glTexImage2D convert alpha to rgba openGL ES - c++

Question

I'm trying to convert working openGL code to openGL ES. After some digging, I've concluded the following function doesn't work in ES because converting between format and internalFormat isn't supported (i.e. the source and destination formats need to be the same). The easiest fix seems to be converting the alpha data to rgba where r=g=b=0 which is what openGL was doing before under the surface. My attached fix doesn't seem to work though, because
I don't think I am understanding how the buffer is formatted to make that conversion manually. Also maybe there is an openGL ES function I can call that will make this copy for me. Not sure if it matters but the file is a TGA file.
void foo( unsigned char *inBytes,
unsigned int inWidth,
unsigned int inHeight ) {
int error;
GLenum internalTexFormat = GL_RGBA;
GLenum texDataFormat = GL_ALPHA;
if( myAttemptedFix ) {
texDataFormat = GL_RGBA;
unsigned char rgbaBytes[inWidth * inHeight * 4];
for(int i=0; i < inWidth * inHeight; i++) {
rgbaBytes[4*i] = 0;
rgbaBytes[4*i + 1] = 0;
rgbaBytes[4*i + 2] = 0;
rgbaBytes[4*i + 3] = inBytes[i];
}
inBytes = &rgbaBytes[0];
}
glBindTexture( GL_TEXTURE_2D, mTextureID );
error = glGetError();
if( error != GL_NO_ERROR ) { // error
printf( "Error binding to texture id %d, error = %d\n",
(int)mTextureID,
error );
}
glPixelStorei( GL_UNPACK_ALIGNMENT, 1 );
if( mRepeat ) {
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );
}
else {
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP );
}
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
glTexEnvf( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE );
glTexImage2D( GL_TEXTURE_2D, 0,
internalTexFormat, inWidth,
inHeight, 0,
texDataFormat, GL_UNSIGNED_BYTE, inBytes );
error = glGetError();
if( error != GL_NO_ERROR ) { // error
printf( "Error setting texture data for id %d, error = %d, \"%s\"\n",
(int)mTextureID, error, glGetString( error ) );
}
}
Edit: When I run my fix it outlines the sprite correctly but also puts a lot of junk at the bottom that kind of looks like braille:

Answer 1

This looks more like a C++ problem. I believe your corrupted data is caused by this (shortened) code structure:
if (myAttemptedFix) {
unsigned char rgbaBytes[inWidth * inHeight * 4];
inBytes = &rgbaBytes[0];
}
The scope of rgbaBytes is the body of the if-statement. So the memory reserved for the array becomes invalid after the closing brace, and its content becomes undefined beyond that point. But you make your inBytes variable point at this memory, and use it after rgbaBytes has gone out of scope.
Since inBytes then points at unreserved memory, it's very likely that the memory is occupied by other variables in the code between this point and the glTexImage2D() call. So the content gets trashed before inBytes is consumed by the glTexImage2D() call.
The easiest way to fix this is to move the rgbaBytes declaration outside the if-statement:
unsigned char rgbaBytes[inWidth * inHeight * 4];
if (myAttemptedFix) {
inBytes = &rgbaBytes[0];
}
You'll probably want to make the code structure a little nicer once you have this all figured out, but this should at least make it functional.