OpenGL + libpng: wrong colors on image

Question

Since last week I've been trying to load and show a PNG on the screen. I used a library libpng to copy png into memory and then used glDrawPixels to show it on screen. The result is following:

The original picture:

Screenshot in the "game":

And the code, I'm using static Image::loadPNG to load PNG into map and it's like the following:

bool Image::loadPNG( std::string _image, FILE * file, unsigned char header[ 54 ] )
{
    fseek( file, 8, SEEK_SET );
    ImageData* imageData = new ImageData( );
    Image* image = new Image( );
    image->type = IMAGETYPE_PNG;
    imageData->image = image;
    png_structp png_ptr = png_create_read_struct( PNG_LIBPNG_VER_STRING, NULL, NULL, NULL );

    if( !png_ptr )
    {
        std::cout << "png_create_read_struct failed. " << std::endl;
        return false;
    }

    png_infop info_ptr = png_create_info_struct(png_ptr);

    if( !info_ptr )
    {
        std::cout << "png_create_info_struct failed. " << std::endl;
        return false;
    }

    if( setjmp( png_jmpbuf( png_ptr ) ) )
    {
        std::cout << "Error during init_io. " << std::endl;
        return false;
    }

    png_init_io( png_ptr, file );
    png_set_sig_bytes( png_ptr, 8 );
    png_read_info( png_ptr, info_ptr );
    png_set_strip_16( png_ptr );
    png_read_update_info(png_ptr, info_ptr);
    int bit_depth;
    int color;
    png_get_IHDR(png_ptr, info_ptr, &image->width, &image->height, &bit_depth, &color, NULL, NULL, NULL);
    png_read_update_info(png_ptr, info_ptr);
    int pitch = png_get_rowbytes(png_ptr, info_ptr);
    image->image = new unsigned char[pitch * image->height];
    png_bytep* row_pointers = new png_bytep[image->height];

    for (int y = 0; y < image->height; y++)
        row_pointers[image->height - 1 - y] = image->image + y * pitch;

    png_read_image(png_ptr, row_pointers);
    delete[] row_pointers;

    // tym na razie dupy sobie nie zawracam

    Image::images.insert( std::pair< std::string, ImageData* >( _image, imageData ) );
    glGenTextures(1, &imageData->texture);
    glBindTexture( GL_TEXTURE_2D, imageData->texture );
    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_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image->width, image->height, 0, GL_BGRA_EXT, GL_UNSIGNED_BYTE, image->image);
    gluBuild2DMipmaps( GL_TEXTURE_2D, 4, image->width, image->height, GL_RGBA, GL_UNSIGNED_BYTE, image->image );
    return true;
}

And finally showing it in OpenGL:

Image* image = Image::images["f.png"]->image;

if( image )
{
    glDisable(GL_DEPTH_TEST);
    glPushMatrix( );
    //glPixelStorei(GL_UNPACK_ALIGNMENT, 4);  

    float nx = 0.0f;
    float ny = 0.0f;

    glLoadIdentity( );
    glTranslatef( 0, 0, -1 );

    if( nx > 0 || ny > 0 )
        glRasterPos2f( nx, ny );

    else
    {
        glRasterPos2f( 0, 0 );
        glBitmap(0,0,0,0, nx, ny, NULL);
    }

    glDrawPixels( image->width, image->height, GL_RGBA, GL_UNSIGNED_BYTE, image->image);
    glPopMatrix( );
    glEnable(GL_DEPTH_TEST);
}

As you can see, the colors are different than they should be. Could anyone help me to figure out how to solve my problem?

Answer 1

It looks like you may have some color channels swapped. In the call to glTexImage2D(), try changing the GL_BGRA_EXT to GL_RGBA, I think. Or, if that doesn't work, change GL_UNSIGNED_BYTE to GL_UNSIGNED_INT_8_8_8_8_REV.

Answer 2

It looks to me like your image is perhaps being drawn using a (multiply?) blend.

Do you have alpha blending enabled? (your image seems to have an alpha channel)

What blend mode have you set?

It should probably just be:

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);