Gamemaker Studio 2 2d Shaders Foundations Exercises

Notes

There are differences between Unity's Shaderlab and GLSL(ES) that GMS2 uses.

• The blending operation is actually not part of the shader language itself, rather is a separate API call. You use either gpu_set_blendmode() or for more complicated modes gpu_set_blendmode_ext() for this purpose. You also want to return the blend mode back to bm_normal afterwards so that other objects don't get drawn with the same blending mode.
• At this time, you cannot change the blend mode operation. You can set the multipliers(what comes after Blend in Shaderlab, like SrcColor, One, and OneMinusSrcAlpha) via the function gpu_set_blendmode_ext(), or for common defaults, you can gpu_set_blendmode() which allows a single constant to be specified. But, the operation between the source and destination(BlendOp in Shaderlab), is currently not accessible in GMS2.

• This leads to an issue trying to reproduce accurately the subtractive blend mode. You can use bm_subtract as the parameter to the call to gpu_set_blendmode() but this is the same as calling gpu_set_blendmode_ext() with parameters bm_zero and bm_inv_src_color. The result is technically a form of subtractive blending, but the result color is not an exact subtraction. For example, subtracting 0.25 source from 0.75 destination should result in 0.5, but actually gives you 0.5625 using GMS2's method.

There is a helper object called objRoomChanger. It is set to persistent. It's purpose is simply to receive the spacebar input and change rooms between the two exercises.

Exercise 1

Shaders

• shdScroller-This shader takes a single float Uniform which is added to the U value of the UV coordinate. The GML code increases this value from 0 to 1 based on the speed set in the code.

objects

• objCityLoop-This draws the looping city background

• Create-creates the scrollSpeed and scrollValue variables
• Step-increases the scrollValue variable by the scrollSpeed, and reduces the scrollValue back to 0 when it reaches 1.
• Draw-calls gpu_set_tex_repeat(true) so that the texture loops instead of clamping(this is a texture import setting in Unity). It also handles setting the shader, sending the uniform variable, and draws a little debug text so you can see the scrollvalue. Note that you have to call shader_reset() to return the shader to the default. If you don't, all the objects that get drawn afterwards will use the same shader.

Rooms

• rmScrolling-contains an instance of objCityLoop.

Exercise 2

Shaders

• There is no separate shader for this exercise, as the blend modes are not part of the shader code in GLSL(ES), unlike Unity's Shaderlab.

Objects

• objAlpha-This does nothing except use the default drawing(no draw event event). GMS2 uses alpha blending by default, so there is no need to change blend modes.
• objAdditive-This draws the sprite with additive blending.

• Draw-sets the blend mode to bm_add which is accurate additive blending. Calling gpu_set_blendmode(bm_add) is the same thing as calling gpu_set_blendmode_ext(bm_one, bm_one). Note that you also call gpu_set_blendmode(bm_normal) after drawing the sprite. This is important because any sprites that get drawn afterwards will use the wrong blend mode.

• objMultiply-This draws the sprite with the multiply blend mode. Identical to objAdditive besides the blend mode.

• Draw-calls gpu_set_blendmode_ext(bm_dest_colour, bm_zero) since this mode is not handled by gpu_set_blendmode().

• objSubtractive-This draws the sprite with the subtractive blend mode. See the notes as to why this isn't an exact subtractive blend. Identical to objAdditive.

• Draw-calls gpu_set_blendmode(bm_subtract), draws the srpite, then resets the blend mode.

Rooms

• rmBlending-Contains instances of objAlpha, objAdditive, objMultiply, and objSubtractive.