Rendering Textures without Perspective Correction

Unity Engine
1

Hi There!

I’m trying to render textured 3D objects without perspecitve correction. My plan is to paint the texture’s perspective myself for some objects.

I tried to find a way to change the interpolation mode of the fragment shader.

GLSL for example has options like:
flat: the value is not interpolated. The value given to the fragment shader is the value from the Provoking Vertex for that primitive.
smooth: performs a perspective correct interpolation.
noperspective: performs a linear interpolation in window space.

Is there anything similar in Unity’s ShaderLab? I found one example on the web which tries to undo perspective correction. Unfortunately it didn’t do the job.

Any help or ideas would be very welcome!!!

2

I think you would need to use a custom vertex shader. Not sure how you would do it with a surface shader. The basic idea is to ensure that the w value of the vertex clip position is 1 or -1.

o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
o.pos /= abs(o.pos.w);
3

Hi! Thanks for answering.
I’ve got a custom vertex and a fragment shader. The object itself should be rendered with perspective, only its texture needs to be rendered without perspective. The texture should look similar to a window into an image (=texture) behind my 3D Object. Then I can make that texture as large as my screen and use Photoshop to paint the details into the right positions of the 3D Objects…

4

Here’s the solution for this problem. Credits to Gibbonator! For more information about the issue I recommend this post: Getting a pixel's screen-space coordinate in a fragment shader - Questions & Answers - Unity Discussions

Vertex Shader:
output.pos = mul(UNITY_MATRIX_MVP, input.vertex)
output.pos/=output.pos.w
output.uv[0]=(output.pos[0]+1)/2;
output.uv[1]=(output.pos[1]+1)/2;

Fragment Shader:
return tex2D(_MainTex, input.uv);

5

I know this is old, but in case someone stumbles over it: just put the keyword ‘noperspective’ in front of the variable declaration.