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.

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:

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

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