Might be possible to see what happens in luashaders through print() or anything similar?

pie

I wanted to try some shader effect, and even if I understood something by tinkering with already working shaders, I still don't get how to do some pretty basic stuff:

Since I managed to color moving relative portions of the screen at my pleasure, I was trying to draw a more "detailed" thing like a grid.
And here I noticed how much I miss having a print function to see what happens there:

My idea was that I could basically draw a black pixel when its x or y coordinate was even, but it seems that my condition is never triggered, and I guess that the issue is that fTexCoord.x and .y are relative values that go from 0 to 1, not "real" pixel coordinates.. but at the same time I think that I should have got at least a weird output instead of the source image..

Any suggestions?

Effect.grid=makeEffect("grid", function (vVertex,vColor,vTexCoord) : Shader local vertex = hF4(vVertex,0.0,1.0) fTexCoord=vTexCoord return vMatrix*vertex end, function () : Shader local frag=lF4(fColor)*texture2D(fTexture, fTexCoord)   if fTexCoord.x%2.0 ==0 or fTexCoord.y%2.0 == 0 then --when x/2 is even frag.rgb=lF3(0.0,0.0,0.0) --this changes the pixel color else frag.rgb=lF3(frag.r,frag.g,frag.b) --this leaves the original pixel color end if (frag.a==0.0) then discard() end --discard transparent pixels return frag end)

thank you!

keszegh

@pie, as far as i understood shaders cannot be easily debugged, that's their main drawback but otherwise they would be not as fast as they are.
on the other hand you can use e.g. https://www.shadertoy.com/ to prototype much faster perhaps than in gideros/lua and when you are happy with the result then you can translate your code to gideros glsl or lua shader (although there will be hassles with texture coordinates etc. at least for me it also takes time and trial and error to translate a shadertoy code so that it works as i expect).

keszegh

some further answers (they won't help you within gideros except maybe the last trick when you add the debug info into the pixel values directly):
https://computergraphics.stackexchange.com/questions/96/how-can-i-debug-glsl-shaders

hgy29

@pie, Unfortunately no, shaders don't have a way to output textual debug information. They are run on GPU and for every pixel, so thta'd make a lot of debug if that was possible. But as @keszegh pointed out, there some tools that can help diagnose shaders issues. Xcode has a very nice GPU capture tool for iOS, and there is a plugin on firefox (Spector.js) that could help understanding what happens too.

hgy29

Actually, when I need to understand why a shader doesn’t work as expected, I return intermediary computations in the pixel value, in r g or b.

pie

hgy29 said:

Actually, when I need to understand why a shader doesn’t work as expected, I return intermediary computations in the pixel value, in r g or b.

how do you read it when it's rendered? A color picker?

hgy29

No, I just scale my values so that I can understand just by having a look at what is rendered