Optimizing Blender’S Render Engines For Transparent Png Output

Understanding PNG Image Format

The PNG (Portable Network Graphics) format is a popular lossless image format that supports transparency. Key attributes of PNGs include:

• Lossless compression – Preserves all image data and quality on save
• Transparency support – Has an alpha channel for transparency and anti-aliasing
• Variable transparency – Supports 256 levels of transparency per pixel
• Small file sizes – More efficient compression than lossless JPEG
• Wide application support – Displayed properly in web browsers and programs

The alpha channel in a PNG determines the transparency level per pixel. White indicates fully opaque, black indicates fully transparent. Shades of gray provide variable semi-transparency between those extremes.

When rendering transparent PNGs, it’s vital Blender can accurately capture and preserve transparency information in the alpha channel. Optimized settings help minimize errors like banding while keeping file sizes small.

Blender’s Render Engines and PNG Support

Blender provides multiple render engines for generating images and animations. The two most commonly used for transparent PNG rendering are Cycles and Eevee.

Cycles Render Engine

Cycles is Blender’s unbiased photorealistic path tracer render engine. Key attributes related to PNG transparency include:

• Physically-based – Simulates realistic light transport and materials
• Unbiased – Noise-free images given enough render time
• Full PNG alpha support – Accurately preserves transparency
• Quality focused – Higher render times for accuracy

Cycles can produce extremely clean transparency and anti-aliasing in PNGs. The raytracing process accurately preserves variable alpha levels without banding or noise. The tradeoff is much higher render times.

Eevee Render Engine

Eevee is Blender’s real-time render engine. Key attributes related to PNG transparency:

• Real-time – Very fast render times by approximation
• Fixed transparency – Limited accuracy preserving variable alpha
• Quality vs speed – Sacrifices accuracy for fast rendering
• Limited anti-aliasing – Can cause jagged edges on transparency

Eevee uses shortcuts that can create artifacts in transparency like banding, noise, and jagged edges. Quality is traded for extreme render speed. Extra samples and manual anti-aliasing can improve results.

Key Render Settings for Optimized Transparent PNGs

Certain render settings have a major impact getting clean alpha channels in transparent PNG images. Optimizing these helps minimize file size while preserving quality:

Film Transparent

Enabling Film > Transparent in render properties automatically sets alpha in empty areas of the rendered image. Pixels with nothing rendered show as fully transparent rather than default black.

Anti-Aliasing

Higher anti-aliasing smooths the edges of transparency to avoid jagged edges and noise. Cycles handles this automatically. For Eevee, post-processing anti-aliasing filters are required.

Bit Depth

16-bit PNGs provide 64 times more levels of transparency per pixel than 8-bit for smoother alpha gradients. This prevents banding but increases file size. 8-bit is often adequate for Eevee due to approximation.

Z-Buffer Discarding

Enabling Z-buffer discarding avoids fully rendering hidden objects behind closer transparent ones. This speeds up render times significantly with minimal quality loss.

Setting Up Transparency and Alpha in Materials

For accurately rendered transparency in PNG images, materials must enable transparency in the Surface section. Key material settings include:

Transparency Settings

The Transparency > Alpha clipping option sets fully transparent pixels to alpha 0 rather than continuously varying values. This is useful to reduce file size and prevent background bleed.

Transmission

The Transmission value sets how transparent a material is with 1.0 being fully see-through. Values above 1 simulate light amplification in glass. This controls the alpha gradient range.

Glass BSDF

The Glass BSDF shader node is ideal for creating transparent materials in Cycles and Eevee. It allows light to pass through based on transmission while still interacting with the surface.

Optimizing Samples and Passes for Clean Transparency

Sufficient render samples are vital for smoothing noise and preventing banding in the alpha channel:

Samples

Increasing overall Sample count improves anti-aliasing for Cycles and Eevee. More samples smooth out noise and transitions. Too few samples creates unwanted grain or banding.

Transparency Passes

Dedicated transparency passes change sample counts specifically for transparent areas. This focuses effort on the hard-to-render alpha channel to improve quality.

Transparent Max bounces boosts rays for Cycles to improve accuracy in fragile light transport behind glass and translucent surfaces.

Avoiding Banding and Noise in Transparent Areas

Banding in alpha channels stems from insufficient sampling failing to capture smooth transparency gradients. Noise originates from render shortcuts and approximations disrupting light continuity through surfaces.

Clamping

Indirect clamping in Cycles reduces noise by approximating indirect light contributions after a certain level. A value between 1 and 10 is recommended for preserving transparency accuracy.

Portal Sampling

Increasing portal sample count in Eevee generates cleaner transparency by better capturing light passing through glass windows and other openings.

Depth of Field

Limit depth of field blur for transparent objects in Eevee renders. Excess blur sometimes creates unwanted scatter and noise specifically in translucent alpha channels.

Outputting and Compositing PNGs with Alpha

To preserve transparency in finished PNG images and animations, alpha channels must remain intact for compositing:

PNG Output Format

Using PNG as the output format for both still images and animations retains the alpha channel with transparency. Be sure to enable it in output properties.

Alpha Transparency in Compositor

Connect the alpha output of render layers straight into composite node alpha inputs. This ensures transparency passes through unaltered when mixing and layering rendered elements.

Alpha Over as Comp Mode

The alpha over compositing mode displays images with variable transparency properly blended together. Alpha under fails for variable transparency, showing only binary on/off.

Example Node Setups and Render Settings for Transparent PNGs

Below are some recommended node, material, and render configurations tailored for capturing clean alpha channels in transparent PNG images.

Glass Material with Accurate Caustics

• Cycles Render Engine
• Glass BSDF – IOR 1.45, Roughness 0.0, Transmission 1.0
• 3000 Samples – Reduces noise for clean transparency
• Caustics enabled – Captures realistic refracted light
• Transparent bounces 8 – Smooths rays through glass

Smoothed Eevee Glass for Performance

• Eevee Render Engine
• Glass BSDF – IOR 1.45, Roughness 0.0, Transmission 1.0
• Portal Samples 40 – Smoothes transparency noise
• Soft Shadows – Reduces jagged edges on glass
• Post AA – Enables custom anti-aliasing passes

Translucent Material Setup

• Cycles or Eevee Engine
• Translucent BSDF – IOR 1.45
• Subsurface Scattering – Adds realistic diffusion
• Transmission Rays 64 – Accurate in Cycles
• Portal Samples 30 – Smooth in Eevee