Xjyuk

I would like to have a rectangle that has some hatch. Furthermore, the hatch should have some shading. But, I am only able to get a transition of the colors in the background. Does anyone know how to only have a color gradient in the hatch with only a white background?

Thanks a lot.

documentclassstandalone
usepackagetikz
usetikzlibrarypatterns
begindocument
begintikzpicture 
fill[left color=blue!40, right color=white, postaction=pattern=north west lines, pattern color=blue!40] (0,0) rectangle (3,3);
endtikzpicture
enddocument

This is conceptually the same as Ignasi's nice answer in that the pattern is in the background color, so it determines what should be left unshaded. And in order to customize the pattern I use this answer. And of course you need to dial a shading angle to make the shading follow the north east lines.

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarypatterns
% defining the new dimensions and parameters
newlengthhatchspread
newlengthhatchthickness
newlengthhatchshift
newcommandhatchcolor
% declaring the keys in tikz
tikzsethatchspread/.code=setlengthhatchspread#1,
 hatchthickness/.code=setlengthhatchthickness#1,
 hatchshift/.code=setlengthhatchshift#1,% must be >= 0
 hatchcolor/.code=renewcommandhatchcolor#1
% setting the default values
tikzsethatchspread=3pt,
 hatchthickness=0.4pt,
 hatchshift=0pt,% must be >= 0
 hatchcolor=black
% declaring the pattern
pgfdeclarepatternformonly[hatchspread,hatchthickness,hatchshift,hatchcolor]% variables
 custom north west lines% name
 pgfqpointdimexpr-2hatchthicknessdimexpr-2hatchthickness% lower left corner
 pgfqpointdimexprhatchspread+2hatchthicknessdimexprhatchspread+2hatchthickness% upper right corner
 pgfqpointdimexprhatchspreaddimexprhatchspread% tile size
 % shape description
 pgfsetlinewidthhatchthickness
 pgfpathmovetopgfqpoint-1.5hatchthicknessdimexpr1.5hatchthickness+hatchspread+hatchshift
 pgfpathlinetopgfqpointdimexprhatchspread+1pt+hatchshift-1pt
 ifdim hatchshift > 0pt
 pgfpathmovetopgfqpoint0pthatchshift
 pgfpathlinetopgfqpointdimexpr1pt+hatchshift-1pt
 fi
 pgfsetstrokecolorhatchcolor
 pgfusepathstroke
 

begindocument
begintikzpicture 
fill[left color=blue!40, right color=white,shading angle=45,line width=2pt,
postaction=pattern=custom north west lines, 
hatchthickness=1pt,hatchcolor=white] (0,0) rectangle (3,3);
endtikzpicture
enddocument

Only for fun!

documentclass[border=3mm]standalone
usepackagetikz
begindocument

begintikzpicture
defa3
deflw0.2
draw[blue, thick] (0,0) rectangle (a,a);
foreach x [count=i] in0,0.1,0.2,...,a
draw [blue,line width=lw mm,opacity=i/45](x,0)--(0,x) (a,a-x)--(a-x,a);
endtikzpicture

enddocument