Loading a BMP into an OpenGL textures switches the red and blue colors. (C++/Windows)

Question

I\'m trying to load a bitmap into an OpenGL texture and display it to the screen, but when I do so, the red and blue values seem to switch (e.g.: a blue image appears orange

Answer 1

You've got your RGB and BGR backwards.

glTexImage2D(GL_TEXTURE_2D, 0, 3, ilGetInteger(IL_IMAGE_WIDTH), ilGetInteger(IL_IMAGE_HEIGHT), \
    0, GL_RGB, GL_UNSIGNED_BYTE, ilGetData());

Is there a GL_BGR you can specify on the second line, instead of GL_RGB? That should fix it.

The reason a PNG causes the image to flip is because of how a BMP is stored: BMPs are stored bottom up, the first pixels in the file are the bottom row of the image.

Answer 2

I know this is an old topic but I found the easiest way was to flip the z and x in the vec4 that texture2D returns in the fragment shader, not glTexture2d. It doesn't rely on extensions that may not exist. Solved it on an GLES 2 project im working on.

                "varying vec2 v_texCoord;                         "
                "uniform sampler2d s_texture;                     "
                "vec4 v_bgr;                                      "
                "void main()                                      "
                "{                                                "
                "    v_bgr = texture2D( s_texture, v_texCoord );  "
                "  gl_FragColor = v_bgr.zyxw;                     "
                "}                                                ";

Answer 3

You can switch GL_RGB to GL_BGR_EXT. In some cases GL_BGR isn't recognized.

Answer 4

I had a similar issue a while back. Try setting GL_RBG in glTexImage2D to GL_BGR.

Answer 5

You'll normally have a call to glTexImage2D that specifies the external format of the pixels. From the sounds of things, you need to check that and switch from GL_RGB to GL_BGR.