Question

我使用此方法调整png图像的大小：

Method for converting PNGs to premultiplied alpha

但这张照片仍然失去了透明度：

import Image, numpy

def resize(filename, img,height, width):
    if filename.endswith(".png"):
        img = img.convert('RGBA')
        premult = numpy.fromstring(img.tostring(), dtype=numpy.uint8)
        alphaLayer = premult[3::4] / 255.0
        premult[::4] *= alphaLayer
        premult[1::4] *= alphaLayer
        premult[2::4] *= alphaLayer
        img = Image.fromstring("RGBA", img.size, premult.tostring())
    img = img.resize((height,width), Image.ANTIALIAS)
    return img

这

至

Answer 1

问题与链接问题无关，而是必须修补PIL，以便正确读取tRNS PNG块。 PIL假定此块的单个值，但显示的此图像具有调色板中每个值的透明度描述。处理之后，解决问题很简单：将图像转换为'LA'模式并调整大小：

import sys
from PIL import Image

img = Image.open(sys.argv[1])
pal = img.getpalette()
width, height = img.size
actual_transp = img.info['actual_transparency'] # XXX This will fail.

result = Image.new('LA', img.size)

im = img.load()
res = result.load()
for x in range(width):
    for y in range(height):
        t = actual_transp[im[x, y]]
        color = pal[im[x, y]]
        res[x, y] = (color, t)

result.resize((64, 64), Image.ANTIALIAS).save(sys.argv[2])

所以我们从这个 enter image description here 转到：

针对这种特定情况的PIL补丁实际上非常简单。打开PIL/PngImagePlugin.py，转到功能chunk_tRNS，输入检查if的{{1}}语句，然后检查im_mode == "P"，然后添加行{ {1}}。

Answer 2

这是一个更复杂的函数来调整大小（保持透明度）：它允许您只使用新的宽度值，新的宽度和高度值或参考文件来获得新的大小。您也可以更改重新采样方法：

## PngResizeTransparency.py
#
## Resize PNG image by keeping the transparency
## thanks to https://stackoverflow.com/users/1453719/nicolas-barbey
#
## Use:
##   - using a reference file to get new sizes:
#        PNG_ResizeKeepTransparency(SourceFile, ResizedFile, RefFile ='YourRefFile.png')
##   - using only the resized width:
#        PNG_ResizeKeepTransparency(SourceFile, ResizedFile, new_width)
##   - using resized width and hight:
#        PNG_ResizeKeepTransparency(SourceFile, ResizedFile, new_width, new_height)
##   - using resample mode: add param resample="NEAREST"/"BILINEAR"/"BICUBIC"/"ANTIALIAS"

from PIL import Image

def PNG_ResizeKeepTransparency(SourceFile, ResizedFile, new_width=0, new_height=0, resample="ANTIALIAS", RefFile =''):
    # needs PIL
    # Inputs:
    #   - SourceFile  = initial PNG file (including the path)
    #   - ResizedFile = resized PNG file (including the path)
    #   - new_width   = resized width in pixels; if you need % plz include it here: [your%] *initial width
    #   - new_height  = resized hight in pixels ; default = 0 = it will be calculated using new_width
    #   - resample = "NEAREST", "BILINEAR", "BICUBIC" and "ANTIALIAS"; default = "ANTIALIAS"
    #   - RefFile  = reference file to get the size for resize; default = ''

    img = Image.open(SourceFile) # open PNG image path and name
    img = img.convert("RGBA")    # convert to RGBA channels
    width, height = img.size     # get initial size

    # if there is a reference file to get the new size
    if RefFile != '':
        imgRef = Image.open(RefFile)
        new_width, new_height = imgRef.size
    else:
        # if we use only the new_width to resize in proportion the new_height
        # if you want % of resize please use it into new_width (?% * initial width)
        if new_height == 0:
            new_height = new_width*width/height

    # split image by channels (bands) and resize by channels
    img.load()
    bands = img.split()
    # resample mode
    if resample=="NEAREST":
        resample = Image.NEAREST
    else:
        if resample=="BILINEAR":
            resample = Image.BILINEAR
        else:
            if resample=="BICUBIC":
                resample = Image.BICUBIC
            else:
                if resample=="ANTIALIAS":
                    resample = Image.ANTIALIAS
    bands = [b.resize((new_width, new_height), resample) for b in bands]
    # merge the channels after individual resize
    img = Image.merge('RGBA', bands)
    # save the image
    img.save(ResizedFile)
    return

#######################################################

if __name__ == "__main__":
    sFile = './autumn-png-leaf.png'
    # resize using new width value (new height is calculated by keeping image aspect)
    PNG_ResizeKeepTransparency(sFile, sFile[:-4]+'_resized.png', 400)
    # resize using a reference file to get the new image dimension 
    PNG_ResizeKeepTransparency(sFile, sFile[:-4]+'_resized.png', RefFile = 'autumn-png-leaf_starry-night-van-gogh_fchollet_10.png')

Answer 3

我已经解决了这个问题，不再需要来自mmgp的好的黑客攻击。 PIL现在将正确阅读并应用透明度。

https://github.com/d-schmidt/Pillow/commit/5baa1ac1b8d41fcedce7b12ed1c4a8e87b4851bc

使用PIL调整png图像大小会失去透明度

3 个答案: