Custom shader fundamentals in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output color of this simple shader?

Consider this Unity shader code snippet that sets the output color. What color will the shader output on the object?

Unity

Shader "Custom/SimpleColor" {
  SubShader {
    Pass {
      CGPROGRAM
      #pragma vertex vert
      #pragma fragment frag

      struct appdata {
        float4 vertex : POSITION;
      };

      struct v2f {
        float4 pos : SV_POSITION;
      };

      v2f vert(appdata v) {
        v2f o;
        o.pos = UnityObjectToClipPos(v.vertex);
        return o;
      }

      fixed4 frag(v2f i) : SV_Target {
        return fixed4(0.0, 1.0, 0.0, 1.0); // green color
      }
      ENDCG
    }
  }
}

A The object will appear solid red.

B The object will appear solid green.

C The object will appear solid blue.

D The object will be fully transparent.

💡 Hint

Look at the color values in the fragment shader's return statement. The format is (red, green, blue, alpha).

The fragment shader returns fixed4(0.0, 1.0, 0.0, 1.0), which means red=0, green=1, blue=0, alpha=1 (fully opaque). So the output color is solid green.

🧠 Conceptual

intermediate

Which shader stage transforms vertex positions to clip space?

In a Unity shader, which stage is responsible for converting vertex positions from object space to clip space?

A Compute shader

B Fragment shader

C Geometry shader

D Vertex shader

💡 Hint

Think about which shader stage handles the shape and position of the object before pixels are drawn.

The vertex shader processes each vertex and transforms its position from object space to clip space, preparing it for rasterization.

🔧 Debug

advanced

Why does this shader produce a black screen?

Examine the following fragment shader code. Why does the rendered object appear completely black?

Unity

fixed4 frag(v2f i) : SV_Target {
  fixed4 color = fixed4(1.0, 0.5, 0.0, 0.0);
  return color;
}

A The fragment shader is missing a return statement.

B The RGB values are invalid and cause the shader to fail.

C Alpha is zero, so the output is fully transparent and appears black.

D The color is orange, so the object should appear orange, not black.

💡 Hint

Consider if the alpha value affects the rendering. Is there a Blend mode specified in the Pass?

The fragment shader returns fixed4(1.0, 0.5, 0.0, 0.0), an orange color with alpha=0. If the shader or material uses blending, the alpha=0 causes full transparency, making the object appear black (or invisible). Without a Blend mode, opaque rendering ignores alpha, but the question implies the black screen is due to transparency.

📝 Syntax

advanced

Which option correctly declares a float4 color variable in HLSL?

Choose the correct syntax to declare a float4 variable named color with red=1, green=0, blue=0, alpha=1 in a Unity shader.

A float4 color = float4(1, 0, 0, 1);

B float4 color = (1, 0, 0, 1);

C float4 color = {1, 0, 0, 1};

D float4 color = [1, 0, 0, 1];

💡 Hint

Remember the constructor syntax for vector types in HLSL.

In HLSL, vector types like float4 are constructed using the type name followed by parentheses enclosing the values.

🚀 Application

expert

What is the output of this shader code with conditional color?

Given this fragment shader code, what color will be output when i.pos.x is 0.3?

Unity

fixed4 frag(v2f i) : SV_Target {
  fixed4 color = (i.pos.x > 0.5) ? fixed4(0, 0, 1, 1) : fixed4(1, 1, 0, 1);
  return color;
}

A Yellow color (1,1,0,1)

B Blue color (0,0,1,1)

C Transparent color (0,0,0,0)

D Black color (0,0,0,1)

💡 Hint

Check the condition and compare the value of i.pos.x to 0.5.

Since 0.3 is not greater than 0.5, the condition is false, so the color is fixed4(1,1,0,1), which is yellow.

Practice

1. What is the main purpose of a custom shader in Unity?

easy

A. To control exactly how objects look by writing small programs

B. To create 3D models from scratch

C. To write game logic and rules

D. To manage user input events

Solution

1. Step 1: Understand the role of shaders

   Shaders define how surfaces appear by controlling colors, lighting, and textures.

2. Step 2: Identify what custom shaders do

   Custom shaders let you write small programs to control object appearance beyond built-in options.

3. Final Answer:

   To control exactly how objects look by writing small programs -> Option A

4. Quick Check:

   Custom shader = control object look [OK]

Hint: Shaders control object appearance with code [OK]

Common Mistakes:

• Confusing shaders with 3D modeling
• Thinking shaders handle game logic
• Assuming shaders manage input

2. Which of the following is the correct way to start a simple Unity shader code block?

easy

A. ShaderBegin

B. StartShader

C. CGPROGRAM

D. ProgramCG

Solution

1. Step 1: Recall Unity shader syntax

   Unity shaders use CGPROGRAM to start the programmable shader code block.

2. Step 2: Check options for correct keyword

   Only CGPROGRAM is the valid keyword to begin the shader code section.

3. Final Answer:

   CGPROGRAM -> Option C

4. Quick Check:

   Shader code block starts with CGPROGRAM [OK]

Hint: Look for CGPROGRAM to start shader code [OK]

Common Mistakes:

• Using made-up keywords like StartShader
• Confusing CGPROGRAM with other terms
• Misspelling the keyword

3. What will be the output color of this simple shader fragment code?

fixed4 frag() : SV_Target {
    return fixed4(1, 0, 0, 1);
}

medium

A. Solid red color

B. Solid green color

C. Solid blue color

D. Transparent black color

Solution

1. Step 1: Analyze the returned color vector

   The function returns fixed4(1, 0, 0, 1), which means red=1, green=0, blue=0, alpha=1.

2. Step 2: Interpret color components

   Red is full intensity, green and blue are zero, alpha is fully opaque.

3. Final Answer:

   Solid red color -> Option A

4. Quick Check:

   fixed4(1,0,0,1) = red [OK]

Hint: RGBA (1,0,0,1) means solid red [OK]

Common Mistakes:

• Mixing up color channel order
• Ignoring alpha channel
• Assuming zero means white

4. Identify the error in this shader snippet:

Shader "Custom/Simple" {
    Properties {
        _Color ("Color", Color) = (1,1,1,1)
    }
    SubShader {
        Pass {
            CGPROGRAM
            fixed4 frag() : SV_Target {
                return _Color;
            }
            ENDCG
        }
    }
}

medium

A. Missing semicolon after _Color property declaration

B. _Color cannot be used directly in fragment shader

C. frag function missing input parameters

D. Incorrect use of ENDCG keyword

Solution

1. Step 1: Check Properties block syntax

   The property declaration is correct: no semicolon needed, parentheses balanced.

2. Step 2: Verify shader code block

   Properties like _Color must be declared inside CGPROGRAM as 'uniform fixed4 _Color;' to be used; direct use causes undeclared identifier error.

3. Final Answer:

   _Color cannot be used directly in fragment shader -> Option B

4. Quick Check:

   Properties need uniform declaration [OK]

Hint: Declare Properties as uniforms in CGPROGRAM [OK]

Common Mistakes:

• Assuming semicolons are needed in Properties
• Not declaring uniform for _Color
• Thinking ENDCG is incorrect

5. You want to write a custom shader that colors an object red only if its x-coordinate is positive, otherwise blue. Which fragment shader code correctly implements this?

hard

A. fixed4 frag() : SV_Target {
if (POSITION.x > 0) return fixed4(1,0,0,1);
else return fixed4(0,0,1,1);
}

B. fixed4 frag(float3 pos) : SV_Target {
if (pos.x > 0) return fixed4(1,0,0,1);
else return fixed4(0,0,1,1);
}

C. fixed4 frag(float3 pos : TEXCOORD0) : SV_Target {
return pos.x > 0 ? fixed4(1,0,0,1) : fixed4(0,0,1,1);
}

D. fixed4 frag(float3 pos : POSITION) : SV_Target {
return pos.x > 0 ? fixed4(1,0,0,1) : fixed4(0,0,1,1);
}

Solution

1. Step 1: Understand input semantics

   To access vertex position in fragment shader, use input parameter with semantic POSITION.

2. Step 2: Check code correctness

   fixed4 frag(float3 pos : POSITION) : SV_Target { return pos.x > 0 ? fixed4(1,0,0,1) : fixed4(0,0,1,1); } correctly declares pos with POSITION semantic and uses conditional operator to return red or blue.

3. Step 3: Identify errors in other options

   fixed4 frag(float3 pos) : SV_Target { if (pos.x > 0) return fixed4(1,0,0,1); else return fixed4(0,0,1,1); } lacks semantic, fixed4 frag(float3 pos : TEXCOORD0) : SV_Target { return pos.x > 0 ? fixed4(1,0,0,1) : fixed4(0,0,1,1); } uses TEXCOORD0 which is wrong for position, fixed4 frag() : SV_Target { if (POSITION.x > 0) return fixed4(1,0,0,1); else return fixed4(0,0,1,1); } uses POSITION as variable incorrectly.

4. Final Answer:

   fixed4 frag(float3 pos : POSITION) : SV_Target { return pos.x > 0 ? fixed4(1,0,0,1) : fixed4(0,0,1,1); } -> Option D

5. Quick Check:

   Use POSITION semantic for position input [OK]

Hint: Use POSITION semantic to get vertex position [OK]

Common Mistakes:

• Missing POSITION semantic in input
• Using wrong semantic like TEXCOORD0
• Referencing POSITION as variable directly