opengl : glGetTexImage for 2d textures and 1d texture array confusion

Question

For texture2D i extract the texture for an mipmap as follows

pixels=ByteBuffer.allocateDirect(4*width*height);
    
GL11.glGetTexImage(
                    GL11.GL_TEXTURE_2D,mipMap
                   ,GL11.GL_RGB,GL11.GL_UNSIGNED_BYTE
                   ,pixels
                  );

As per the documentation it writes 4 bytes per pixels setting the required components to zero if the texture is not 4 components.

Later i can create an 2D texture using the pixels as follows

GL11.glPixelStorei(GL11.GL_UNPACK_ALIGNMENT,4); //4 bytes since RGBA bytes were read from texture

GL11.glTexImage2D(GL11.GL_TEXTURE_2D,0
                 ,GL11.GL_RGB,width,height,0
                 ,GL11.GL_RGB,GL11.GL_UNSIGNED_BYTE,pixels);

So this works perfectly for 2D textures.

Now jump to texture 1D Arrays i read the images for all layers of an specific mipmap as follows

pixels=ByteBuffer.allocateDirect(4*image.width*layers); //again creating RGBA byte buffer because thats the default behaviour

GL11.glGetTexImage(                                     //this will return the texture images of all layers of mipmap level 0
                   GL30.GL_TEXTURE_1D_ARRAY,0
                  ,GL11.GL_RGB,GL11.GL_UNSIGNED_BYTE
                  ,pixels
                 );

ByteBuffer levelN=ByteBuffer.allocateDirect(4*image.width);

int offset=4*image.width*level;                         //level is layer 0,layer 1 so on this part reads only the texels of an specific layer
for(int i=offset;i<offset+(image.width*4);i++){levelN.put(pixels.get(i));}

pixels=levelN;

But later when i create my texture1D array as follows

ByteBuffer[] layers=//read using above method

 GL11.glPixelStorei(GL11.GL_UNPACK_ALIGNMENT,4); //4 bytes since RGBA bytes were read from texture

GL11.glTexImage2D(GL30.GL_TEXTURE_1D_ARRAY,0                                //allocate enough storage for all layers
                 ,GL11.GL_RGB,width,layers.length,0
                 ,GL11.GL_RGB,GL11.GL_UNSIGNED_BYTE,(ByteBuffer)null);



for(int layer=0;i<layers.length;layer++)
{
 ByteBuffer image=layers[i];
 
 GL11.glTexSubImage2D(GL30.GL_TEXTURE_1D_ARRAY,0                              //Update individual layers using texSubImage
                      ,0,layer,width,1
                      ,GL11.GL_RGB,GL11.GL_UNSIGNED_BYTE,image);
}

The Colors come out all incorrect and even changing the texture format to GL_RGBA also didn't solve the problem.but when i change the constant from 4 to 3[read only 3 bytes per pixel in the readMethod() of texture 1d array] everything works correctly again. So i am really confused here because all my test textures are RGB format and what i observed was

->For 2D Textures reading 4 bytes per pixel in glGetTexImage() but then later specifying only RGB for the texture format worked

->For 1D Texture Arrays reading 3 bytes per pixel in the glGetTexImage() but then later specifying only RGB for texture format worked

But the specs say that it default reads 4 bytes per pixel for all texture types unless you the change that behaviour using pixelStorei()

and i am only using that method for creating 2D textures not anywhere else.

Can someone please explain why the diffrences?

*"As per the documentation it writes 4 bytes per pixels " - No it does not. glGetTexImage(...GL_RGB, GL_UNSIGNED_BYTE...) reads 3 bytes per pixel. The length of a line is aligned to 4 bytes (if GL_PACK_ALIGNMENT is 4). — Rabbid76
But is says that it fills out the alpha to zero if the texture is RGB format right?Even then why did 4 bytes work for my tex2D even though it was an RGB texture? — Sync it
It says: "If the selected texture image does not contain four components [...]". That means if you try to read an RGB texture to an RGBA target buffer, the alpha channel will be set 255. The arguments to glGetTexImage do not specify the format of the source texture, but they specify the format of the target buffer. — Rabbid76
I see but I specified unpack allignment to 4 for my 1D texture array[forgot to include in my question] and it did nothing. My buffer is RGBA and my allignment is also 4 now but that didn't solve anything — Sync it
Anyway I will try playing around with the buffer a little more tomorrow and will edit the results in my question — Sync it

Rabbid76 Rabbid76 · Accepted Answer · 2020-12-07T17:46:37

As per the documentation it writes 4 bytes per pixels [...]

No it does not. glGetTexImage(...GL_RGB, GL_UNSIGNED_BYTE...) reads 3 bytes per pixel. The length of a line is aligned to 4 bytes (if GL_PACK_ALIGNMENT is 4).
The arguments format and _type of glGetTexImage do not specify the format of the source texture, but the format of the target buffer.

opengl : glGetTexImage for 2d textures and 1d texture array confusion

1 Answers