[2]; // calculate TGA height (highbyte*256+lowbyte)


if(tgaFile->Width <= 0 || // width less than or equal to 0?

tgaFile->Height <= 0 || // height less than or equal to 0?

(header[4] != 24 && header[4] !=32 )) // is the file not 24 or 32 bit?

{
fclose(fp); // something went wrong, so close

return false; // quit

}

tgaFile->BitDepth = header[4]; // get TGA's bit depth

bytesPerPixel = tgaFile->BitDepth/8; // divide by 8 to get bytes per pixel

imageSize = tgaFile->Width*tgaFile->Height*bytesPerPixel; // determine necessary memory for image data


tgaFile->ImageData=(GLubyte *)malloc(imageSize); // allocate memory for TGA data


if(tgaFile->ImageData == NULL || // storage memory doesn't exist?

fread(tgaFile->ImageData,1,imageSize,fp)!=imageSize) // memory size doesn't match image size?

{
if(tgaFile->ImageData!=NULL) // if we had loaded the data...

free(tgaFile->ImageData); // release it


fclose(fp); // close the file

return false; // quit

}

for(GLuint i=0;i < int(imageSize);i += bytesPerPixel) // loop to convert from BGR(A) to RGB(A)

{ // swap the first and third bytes (red and blue, green doesn't go anywhere)

temp = tgaFile->ImageData[i]; // temporarily store the value of the first byte

tgaFile->ImageData[i] = tgaFile->ImageData[i + 2]; // first byte gets the third byte

tgaFile->ImageData[i + 2] = temp; // third byte gets temp (which is the first byte)

}

fclose(fp); // close the file

return true; // good to go

}


Edited by - Irrelevant on February 16, 2002 8:08:47 PM

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duh. thanks. i''ll delete my posts to get rid of my idiotic spam. unless someone else finds all of you folks'' advice helpful.

*smacks self in the forehead*

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Guest Anonymous Poster
Just use DevIL, formerly OpenIL, to load pictures (including Targa). That way you don''t have to worry about your loading code having bugs and it supports a ton of picture formats.

http://openil.sourceforge.net/

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quote:
Original post by Irrelevant
duh. thanks. i''ll delete my posts to get rid of my idiotic spam. unless someone else finds all of you folks'' advice helpful.

Don''t delete it. You never know, maybe one day, beginners will learn to use the search feature, and find things like this useful

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