将大 1 位 tiff 缩小为 8 位灰度/24 位
假设我有一个 100000x100000 1 位（K 通道）tiff，dpi 为 2000，我想将其缩小到 dpi 200。我生成的图像将是 10000x10000 图像。这是否意味着 1 位图像中的每 10 位对应于新图像中的 1 个像素？顺便说一句，我正在使用 libtiff 并使用 tiffreadscanline 读取 1 位 tiff。谢谢！
Let's say i have a 100000x100000 1 bit (K channel) tiff with a dpi of 2000 and i want to downscale this to a dpi of 200. My resulting image would be 10000x10000 image. Does this mean that every 10 bits in the 1 bit image correspond to 1 pixel in the new image? By the way, i am using libtiff and reading the 1 bit tiff with tiffreadscanline. Thanks!
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这意味着 1 位图像中的每 100 位对应于新图像中的 1 个像素。您需要对 10x10 1 位像素区域的值进行平均。为了获得更平滑的灰度，您最好对 n 位进行平均，其中 n 是目标像素的位深度，将计算出的区域与相邻区域部分重叠（16x16 像素的正方形，间隔 10x10 像素） ，因此它们的边框重叠，以获得平滑的 8 位灰度。）
That means every 100 bits in the 1 bit image correspond to 1 pixel in the new image. You'd need to average the value over 10x10 1bit pixel area. For smoother greyscales, you'd better average over n bits where n is the bit depth of your target pixel, overlying the calculated area partially with neighbor areas (16x16px squares 10x10px apart, so their borders overlay, for a smooth 8-bit grayscale.)
重要的是要了解为什么要缩小规模（因为输出介质还是因为文件大小？）。正如 SF 指出的，颜色/灰度在某种程度上可以与分辨率互换。如果它只是关于文件大小，无损/有损压缩也值得一看。
另一件事是了解源图像的一些特征。例如，如果源图像被光栅化（对于报纸图像），您可能会得到奇怪的图案，因为点阵被弄乱了。我曾经尝试过修复旧报纸图像，但发现工作量很大。在增强图像之前，我最终先将其转换为灰度。
我建议用 VIPS 或 Irfanview 进行一些实验，以找到最佳结果（即是某种重采样算法对图像质量的影响）。使用这些程序（例如 Photoshop）的原因是您可以尝试 GUI/命令行，同时了解其背后算法的名称/参数。使用 VIPS，您可以控制大多数（如果不是全部）参数。
[编辑]
TiffDump（与 LibTiff 二进制文件一起提供）是一个有价值的信息源。它会告诉你有关字节排序等的信息。我所做的是从一个已知的图像开始。例如，LibTIFF.NET 附带了许多测试图像，包括黑白（有些带有 0=黑色，有些 1=黑色）。 [/编辑]
It is important to understand why you want to downscale (because of output medium or because of file size?). As SF pointed out, colors/grayscale are somewhat interchangeable with resolution. If it is only about file size losless/lossy compression is also worth to look at..
The other thing is to understand a bit of the characteristics of your source image. For instance, if the source image is rasterized (as for newspaper images) you may get akward patterns because the dot-matrix is messed up. I have once tried to restore an old news-paper image, and I found it a lot of work. I ended up converting it to gray scale first before enhancing the image.
I suggest to experiment a bit with VIPS or Irfanview to find the best results (i.e. what is the effect of a certain resampling algorithm on your image quality). The reason for these programs (over i.e. Photoshop) is that you can experiment with GUI/command line while being aware of name/parameters of the algorithms behind it. With VIPS you can control most if not all parameters.
[Edit]
TiffDump (supplied with LibTiff binaries) is a valuable source of information. It will tell you about byte ordering etc. What I did was to start with a known image. For instance, LibTIFF.NET comes with many test images, including b&w (some with 0=black, some with 1=black). [/Edit]