Shapes and GL states

EG1

Draw a white square

Notice: GL state leakage is being handled implicitly.

GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
GlStateManager.disableAlpha();
GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

int x = 10, y = 10, width = 20, height = 20;

GL11.glBegin(GL11.GL_QUADS);
GL11.glVertex2f(x, y);
GL11.glVertex2f(x + width, y);
GL11.glVertex2f(x + width, y + height);
GL11.glVertex2f(x, y + height);
GL11.glEnd();

• A white square is being drawn

---

EG2

An alternative way

GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
GlStateManager.disableAlpha();
GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

int x = 10, y = 10, width = 20, height = 20;
        
Tessellator tessellator = Tessellator.getInstance();
BufferBuilder bufferbuilder = tessellator.getBuffer();
bufferbuilder.begin(GL11.GL_QUADS, DefaultVertexFormats.POSITION);
bufferbuilder.pos(x, y, 0).endVertex();
bufferbuilder.pos(x + width, y, 0).endVertex();
bufferbuilder.pos(x + width, y + height, 0).endVertex();
bufferbuilder.pos(x, y + height, 0).endVertex();
tessellator.draw();

• This example is totally equivalent to the first one
• But we use the Minecraft class Tessellator instead
• It's recommended to use Tessellator & BufferBuilder instead because glBegin...glEnd is slow

---

EG3

Without disableTexture2D

//GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
GlStateManager.disableAlpha();
GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

// draw that square
// code from EG2

• Our square becomes unexpected without disableTexture2D
• It's because a texture is being used

---

EG4

Without enableBlend

GlStateManager.disableTexture2D();
//GlStateManager.enableBlend();
GlStateManager.disableAlpha();
GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

// draw that square
// code from EG2

• We still get a white square
• Because we don't intend to draw anything transparent
• So actually no need to blend

---

EG5

Without disableAlpha

GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
//GlStateManager.disableAlpha();
GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

// draw that square
// code from EG2

• Still, nothing happens
• We use alpha test to discard pixels based on their alpha values
• In this case, we disable alpha test so we don't discard anything
• And also, this white square doesn't even have alpha values

---

EG6

Without tryBlendFuncSeparate

GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
GlStateManager.disableAlpha();
//GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

// draw that square
// code from EG2

• Still, nothing happens
• Because we don't care how we blend things, and we don't even blend

---

EG7

Without disableCull

GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
GlStateManager.disableAlpha();
GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
//GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

int x = 10, y = 10, width = 20, height = 20;
        
Tessellator tessellator = Tessellator.getInstance();
BufferBuilder bufferbuilder = tessellator.getBuffer();
bufferbuilder.begin(GL11.GL_QUADS, DefaultVertexFormats.POSITION);
bufferbuilder.pos(x, y, 0).endVertex();
bufferbuilder.pos(x + width, y, 0).endVertex();
bufferbuilder.pos(x + width, y + height, 0).endVertex();
bufferbuilder.pos(x, y + height, 0).endVertex();
tessellator.draw();

• Nothing is being rendered
• Let me explain
• By default, OpenGL uses counterclockwise vertex order for the front face of polygons. If your square's vertices are defined in clockwise order, OpenGL will consider it a "back" face and won't render it when culling is enabled, because culling removes the back faces.

---

EG8

Still without disableCull, but now it's a different story

GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
GlStateManager.disableAlpha();
GlStateManager.tryBlendFuncSeparate(GlStateManager.SourceFactor.SRC_ALPHA, GlStateManager.DestFactor.ONE_MINUS_SRC_ALPHA, GlStateManager.SourceFactor.ONE, GlStateManager.DestFactor.ZERO);
//GlStateManager.disableCull();
GlStateManager.color(1, 1, 1, 1);

int x = 10, y = 10, width = 20, height = 20;
        
Tessellator tessellator = Tessellator.getInstance();
BufferBuilder bufferbuilder = tessellator.getBuffer();
bufferbuilder.begin(GL11.GL_QUADS, DefaultVertexFormats.POSITION);
bufferbuilder.pos(x, y, 0).endVertex();
bufferbuilder.pos(x, y + height, 0).endVertex();
bufferbuilder.pos(x + width, y + height, 0).endVertex();
bufferbuilder.pos(x + width, y, 0).endVertex();
tessellator.draw();

• Square is being rendered correctly without disableCull
• Because I defined my square in a counterclockwise order

Contributors

Nullpinter

Last edited about 1 hour ago

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