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Tiny Shader

Modern GPUs use fragment shaders to determine the final color for each pixel. Thousands of shading units run in parallel to speed up this process and ensure that a high FPS ratio can be achieved.

Tiny Shader mimics such a shading unit and executes a shader with 10 instructions for each pixel. No framebuffer is used, the color values are generated on the fly. Tiny Shader also offers an SPI interface via which a new shader can be loaded. The final result can be viewed via the VGA output at 640x480 @ 60 Hz, although at an internal resolution of 64x48 pixel.

Examples

These images and many more can be generated with Tiny Shader. Note, that shaders can even be animated by acessing the user or time register.

test2.png test4.png test5.png test7.png 
GETX R0
SINE R1
SETR R1
GETY R0
SINE R1
SETG R1
 
GETX R0
GETY R1
XOR R0 R1
SETRGB R0
 
GETY R1
LDI 1
AND R1 R0
IFNE R1
LDI 63
IFEQ R1
LDI 0
SETRGB R0
 
CLEAR R3
GETX R0
GETUSER R1
ADD R0 R1
SETRGB R0
SINE R0
HALF R0
GETY R1
IFGE R1
SETRGB R3

Here is an animation using the time register:

animation.gif

Architecture

Tiny Shader has four (mostly) general purpose registers, REG0 to REG3. REG0 is special in a way as it is the target or destination register for some instructions. All registers are 6 bit wide.

Input

The shader has four sources to get input from:

  • X - X position of the current pixel
  • Y - Y position of the current pixel
  • TIME - Increments at 7.5 Hz, before it overflow it counts down again.
  • USER - Register that can be set via the SPI interface.

Output

The goal of the shader is to determine the final output color:

  • RGB - The output color for the current pixel. Channel R, G and B can be set individually. If not set, the color of the previous pixel is used.

Sine Look Up Table

Tiny Shader contains a LUT with 16 6-bit sine values for a quarter of a sine wave. When accesing the LUT, the entries are automatically mirrored to form one half of a sine wave with a total of 32 6-bit values.

Instructions

The following instructions are supported by Tiny Shader. A program consists of 10 instructions and is executed for each pixel individually. The actual resolution is therefore one tenth of the VGA resolution (64x48 pixel).

Output

Instruction Operation Description
SETRGB RA RGB <= RA Set the output color to the value of the specified register.
SETR RA R <= RA[1:0] Set the red channel of the output color to the lower two bits of the specified register.
SETG RA G <= RA[1:0] Set the green channel of the output color to the lower two bits of the specified register.
SETB RA B <= RA[1:0] Set the blue channel of the output color to the lower two bits of the specified register.

Input

Instruction Operation Description
GETX RA RA <= X Set the specified register to the x position of the current pixel.
GETY RA RA <= Y Set the specified register to the y position of the current pixel.
GETTIME RA RA <= TIME Set the specified register to the current time value, increases with each frame.
GETUSER RA RA <= USER Set the specified register to the user value, can be set via the SPI interface.

Branches

Instruction Operation Description
IFEQ RA TAKE <= RA == R0 Execute the next instruction if RA equals R0.
IFNE RA TAKE <= RA != R0 Execute the next instruction if RA does not equal R0.
IFGE RA TAKE <= RA >= R0 Execute the next instruction if RA is greater then or equal R0.
IFLT RA TAKE <= RA < R0 Execute the next instruction if RA is less than R0.

Arithmetic

Instruction Operation Description
DOUBLE RA RA <= RA * 2 Double the value of RA.
HALF RA RA <= RA / 2 Half the value of RA.
ADD RA RB RA <= RA + RB Add RA and RB, result written into RA.

Load

Instruction Operation Description
CLEAR RA RA <= 0 Clear RA by writing 0.
LDI IMMEDIATE RA <= IMMEDIATE Load an immediate value into RA.

Special

Instruction Operation Description
SINE RA RA <= SINE[R0[4:0]] Get the sine value for R0 and write into RA. The sine value LUT has 32 entries.

Boolean

Instruction Operation Description
AND RA RB RA <= RA & RB Boolean AND of RA and RB, result written into RA.
OR RA RB RA <= RA | RB Boolean OR of RA and RB, result written into RA.
NOT RA RB RA <= ~RB Invert all bits of RB, result written into RA.
XOR RA RB RA <= RA ^ RB XOR of RA and RB, result written into RA.

Move

Instruction Operation Description
MOV RA RB RA <= RB Move value of RB into RA.

Shift

Instruction Operation Description
SHIFTL RA RB RA <= RA << RB Shift RA with RB to the left, result written into RA.
SHIFTR RA RB RA <= RA >> RB Shift RA with RB to the right, result written into RA.

Pseudo

Instruction Operation Description
NOP R0 <= R0 & R0 No operation.

Tiny Tapeout

This project was designed for Tiny Tapeout.