Getting black images while applying discrete cosine transform (DCT) on image

def perform_dct(image_path):
    # Load the image in grayscale
    img = cv2.imread(image_path, cv2.IMREAD_GRAYSCALE)
    if img is None:
        raise ValueError(f"Image at {image_path} could not be loaded.")
    # Resize the image to 224x224 (size expected by ViT model)


    img = cv2.resize(img, (224, 224))
    # Perform DCT
    dct_img = cv2.dct(np.float32(img))

       # Normalize the DCT image to the range [0, 1]
    dct_img_min = np.min(dct_img)
    dct_img_max = np.max(dct_img)
    dct_img_normalized = (dct_img - dct_img_min) / (dct_img_max - dct_img_min)
    
    # Convert to 3-channel RGB image
    dct_img_normalized = (dct_img_normalized * 255).astype(np.uint8)
    dct_img_rgb = cv2.merge([dct_img_normalized, dct_img_normalized, dct_img_normalized])

    plt.imshow(dct_img_rgb)
    plt.title("DCT of Imagesss")
    plt.show()

    return dct_img_rgb

Trying to get discrete cosine transform (DCT) image but getting whole black image , I am not able to find error in my code . If you can Please help me