代码示例 / 计算机视觉 / 使用 🤗Transformers 的 Segment Anything 模型

使用 🤗Transformers 的 Segment Anything 模型

作者: Merve Noyan & Sayak Paul
创建时间 2023/07/11
上次修改时间 2023/07/11
描述:使用 Keras 和 🤗 Transformers 对 Segment Anything 模型进行微调。

ⓘ 此示例使用 Keras 2

在 Colab 中查看 GitHub 源代码


简介

大型语言模型 (LLM) 使最终用户能够轻松地通过“提示”将它们应用于各种应用程序。例如,如果我们希望 LLM 预测以下句子的情感——“那部电影很棒,我非常喜欢它”——我们将使用类似以下内容的提示来提示 LLM

以下句子的情感是什么:“那部电影很棒,我非常喜欢它”?

作为回报,LLM 将返回情感标记。

但是,当涉及到视觉识别任务时,我们如何设计“视觉”线索来提示基础视觉模型?例如,我们可以有一个输入图像,并使用该图像上的边界框提示模型,并要求它执行分割。边界框将在这里充当我们的视觉提示。

Segment Anything 模型(称为 SAM)中,Meta 的研究人员将语言提示的空间扩展到视觉提示。SAM 能够使用提示输入执行零样本分割,其灵感来自大型语言模型。此处的提示可以是一组前景/背景点、自由文本、框或掩码。有许多下游分割任务,包括语义分割和边缘检测。SAM 的目标是通过提示启用所有这些下游分割任务。

在本例中,我们将学习如何使用来自 🤗 Transformers 的 SAM 模型进行推理和微调。


安装

!!pip install -q git+https://github.com/huggingface/transformers
[]

让我们导入本示例所需的一切。

from tensorflow import keras
from transformers import TFSamModel, SamProcessor
import numpy as np
import tensorflow as tf
import matplotlib.pyplot as plt
from tensorflow.python.ops.numpy_ops import np_config
from PIL import Image
import requests
import glob
import os
/Users/mervenoyan/miniforge3/envs/py310/lib/python3.10/site-packages/tqdm/auto.py:21: TqdmWarning: IProgress not found. Please update jupyter and ipywidgets. See https://ipywidgets.readthedocs.io/en/stable/user_install.html
  from .autonotebook import tqdm as notebook_tqdm

SAM 简述

SAM 具有以下组件

图片来自官方 SAM 博客文章

图像编码器负责计算图像嵌入。与 SAM 交互时,我们一次计算图像嵌入(因为图像编码器很重),然后将其与上面提到的不同提示(点、边界框、掩码)一起重复使用。

点和框(所谓的稀疏提示)通过轻量级提示编码器,而掩码(密集提示)通过卷积层。我们将从图像编码器中提取的图像嵌入和提示嵌入结合起来,两者都进入轻量级掩码解码器。解码器负责预测掩码。

图片来自 SAM 论文

SAM 经过预训练以预测任何可接受提示的有效掩码。此要求允许 SAM 即使在提示难以理解时也能输出有效掩码——这使得 SAM 能够感知模糊性。此外,SAM 为单个提示预测多个掩码。

我们强烈建议您查看 SAM 论文博客文章,以了解有关 SAM 的更多详细信息以及用于预训练它的数据集。


使用 SAM 进行推理

SAM 有三个检查点

我们在 TFSamModel 中加载 sam-vit-base。我们还需要 SamProcessor 用于关联的检查点。

model = TFSamModel.from_pretrained("facebook/sam-vit-base")
processor = SamProcessor.from_pretrained("facebook/sam-vit-base")
All model checkpoint layers were used when initializing TFSamModel.
All the layers of TFSamModel were initialized from the model checkpoint at facebook/sam-vit-base.
If your task is similar to the task the model of the checkpoint was trained on, you can already use TFSamModel for predictions without further training.

接下来,我们编写一些用于可视化的实用函数。大多数这些函数都取自 此笔记本

np_config.enable_numpy_behavior()


def show_mask(mask, ax, random_color=False):
    if random_color:
        color = np.concatenate([np.random.random(3), np.array([0.6])], axis=0)
    else:
        color = np.array([30 / 255, 144 / 255, 255 / 255, 0.6])
    h, w = mask.shape[-2:]
    mask_image = mask.reshape(h, w, 1) * color.reshape(1, 1, -1)
    ax.imshow(mask_image)


def show_box(box, ax):
    x0, y0 = box[0], box[1]
    w, h = box[2] - box[0], box[3] - box[1]
    ax.add_patch(
        plt.Rectangle((x0, y0), w, h, edgecolor="green", facecolor=(0, 0, 0, 0), lw=2)
    )


def show_boxes_on_image(raw_image, boxes):
    plt.figure(figsize=(10, 10))
    plt.imshow(raw_image)
    for box in boxes:
        show_box(box, plt.gca())
    plt.axis("on")
    plt.show()


def show_points_on_image(raw_image, input_points, input_labels=None):
    plt.figure(figsize=(10, 10))
    plt.imshow(raw_image)
    input_points = np.array(input_points)
    if input_labels is None:
        labels = np.ones_like(input_points[:, 0])
    else:
        labels = np.array(input_labels)
    show_points(input_points, labels, plt.gca())
    plt.axis("on")
    plt.show()


def show_points_and_boxes_on_image(raw_image, boxes, input_points, input_labels=None):
    plt.figure(figsize=(10, 10))
    plt.imshow(raw_image)
    input_points = np.array(input_points)
    if input_labels is None:
        labels = np.ones_like(input_points[:, 0])
    else:
        labels = np.array(input_labels)
    show_points(input_points, labels, plt.gca())
    for box in boxes:
        show_box(box, plt.gca())
    plt.axis("on")
    plt.show()


def show_points_and_boxes_on_image(raw_image, boxes, input_points, input_labels=None):
    plt.figure(figsize=(10, 10))
    plt.imshow(raw_image)
    input_points = np.array(input_points)
    if input_labels is None:
        labels = np.ones_like(input_points[:, 0])
    else:
        labels = np.array(input_labels)
    show_points(input_points, labels, plt.gca())
    for box in boxes:
        show_box(box, plt.gca())
    plt.axis("on")
    plt.show()


def show_points(coords, labels, ax, marker_size=375):
    pos_points = coords[labels == 1]
    neg_points = coords[labels == 0]
    ax.scatter(
        pos_points[:, 0],
        pos_points[:, 1],
        color="green",
        marker="*",
        s=marker_size,
        edgecolor="white",
        linewidth=1.25,
    )
    ax.scatter(
        neg_points[:, 0],
        neg_points[:, 1],
        color="red",
        marker="*",
        s=marker_size,
        edgecolor="white",
        linewidth=1.25,
    )


def show_masks_on_image(raw_image, masks, scores):
    if len(masks[0].shape) == 4:
        final_masks = tf.squeeze(masks[0])
    if scores.shape[0] == 1:
        final_scores = tf.squeeze(scores)

    nb_predictions = scores.shape[-1]
    fig, axes = plt.subplots(1, nb_predictions, figsize=(15, 15))

    for i, (mask, score) in enumerate(zip(final_masks, final_scores)):
        mask = tf.stop_gradient(mask)
        axes[i].imshow(np.array(raw_image))
        show_mask(mask, axes[i])
        axes[i].title.set_text(f"Mask {i+1}, Score: {score.numpy().item():.3f}")
        axes[i].axis("off")
    plt.show()

我们将使用点提示对汽车图像进行分割。在调用处理器时,请确保将 return_tensors 设置为 tf

让我们加载一张汽车图像并对其进行分割。

img_url = "https://hugging-face.cn/ybelkada/segment-anything/resolve/main/assets/car.png"
raw_image = Image.open(requests.get(img_url, stream=True).raw).convert("RGB")
plt.imshow(raw_image)
plt.show()

png

现在让我们定义一组我们将用作提示的点。

input_points = [[[450, 600]]]

# Visualize a single point.
show_points_on_image(raw_image, input_points[0])

png

并进行分割

# Preprocess the input image.
inputs = processor(raw_image, input_points=input_points, return_tensors="tf")

# Predict for segmentation with the prompt.
outputs = model(**inputs)

outputs 有两个我们感兴趣的属性

  • outputs.pred_masks:表示预测的掩码。
  • outputs.iou_scores:表示与掩码关联的 IoU 分数。

让我们后处理掩码并将其与它们的 IoU 分数一起可视化

masks = processor.image_processor.post_process_masks(
    outputs.pred_masks,
    inputs["original_sizes"],
    inputs["reshaped_input_sizes"],
    return_tensors="tf",
)

show_masks_on_image(raw_image, masks, outputs.iou_scores)

png

就是这样!

可以注意到,所有掩码都是我们提供的点提示的有效掩码。

SAM 足够灵活,可以支持不同的视觉提示,我们鼓励您查看 此笔记本 以了解更多信息!


微调

我们将使用 此数据集,该数据集包含乳腺癌扫描。在医学影像领域,能够分割包含恶性细胞的细胞是一项重要的任务。

数据准备

让我们首先获取数据集。

remote_path = "https://hf.co/datasets/sayakpaul/sample-datasets/resolve/main/breast-cancer-dataset.tar.gz"
dataset_path = keras.utils.get_file(
    "breast-cancer-dataset.tar.gz", remote_path, untar=True
)

现在让我们可视化数据集中的一个样本。

show_mask() 实用程序取自 此笔记本

def show_mask(mask, ax, random_color=False):
    if random_color:
        color = np.concatenate([np.random.random(3), np.array([0.6])], axis=0)
    else:
        color = np.array([30 / 255, 144 / 255, 255 / 255, 0.6])
    h, w = mask.shape[-2:]
    mask_image = mask.reshape(h, w, 1) * color.reshape(1, 1, -1)
    ax.imshow(mask_image)


# Load all the image and label paths.
image_paths = sorted(glob.glob(os.path.join(dataset_path, "images/*.png")))
label_paths = sorted(glob.glob(os.path.join(dataset_path, "labels/*.png")))

# Load the image and label.
idx = 15
image = Image.open(image_paths[idx])
label = Image.open(label_paths[idx])
image = np.array(image)
ground_truth_seg = np.array(label)

# Display.
fig, axes = plt.subplots()
axes.imshow(image)
show_mask(ground_truth_seg, axes)
axes.title.set_text(f"Ground truth mask")
axes.axis("off")
plt.show()

tf.shape(ground_truth_seg)

png

<tf.Tensor: shape=(2,), dtype=int32, numpy=array([256, 256], dtype=int32)>

准备 tf.data.Dataset

现在,我们编写一个生成器类,使用上面使用的 processor 来准备图像和分割掩码。我们将利用此生成器类通过使用 tf.data.Dataset.from_generator() 为我们的训练集创建一个 tf.data.Dataset 对象。此类的实用程序已从 此笔记本 中改编而来。

生成器负责生成预处理的图像和分割掩码,以及 SAM 模型所需的一些其他元数据。

class Generator:
    """Generator class for processing the images and the masks for SAM fine-tuning."""

    def __init__(self, dataset_path, processor):
        self.dataset_path = dataset_path
        self.image_paths = sorted(
            glob.glob(os.path.join(self.dataset_path, "images/*.png"))
        )
        self.label_paths = sorted(
            glob.glob(os.path.join(self.dataset_path, "labels/*.png"))
        )
        self.processor = processor

    def __call__(self):
        for image_path, label_path in zip(self.image_paths, self.label_paths):
            image = np.array(Image.open(image_path))
            ground_truth_mask = np.array(Image.open(label_path))

            # get bounding box prompt
            prompt = self.get_bounding_box(ground_truth_mask)

            # prepare image and prompt for the model
            inputs = self.processor(image, input_boxes=[[prompt]], return_tensors="np")

            # remove batch dimension which the processor adds by default
            inputs = {k: v.squeeze(0) for k, v in inputs.items()}

            # add ground truth segmentation
            inputs["ground_truth_mask"] = ground_truth_mask

            yield inputs

    def get_bounding_box(self, ground_truth_map):
        # get bounding box from mask
        y_indices, x_indices = np.where(ground_truth_map > 0)
        x_min, x_max = np.min(x_indices), np.max(x_indices)
        y_min, y_max = np.min(y_indices), np.max(y_indices)

        # add perturbation to bounding box coordinates
        H, W = ground_truth_map.shape
        x_min = max(0, x_min - np.random.randint(0, 20))
        x_max = min(W, x_max + np.random.randint(0, 20))
        y_min = max(0, y_min - np.random.randint(0, 20))
        y_max = min(H, y_max + np.random.randint(0, 20))
        bbox = [x_min, y_min, x_max, y_max]

        return bbox

get_bounding_box() 负责将地面真实分割图转换为边界框。这些边界框在微调期间作为提示(以及原始图像)馈送到 SAM,然后训练 SAM 以预测有效掩码。

首先创建一个生成器,然后使用它来创建 tf.data.Dataset 的优势在于灵活性。有时,我们可能需要使用其他库(例如 albumentations)中的实用程序,这些实用程序可能没有 TensorFlow 本身实现。通过使用此工作流程,我们可以轻松地适应此类用例。

但是,非 TF 对等体可能会引入性能瓶颈。但是,对于我们的示例,它应该可以正常工作。

现在,我们从训练集中准备 tf.data.Dataset

# Define the output signature of the generator class.
output_signature = {
    "pixel_values": tf.TensorSpec(shape=(3, None, None), dtype=tf.float32),
    "original_sizes": tf.TensorSpec(shape=(None,), dtype=tf.int64),
    "reshaped_input_sizes": tf.TensorSpec(shape=(None,), dtype=tf.int64),
    "input_boxes": tf.TensorSpec(shape=(None, None), dtype=tf.float64),
    "ground_truth_mask": tf.TensorSpec(shape=(None, None), dtype=tf.int32),
}

# Prepare the dataset object.
train_dataset_gen = Generator(dataset_path, processor)
train_ds = tf.data.Dataset.from_generator(
    train_dataset_gen, output_signature=output_signature
)

接下来,我们配置数据集以提高性能。

auto = tf.data.AUTOTUNE
batch_size = 2
shuffle_buffer = 4

train_ds = (
    train_ds.cache()
    .shuffle(shuffle_buffer)
    .batch(batch_size)
    .prefetch(buffer_size=auto)
)

获取一批数据并检查其中元素的形状。

sample = next(iter(train_ds))
for k in sample:
    print(k, sample[k].shape, sample[k].dtype, isinstance(sample[k], tf.Tensor))
pixel_values (2, 3, 1024, 1024) <dtype: 'float32'> True
original_sizes (2, 2) <dtype: 'int64'> True
reshaped_input_sizes (2, 2) <dtype: 'int64'> True
input_boxes (2, 1, 4) <dtype: 'float64'> True
ground_truth_mask (2, 256, 256) <dtype: 'int32'> True

训练

我们现在将编写 DICE 损失。此实现基于 MONAI DICE 损失

def dice_loss(y_true, y_pred, smooth=1e-5):
    y_pred = tf.sigmoid(y_pred)
    reduce_axis = list(range(2, len(y_pred.shape)))
    if batch_size > 1:
        # reducing spatial dimensions and batch
        reduce_axis = [0] + reduce_axis
    intersection = tf.reduce_sum(y_true * y_pred, axis=reduce_axis)
    y_true_sq = tf.math.pow(y_true, 2)
    y_pred_sq = tf.math.pow(y_pred, 2)

    ground_o = tf.reduce_sum(y_true_sq, axis=reduce_axis)
    pred_o = tf.reduce_sum(y_pred_sq, axis=reduce_axis)
    denominator = ground_o + pred_o
    # calculate DICE coefficient
    loss = 1.0 - (2.0 * intersection + 1e-5) / (denominator + 1e-5)
    loss = tf.reduce_mean(loss)

    return loss

微调 SAM

我们现在将微调 SAM 的解码器部分。我们将冻结视觉编码器和提示编码器层。

# initialize SAM model and optimizer
sam = TFSamModel.from_pretrained("facebook/sam-vit-base")
optimizer = keras.optimizers.Adam(1e-5)

for layer in sam.layers:
    if layer.name in ["vision_encoder", "prompt_encoder"]:
        layer.trainable = False


@tf.function
def train_step(inputs):
    with tf.GradientTape() as tape:
        # pass inputs to SAM model
        outputs = sam(
            pixel_values=inputs["pixel_values"],
            input_boxes=inputs["input_boxes"],
            multimask_output=False,
            training=True,
        )

        predicted_masks = tf.squeeze(outputs.pred_masks, 1)
        ground_truth_masks = tf.cast(inputs["ground_truth_mask"], tf.float32)

        # calculate loss over predicted and ground truth masks
        loss = dice_loss(tf.expand_dims(ground_truth_masks, 1), predicted_masks)
        # update trainable variables
        trainable_vars = sam.trainable_variables
        grads = tape.gradient(loss, trainable_vars)
        optimizer.apply_gradients(zip(grads, trainable_vars))

        return loss
All model checkpoint layers were used when initializing TFSamModel.
All the layers of TFSamModel were initialized from the model checkpoint at facebook/sam-vit-base.
If your task is similar to the task the model of the checkpoint was trained on, you can already use TFSamModel for predictions without further training.
WARNING:absl:At this time, the v2.11+ optimizer [`tf.keras.optimizers.Adam`](https://tensorflowcn.cn/api_docs/python/tf/keras/optimizers/Adam) runs slowly on M1/M2 Macs, please use the legacy Keras optimizer instead, located at [`tf.keras.optimizers.legacy.Adam`](https://tensorflowcn.cn/api_docs/python/tf/keras/optimizers/legacy/Adam).

我们现在可以运行三个 epoch 的训练。我们可能会收到关于掩码解码器的 IoU 预测头部不存在梯度的警告,我们可以安全地忽略它。

# run training
for epoch in range(3):
    for inputs in train_ds:
        loss = train_step(inputs)
    print(f"Epoch {epoch + 1}: Loss = {loss}")
WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

WARNING:tensorflow:Gradients do not exist for variables ['tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_in/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/proj_out/bias:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/kernel:0', 'tf_sam_model_1/mask_decoder/iou_prediction_head/layers_._0/bias:0'] when minimizing the loss. If you're using `model.compile()`, did you forget to provide a `loss` argument?

Epoch 1: Loss = 0.08322787284851074
Epoch 2: Loss = 0.05677264928817749
Epoch 3: Loss = 0.07764029502868652

序列化模型

我们序列化了模型并为您推送到下方。push_to_hub 方法序列化模型,生成模型卡片并将其推送到 Hugging Face Hub,以便其他人可以使用 from_pretrained 方法加载模型以进行推理或进一步微调。我们还需要将相同的预处理器推送到存储库中。在 此处 找到模型和预处理器。

# sam.push_to_hub("merve/sam-finetuned")
# processor.push_to_hub("merve/sam-finetuned")

我们现在可以使用模型进行推理。

# Load another image for inference.
idx = 20
raw_image_inference = Image.open(image_paths[idx])

# process the image and infer
preprocessed_img = processor(raw_image_inference)
outputs = sam(preprocessed_img)

最后,我们可以可视化结果。

infer_masks = outputs["pred_masks"]
iou_scores = outputs["iou_scores"]
show_masks_on_image(raw_image_inference, masks=infer_masks, scores=iou_scores)
WARNING:matplotlib.image:Clipping input data to the valid range for imshow with RGB data ([0..1] for floats or [0..255] for integers).
WARNING:matplotlib.image:Clipping input data to the valid range for imshow with RGB data ([0..1] for floats or [0..255] for integers).
WARNING:matplotlib.image:Clipping input data to the valid range for imshow with RGB data ([0..1] for floats or [0..255] for integers).

png